Audio Source Enhancer

Vinyl or CD: which has the better sound? It’s  a question still hotly debated among audiophiles everywhere. We will try to shed a little light on what lies behind the question and  look at a simple circuit that can significantly  enhance the sound from a CD player.  Sometimes, on first hearing a new low- to  mid-range CD player, the sound is not altogether  convincing  when  compared  to  a  record  player.  It  is  worth  looking  at  the  recording and replay processes as a whole  for  both  CDs  and  records  to  see  why  this  might be. Assuming that we start from the  same source, or master recording, of a given  piece of music, the differences are broadly as  follows.

 

Circuit diagram :

 

Audio Source Enhancer Circuit Diagram

 

Records and CDs use very different recording  technologies.  For  records  the  signal  first  undergoes  pre - emphasis  similar  to  that  used  in  FM  radio,  where  the  higher  frequency  components  of  the  signal  are  amplified. The resulting signal is cut into  the lacquer master disc that will be used  for pressing. Unlike CD manufacture, this is  an entirely analogue process, and it introduces a phase shift into the signal. To compensate for the pre-emphasis the preamplifier in a record player includes a de-emphasis (or ‘RIAA’) filter which attenuates the  higher  frequency  components.  The  purpose  of  pre - emphasis  is  to  improve  the  overall signal-to-noise ratio of the signal as  played back, reducing hiss and crackle. De-emphasis introduces further phase shifts,  and as a result the final signal is rather different from that produced by a CD player.  The processing involved in CD manufacture  and playback can be entirely digital (in the  case of ‘DDD’ recordings) and phase errors  are reduced practically to zero.

 

The circuit shown here uses a quad opamp  (two opamps per channel) to produce ‘record-like’ phase shifts. In the author’s experience  low- and mid-range CD players tend to have greatly attenuated output at higher frequencies, and the circuit therefore also offers the  facility to boost these components to taste.  The value of capacitors C8 and C14 may be  anywhere between 100 pF and 10 nF according to the frequency response desired. At the low-frequency end the response is  more than adequate, thanks to the large coupling capacitors used. The circuit also functions as a buffer or impedance converter,  which can help to reduce the effect of cable  capacitances. With CD players that have an output impedance of 1 kΩ or more the difference between cheap cables and more expensive low-capacitance cables can be noticeable. This circuit has an output impedance of  just 100 Ω and so cheaper cables should normally be more than adequate.

 

The circuit can of course also be used with  other digital audio sources such as minidisc  players, hard disk recorders, DAB tuners, dig-ital terrestrial and satellite television receivers and so on. The supply voltage can be any-where from 10 V to 30 V. It will often be possible to take power from the CD player’s own supply; if not, a separate  AC power adaptor can be used. The output signal for each channel is inverted (i.e., is subjected to a  180 degree phase shift) by the second opamp (IC1.B and IC1.D). This  does not affect the operation of the circuit. By changing the value of  feedback resistors R4 (for IC1.B) and R12 (for IC1.D) the overall gain of  the circuit can be adjusted so that the output level matches that of  other components in the audio system.

 

Author :Thorsten Steurich  – Copyright : Elektor

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Novel Liquid-Level Sensor

Normally, the level of a liquid in a container is determined by sensing changes in the capacitance or resistance between a pair of electrodes that are immersed in the liquid. Generally speaking, this technique requires fairly complicated circuitry to protect the electrodes against electrolysis (and associated corrosion). In addition, in many cases the liquid must be conductive for the measurement principle to actually be usable. The circuit presented here shows that an alternative approach is possible.

Circuit diagram:

Novel Liquid-Level Sensor Circuit Diagram

Here we utilise the fact that a PTC resistor warms up in pro-portion to the amount of current flowing through it, with the result that its resistance increases. If a PTC resistor is immersed in a liquid, the additional warmth is dissipated in the liquid and the resistance remains nearly constant. If the level of the liquid drops below the immersion depth of the resistor, the change in the resistance can be easily sensed by a subsequent comparator stage. The PTC resistor should be isolated from the fluid into which it is immersed, in order to prevent undesirable electrolytic processes from taking place. A further improvement in the characteristics of the circuit can be achieved by using a logic circuit such as a microcontroller to apply power to the circuit only at predefined times and then switch off the power after sampling the comparator output.

Copyright : Elektor

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Dual High Side Switch Controller

Circuit diagram :

Dual High Side Switch Controller Circuit Diagram

One of the most frequent uses of n-channel MOSFET’s is as a voltage controlled switch. To ensure that the MOSFET delivers the full supply voltage to the load it is necessary for the gate voltage to be a few volts above the supply voltage level. This can be a problem if no other suitable higher volt-age sources are available for use elsewhere in the circuit. The LTC 1982 dual high-side switch controller from Lin-ear Technology (www.linear-tech.com) solves this problem by incorporating a voltage tripler circuit in the gate driver stage. The gate voltage is limited to +7.5 V which is 2.0 V above the IC’s maximum operating voltage. It can directly drive the gate of logic-level MOSFET with a VGS(th) from 1.0 V to 2.0 V. A suitable n-channel logic level MOSFET would be the BSP 295. This device can switch up to 1.5 A and is available in an SOT 233 SMD package.

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LDO Regulator

Recently the author had to adapt  a standard circuit configuration  (which often uses an npn bipolar) so as to operate as a low-dropout (LDO) regulator. The circuit shown here uses that rarity,  a depletion-mode MOSFET to implement the LDO function. What to do when you have to derive an analogue supply voltage  (close to +5 V) from an existing ‘digital’ 5-volt rail, ensuring sufficient decoupling between the two? One answer is  to step up and then use a linear regulator to step back down. However, if around 4.5 volts will suffice then an alternative is a home-made LDO regulator. The circuit is usually a fairly  standard shape typically a npn transistor (with base-current limiting resistor) is used.

 

Circuit diagram :

 

LDO Regulator Circuit Diagram

 

Initially, it would appear that this design suffices after all, the text books say the saturation voltage  is around 0.2 V. Unfortunately,  this is no longer true when the collector is tied directly to the positive supply. An enhancement-mode  MOSFET suffers  similar disadvantages: with the drain tied High you need greater than  drain  potential at the gate to achieve low RDS(on). Enter that seldom-used beast the depletion-mode MOSFET! Depletion-mode MOSFETS are ‘on’ even  when  V gs = 0,  and  you have to back-bias the gate  to achieve an increase in channel resistance.In the circuit shown the BSS139,  an NMOS depletion device, operates with the gate forward biased. With a load of 10 mA, the measured FET resistance was  38 ohms.

 

Author :Stephen Bernhoeft - Copyright : Elektor electronics

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Bathroom Fan Controller

Many bathrooms are fitted with a fan to vent  excess humidity while someone is showering. This fan can be connected to the light  switch, but then it runs even if you only want  to brush your teeth. A better solution is to  equip the fan with a humidity sensor. A disadvantage of this approach is that by the time  the humidity sensor switches on the fan, the  room is already too humid. Consequently, we decided to build a circuit  that operates by sensing the temperature of  the hot water line to the shower. The fan runs  as soon as the water line becomes hot. It continues to run for a few minutes after the line  cools down, so that you have considerably  fewer problems with humidity in the bathroom without having the fan run for no reason.

 

Naturally, this is only possible if you can  fit a temperature sensor somewhere on the  hot water line and the line does not become  warm if hot water is used somewhere else. We use an LM335 as the temperature sensor.  It generates an output voltage of 10 mV per  Kelvin. The output voltage is 3.03 V at 30 °C,  3.13 V at 40 °C, 3.23 V at 50 °C, and so on.  We want to have the fan switch on at a temperature somewhere between 40 and 50 °C (approx.100–150 °F). To do this accurately,we first use the opamps in IC2 to improve  the control range. Otherwise we would have  an unstable circuit because the voltage differences at the output of IC1 are relatively  small. IC2a subtracts a voltage of exactly 3.0 V from  the output voltage of IC1.

 

Circuit diagram :

 Bathroom Fan Controller Circuit Diagram

 

It uses Zener diode  D1 for this purpose, so this is not dependent on the value of the supply voltage. The  value of R2 must be selected according to  the actual supply voltage so that the current through D1 is approximately 5 mA. It is  600 Ω with a 6-V supply (560 Ω is also okay),  or 2400 Ω (2.2 kΩ) with a 15-V supply. If you  have to choose between two values, use the  lower value. IC2b amplifies the output voltage of IC2a  by a factor of 16 ((R7 + R8) ÷ R8). As a result,  the voltage at the output of IC2b is 0.48 V at  30 °C, 2.08 V at 40 °C (104 °F), and 3.68 V at  50 °C (122 °F). Comparator IC3a compares this  voltage to a reference voltage set by P1. Due  to variations resulting from the tolerances of  the resistor values, the setting of P1 is best  determined experimentally. A voltage of 2.5 V  on the wiper should be a good starting point  (in theory, this corresponds to 42.6 °C).

 

When  the water line is warm enough, the output of IC3 goes Low. R10  provides  hysteresis  at  the  output  of  IC3a by pulling the voltage on the wiper of  the setting potentiometer down a bit when  the output of IC3a goes Low. IC3b acts as an  inverter so that relay Re1 is energised via T1,  which causes the fan to start running. After  the water line cools down, the relay is de-energised and the fan stops. If this happens  too quickly, you can reduce the value of R11  (to 33 kΩ, for example). This increases the  hysteresis. The circuit does not draw much current, and  the supply voltage is noncritical. A charging  adapter from a discarded mobile phone can  thus be used to power the circuit. If the supply voltage drops slightly when the relay is  energised, this will not create any problem.  In this case the voltage on the wiper of P1 will  also drop slightly, which provides a bit more  hysteresis on IC3a.

 

Author : Heino Peters - Copyright : Elektor

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Electronic Security System

This reliable and easy-to-operate electronic security system can be used in banks, factories, commercial establishments, houses, etc. The system comprises a monitoring system and several sensing zones. Each sensing zone is provided with a closed-loop switch known as sense switch. Sense switches are fixed onthedoors of premises under security and connected to the monitoring system. As long as the doors are closed, sense switches are also closed. The monitoring system can be installed at a convenient central place for easy operation.

Fig. 1 shows the monitoring circuit only for zone 1 along with the common alarm circuit. For other zones, the monitoring circuit is identical, with only the prefixes of components changing as per zone number. Encircled points A, B, and C of each zone monitoring circuit need to be joined to the corresponding points of the alarm circuit (upper half of Fig. 1).

Circuit diagram :

Fig. 1: Monitoring circuit along with the alarm circuit

When zone 1 sensing switch S11, zone switch S1 are all on, pnp transistor T12 reverse biases to go in cut-off condition, with its collector at around 0 volt. When the door fitted with sensor switch S11 is opened, transistor T12 gets forward biased and it conducts. Its collector voltage goes high, which forward biases transistor T10 via resistor R10 to turn it on. (Capacitor C10 serves as a filter capacitor.) As a result, the collector voltage of transistor T10 falls to forward bias transistor T11, which conducts and its collector voltage is sustained at a high level. Under this latched condition, sensor switch S11 and the state of transistor T12 have no effect. In this state, red LED11 of the zone remains lit.

Simultaneously, the high-level voltage from the collector of transistor T11 via diode D10 is applied to VDD pin 5 of siren sound generator IC1 (UM3561) whose pin 2 is grounded. Resistor R3 connected across pins 7 and 8 of IC1 determines the frequency of the in-built oscillator. As a result, IC1 starts generating the audio signal output at pin 3. The output voltage from IC1 is further amplified by Darlington pair of transistors T1 and T2. The amplified output of the Darlington pair drives the loudspeaker whose output volume can be controlled by potentiometer VR1. Capacitor C1 serves as a filter capacitor.

Physical layout :

Fig. 2: Physical layout of sensors and monitoring alarm system

You can alter the alarm sound as desired by changing the connections of IC1 as shown in the table.

The circuit continues to sound the alarm until zone door is closed (to close switch S11) and the reset switch is pressed momentarily (which causes transistor T10 to cut off, returning the circuit to its initial state). The system operates off a 3V DC battery or recharging battery with charging circuit or battery eliminator. If desired, more operating zones can be added. Initially keep the monitoring system switch S1 off. Keep all the zone doors fixed with sensing switches S11, S21, S31, S41, etc closed. This keeps the sensing switches for respective zones in closed position. Also keep zone slide switches S12, S22, S32, S42, etc in ‘on’ position.

This puts the system in operation, guarding all the zone doors.Now, if the door of a particular zone is opened, the monitoring system sounds an audible alarm and the LED corresponding to the zone glows to indicate that the door of the zone is open. The alarm and the LED indication will continue even after that particular door with the sensing switch is immediately closed, or even if that switch is removed/damaged or connecting wire is cut open. Any particular zone in the monitoring system can be put to operation or out of operation by switching on or switching off the corresponding slide switch in the monitoring system.

Author : K. Bharathan - Copyright : Electronicsforu

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Automatic Bicycle Light

T his  automatic  bicycle  light  makes cycling in the dark much  easier (although you still need  to pedal of course). The circuit  takes  the  ambient  light  level  into account and only turns on  the light when it becomes dark.  The light is turned off when no  cycling has taken place for over  a minute or if it becomes light  again. The biggest advantage of  this circuit is that it has no manual controls. This way you can  never ‘forget’ to turn the light  on or off. This makes it ideal for  children and those of a forgetful  disposition.

 

Bicycle Light Image :

To detect when the bicycle is  used (in other words, when the  wheels turn), the circuit uses a  reed switch (S1), mounted on  the frame close to the wheel.  A small magnet is fixed to the  spokes (similar to that used with  most  bicycle  speedometers),  which  closes  the  reed  switch  once for every revolution of the  wheel. Whilst the wheel turns,  pulses are fed to the base of T1  via C1. This charges a small electrolytic capacitor (C2). When it is  dark enough and the LDR there-fore has a high resistance, T2  starts conducting and the lamp  is turned on. With every revolution of the wheel C2 is charged  up again. The charge in C2 ensures that T2  keeps conducting for about a minute after  the wheel stops turning. Almost any type of  light can be connected to the output of the  circuit.

 

Circuit diagram :

Automatic Bicycle Light Circuit Diagram

Part List :

Resistors
R1 = 1MΩ (SMD 0805)
R2,R4 = 100kΩ (SMD 0805)
R3,R6 = 1kΩ (SMD 0805)
R5 = LDR e.g. FW150 Conrad Electronics # 183547

Capacitors

C1 = 1µF 16V (SMD 0805)
C2 = 10µF 16V (SMD chip type)
C3 = 100nF (SMD 0805)

Semiconductors

T1 = BC807 (SMD SOT23)
T2 = STS6NF20V (SMD SO8)

Miscellaneous

S1 = reed switch (not on board) +
2-way right angle pinheader
BT1 = 3–12V (see text)

 

With a supply voltage of 3V the quiescent  current when the reed switch is open is just  0.14 μA. When the magnet happens to be in  a position such that S1 is closed,  the current is 3 μA. In either case  there is no problem using batteries to supply the circuit. The  supply voltage can be anywhere  from 3 to 12 V, depending on the  type of lamp that is connected. Since it is likely that the circuit  will be mounted inside a bicycle light it is important to keep  an eye on its dimensions. The  board has therefore been kept  very compact and use has been made of SMD components. Most  of them come in an 0805 pack-age.  C2 comes in a so called  chip version. The board is single sided with the top also acting as the solder side.

The print outline for the LDR (R5)  isn’t exactly the same as that of  the  outline  of  the  LDR  mentioned  in  the  component  list.  The outline is more a general one  because there is quite a variety  of different LDR packages on the  market. It is therefore possible  to use another type of LDR, if for  example the light threshold isn’t  quite right. The LDR may also be  mounted on the other side of the  board, but that depends on how  the board is mounted inside the  light. For the MOSFET there are also many alternatives available, such as the FDS6064N3 made  by   Fairchild ,  the  SI4864 DY  made by  Vishay Siliconix , the IR F74 0 4 made by IR F or the NTMS 4N01R 2G  made by ONSEMI. The reed switch also  comes in many different shapes and sizes; some of them are even waterproof and come with the wires already attached.

 

For the supply connection and  the connection to the lamp you  can either use PCB pins or solder the wires directly onto the  board. The soldered ends of the  pins can be shortened slightly so that they  don’t stick out from the bottom of the board.  This reduces the chance of shorts with any metal parts of the light. Do take care when you use a dynamo  to  power the circuit the alternating voltage must first be rectified! The same applies to  hub dynamos, which often also output an  alternating voltage.

 

Please Note. Bicycle lighting is subject to legal restrictions, traffic laws and, additionally in  some countries, type approval.

Download : 090102-1 PCB layout (.pdf), from www.elektor.com

Author : Ludwig Libertin (Austria) – Copyright : Elektor

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Audio Controlled Mains Switch

It is often useful for audio or video equipment to be switched off automatically after there has been no input signal for a while. The function of the on-off switch in such equipment is then taken over by switch S2 in the accompanying diagram. It remains, however, possible to  switch off manually by means of Si. Automatic  switch-off occurs after there has been no input  signal for about 2 minutes: this delay makes it possible for a new record or cassette to be placed in the  relevant machine.

 

The audio input to the proposed circuit may be  taken from the output of the relevant TV set, amplifier, or whatever. The input earth is held at + 6 V  with respect to the circuit earth by potential divider  Ri-R2-R3-R4. The two 741s function as comparators: the output of ICi goes high when the in- put signal is greater than + 50 mV, whereas the out- put of IC2 goes high when the input signal  becomes more negative than -50 mV. Resistors  R6, R7, and R8 form an OR gate that drives transistor Ti. If the output of either ICi or IC2 is logic  1, Ti conducts.

 

Circuit diagram :

Audio Controlled Mains Switch Circuit Diagram

 

The 555  operates as a retrigger able monostable,  whose period is determined by Rio and Ci. The  device is triggered when its pin 2 is earthed by the  closing of S2. Its output, pin 3, then remains high  for 1 to 2 minutes, depending on the leakage cur- rent of the 555. The monostable resets itself as soon  as the potential across Ci exceeds a certain value.  As long as there is an input signal to the circuit, Ti conducts and Ci remains uncharged. As soon as  the audio signal ceases, Ti switches off, and Ci  charges until the potential across it is sufficient to  reset the 555. The monostable may also be reset by  closing Si, which connects pin 6 of the 555 to + 12 V.

 

When IC3 is reset, Ci is discharged via its pin 7. Resistor Rrn serves as protection, because without it Ti could short-circuit the supply lines. When the output of IC3 goes high, T2 conducts,  the relay is energized, and the relay contacts switch on the mains voltage as appropriate. To counter the induced potential when the relay contacts close, which could damage T2, diode Di has been connected in parallel with the relay coil.

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Two-wire Lamp Flasher

This circuit was designed to provide that continuous light lamps already wired into a circuit, become flashing. Simply insert the circuit between existing lamp and negative supply. Especially suited for car or panel pilot lights, this device can drive lamps up to 10W.

Circuit diagram :

Two-wire Lamp Flasher Circuit Diagram

Parts:

R1_____6K8  1/4W Resistor
R2_____270K   1/4W Resistor
R3_____22K   1/4W Resistor

C1_____220µF   25V Electrolytic Capacitor
C2_____10µF   25V Electrolytic Capacitor

D1_____1N4002  100V 1A Diode

Q1_____BC557   45V 100mA PNP Transistor
Q2_____BD139   80V 1.5A NPN Transistor

LP1_____Existing filament Lamp: any type in the range 3-24V 10W max.

SW1_____Existing On-Off switch

B1_____Existing V DC source: any type in the range 3-24V suited to the lamp adopted

Notes:

• Break lamp(s) to negative supply connection(s), then insert the circuit between existing lamp(s) connection(s) and negative supply (respecting polarities!).
• C1 value can be varied from 100 to 1000µF or higher, in order to change flashing frequency.
• Although rather oversized, this circuit can also drive any LED, providing a suitable resistor is fitted in series with the light emitting device.
• The resistor should lie in the 47R to 2K2 range, depending on supply voltage.

Source : redcircuits

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Motorbike Alarm

This simple to build alarm can be fitted in bikes to protect them from being stolen. The tiny circuit can be hidden anywhere, without any complicated wiring. Virtually, it suits all bikes as long as they have a battery. It doesn't drain out the battery though as the standby current is zero. The hidden switch S1 can be a small push-to-on switch, or a reed switch with magnet, or any other similar simple arrangement. The circuit is designed around a couple of low-voltage MOSFETs configured as monostable timers. Motorbike key S2 is an ignition switch, while switch S3 is a tilt switch. Motorbike key S2 provides power supply to the gate of MOSFET T2, when turned on.

When you turn ignition off using key S2, you have approximately 15 seconds to get off the bike; this function is performed by resistor R6 to discharge capacitor C3. Thereafter, if anyone attempts to get on the bike or move it, the alarm sounds for approximately15 seconds and also disconnects the ignition circuit. During parking, hidden switch S1 is normally open and does not allow triggering of mosfet T1. But when someone starts the motorbike through ignition switch S2, MOSFET T2 triggers through diode D1 and resistor R5. Relay RL1 (12V, 2C/O) energises to activate the alarm (built around IC1) as well as to disconnect the ignition coil from the circuit. Disconnection of the ignition coil prevents generation of spark from the spark plug. Usually, there is a wire running from the alternator to the ignition coil, which has to be routed through one of the N/C1 contacts of relay RL1 as shown in Fig.1 Fig.2 shows the pin configurations of SCR BT169, MOSFET BS170 and transistor BC548.

Circuit diagram :

 

 Motorbike Alarm Circuit Diagram

Motorbike Alarm-Pin Configurations :

 

Pin configurations of BT169, BS170 and BC548

Also, on disconnection of the coil, sound generator IC UM3561 (IC1) gets power supply through N/O2 contact of relay RL1. This drives the darlington pair built around T3 and T4 to produce the siren sound through loudspeaker LS1.  To start the vehicle, both hidden switch S1 and ignition key S2 should be switched on. Otherwise, the alarm will start sounding. Switching on S1 triggers SCR1, which, in turn, triggers MOSFET T1. MOSFET T1 is configured to disable MOSFET T2 from functioning. As a result, MOSFET T2 does not trigger and relay RL1 remains de-energised, alarm deactivated and ignition coil connected to the circuit.  Connection to the ignition coil helps in generation of spark from the spark plug. Keeping hidden switch S1 accessible only to the owner prevents the bike from pillaging. Tilt switch S3 prevents attempt to move the vehicle without starting it. Glass-and metal-bodied versions of the switch offer bounce-free switching and quick break action even when tilted slowly.

Unless otherwise stated, the angle by which the switch must be tilted to ensure the contact operation (operating angle), must be approximately 1.5 to 2 times the stated differential angle. The differential angle is the measure of the 'just closed' position to the 'just open' position. The tilt switch has characteristics like contacts make and break with vibration, return to the open state at rest, non-position sensitivity, inert gas and hermetic sealing for protection of contacts and tin-plated steel housing. If you find difficulty in getting the tilt switch, you may replace it with a reed switch (N/O) and a piece of magnet. The magnet and the reed switch should be mounted such that the contacts of the switch close when the bike stand is lifted up from rest.

EFY Note. Make sure that while driving, the two internal contacts of the Tilt switch don't touch each other.

 

Author : T.A. BABU - Copyright : electronicsforu

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Simple Automatic Switch For Audio Power Amplifier

Circuit of an automatic switch for audio power amplifier stage is presented here. The circuit uses stereo preamplifier output to detect the presence of audio to switch the audio power amplifier on only when audio is present. The circuit thus helps curtail power wastage. IC1 is used as an inverting adder. The input signals from left and right channels are combined to form a common signal for IC2, which is used as an open loop comparator. IC3 (NE556) is a dual timer. Its second section, i.e., IC3(b), is configured as monostable multivibrator. Output of IC3(b) is used to switch the power amplifier on or off through a Darlington pair formed by transistors T1 and T2. IC3(a) is used to trigger the monostable multivibrator whenever an input signal is sensed.

Circuit diagram:

Automatic Switch For Audio Power Amplifier Circuit Diagram

Under ‘no signal’ condition, pin 3 of IC2 is negative with respect to its pin 2. Hence the output of IC2 is low and as a result output of IC3(a) is high. Since there is no trigger at pin 8 of IC3(b), the output of IC3(b) will be low and the amplifier will be off. When an input singal is applied to IC1, IC2 converts the inverted sum of the input signals into a rectangular waveform by comparing it with a constant voltage which can be controlled by varying potentiometer VR1. When the output of IC2 is high, output pin 5 of IC3 goes low, thus triggering the monostable multivibrator. As soon as the audio input to IC1 stops, pin 5 of IC3 goes high and pin 1 of IC3 discharges through capacitor C3, thus resetting the monostable multivibrator.

Hence, as long as input signals are applied, the amplifier remains ‘on.’ When the input signals are removed, i.e., when signal level is zero, the amplifier switches off after the mono flip-flop delay period determined by the values of resistor R8 and capacitor C3. If no input signals are sensed within this time, the amplifier turns off—else it remains on. Power supply for the circuit can be obtained from the power supply of the amplifier. Hence, the circuit can be permanently fitted in the amplifier box itself. The main switch of the amplifier should be always kept on. Resistors R1 and R2 are used to divide single voltage supply into two equal parts.

Capacitors C1 and C2 are used as regulators and also as an AC bypass for input signals. Diode D1 is used so that loading fluctuations in power amplifier do not affect circuit regulation. Transisitor T2 acts as a high voltage switch which may be replaced by any other high voltage switching transistor satisfying amplifier current requirements. Value of resistor R10 should be modified for large current requirement. The LED glows when the amplifier is on. The circuit is very useful and relieves one from putting the amplifier on and off every time one plays a cassette or radio etc.

Source : EFY

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PC Power Saver

This circuit is designed to help minimise the  quiescent power consumption of PCs and  notebooks, using just our old friend the 555  timer and a relay as the main components. The  circuit itself dissipates around 0.5 W in operation (that is, when the connected PC is on);  when switched off (with the relay not energised) the total power draw is precisely zero. A prerequisite for the circuit is a PC or note book with a USB or PS/2 keyboard socket that  is powered only when the PC is on. The power saver can be used to switch PCs  or even whole multi-way extension leads. The unit can be built  into  an  ordinary  mains  adaptor (which must have an earth  pin!) as the photograph of the  author‘s prototype shows. The  PC is plugged in to the socket  at the output of the power saver  unit, and an extra connection  is made to the control input of  the unit from a PS/2 (keyboard or mouse) socket or USB port. Only  the 5 V supply line of the interface is used.

 

  

PC Power Saver Image

When button S1 on the power saver  is pressed the unit turns on, and the  monostable formed by the 555 timer is  triggered via the network composed by  R4 and C7. This drives relay RE1, whose contacts close. The connected PC is now tentatively powered up via the relay for a period  determined  by  P1  (approximately in the range from 5 s to 10 s). If, during this interval, the PC fails to indicate  that it is alive by supplying 5 V from its USB or  PS/2 connector (that is, if you do not switch  it on), the monostable period will expire, the  relay will drop out and any connected device  will be powered down. No further current will  be drawn from the supply, and, of course, it  will not be possible to turn the PC on. When-ever you want to turn the PC on, you must  always press the button on the power saver  shortly beforehand.

If, however, 5 V is delivered by the PC to the  input of optocoupler IC2 before the monostable times out (which will be the case if the  PC is switched on during that period), the  transistor in the optocoupler will conduct  and discharge capacitor C6. The monostable  will now remain triggered and the relay will  remain energised until the PC is switched off  and power disappears from its USB or PS/2  interface. Then, after the monostable time  period expires, the relay will drop out and the  power saver will disconnect itself from the mains. There is no need to switch anything  else off: just shut down the system and the  power saver will take care of the rest.

 

Circuit diagram :

PC Power Saver Circuit Diagram

 

It is also  possible to leave the machine as it updates its  software, and the power saver will do its job  shortly after the machine shuts down. Power for the unit itself is obtained using a  simple supply circuit based around a miniature transformer. Alternatively, a 12 V mains  adaptor can be used, as long as a relay with a  12 V coil voltage is used for RE1. In his proto-type the author used a relay with a 24 V coil  connected as shown directly to the positive  side of reservoir capacitor C2, the 555 being  powered from 12 V regulated from that sup-ply using R1 and D1. A fixed resistor can of  course be used in place of P1 if desired. If the  adjustment range of P1 is not sufficient (for  example if the PC powers up very slowly) the  monostable period can be increased by using  a larger capacitor at C6.  The relay must have at least two normally-open (or changeover) contacts rated at at  least 8 A. The contact in parallel with S1 is used to supply power to the device  itself, and the other contact carries  all the current for the connected  PC  or  for  the  ex tension  lead  to  which  the  PC  and  peripherals  are  connected.

Pushbutton S1 must be rated for 230 VAC  (US: 120 VAC) operation: this is no place to  make economies. The coil current for the relay  flows through LED D5, which must therefore  be a 20 mA type. If a low-current LED is used,  a 120 Ω resistor can be connected in parallel with it to carry the remaining current.  The Fujitsu FTR-F1CL024R relay used in the  author’s prototype has a rated coil current of  16.7 mA. Optocoupler IC2 provides isolation between  the circuit and the PC, and is protected from  reverse polarity connection by diode D4. The power saver should be built into an insulated enclosure and great care should be  taken to ensure that there is proper isolation  between components and wires carrying the  mains voltage and the other parts of the circuit. In particular, the connection to the PC  and associated components (R6, C5, D4 and  IC2) should be carefully arranged with at least  a 6 mm gap between them and any part of  the circuit at mains potential.

 

Author : Wolfgang Gscheidle (Germany) - Copyright : Elektor

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Test Beeper For Your Stereo

The test beeper generates a sinusoidal signal with a frequency of 1,000 Hz, a common test  frequency for audio amplifiers.  It consists of a classical Wien- Bridge oscillator (also known as  a Wien-Robinson oscillator). The network that determines the  frequency consists here of a series connection of a resistor and  capacitor (R1/C1) and a parallel connection (R2/C2), where  the values of the resistors and  capacitors  are  equal  to  each  other. This network behaves, at  the oscillator frequency (1 kHz  in this case), as two pure resistors. The opamp (IC1) ensures  that the attenuation of the net- work  (3  times)  is  compensated  for.  In  principle  a  gain  of  3 times should have been sufficient to sustain the oscillation,  but  that  is  in  theory.  Because  of tolerances in the values, the  amplification needs to be (automatically) adjusted.

 

Circuit diagram:

Test Beeper For Your Stereo circuit Diagram

 

Instead of an intelligent amplitude  controller  we  chose  for  a  somewhat simpler solution. With  P1, R3 and R4 you can adjust  the gain to the point that oscillation takes place. The range of P1 (±10%) is large enough the cover the tolerance range. To sustain  the oscillation, a gain of slightly  more than 3 times is required,  which  would,  however,  cause  the amplifier to clip (the ‘round-trip’ signal becomes increasingly  larger, after all). To prevent this  from happening, a resistor in se-ries with two anti-parallel diodes  (D1 and D2) are connected in  parallel  with  the  feedback  (P1  and R3). If the voltage increases to the point that the threshold  voltage of the diodes is exceed-ed, then these will slowly start to  conduct.

 

The consequence of this  is that the total resistance of the  feedback  is  reduced  and  with  that  also  the  amplitude  of  the  signal. So D1 and D2 provide a  stabilising function. The distortion of this simple oscillator, after adjustment of P1 and  an output voltage of 100 mV (P2  to  maximum)  is  around  0,1%.  You can adjust the amplitude of  the output signal with P2 as required for the application. The  circuit is powered from a 9-V battery. Because of the low current  consumption  of  only  2 mA  the  circuit will provide many hours  of service.

Author :Ton Giesberts  - Copyright : Elektor Electronics

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A Low Distortion Audio Pre-amplifier

In an audio amplifier the quality of sound depends upon a number of factors, e.g. quality of active and passive components, circuit configuration, and layout. To an extent, the selection of components depends on the constructor’s budget. The discrete active components like transistors have been increasingly replaced by linear ICs, making the task of designer easier. With the passage of time, the general-purpose op-amps like LM741, which were being used in audio/hi-fi circuits, have become The preamplifier circuit presented here is based on a dual precision op-amp for the construction of a low distortion, high quality audio preamplifier.

Circuit diagram :

A Low Distortion Audio Pre-amplifier Circuit Diagram

 

A dual op-amp OPA2604 from Burr-Brown is used for all the stages. The FET input stage op-amp was chosen in this context it is worthwile to mention another popular bi-polar architecture op-amp, the NE5534A. It has, no doubt, an exceptionally low noise figure of 4nV/ÖHz but rest of the specifications compared to OPA2604 are virtually absent in this IC. Also This IC is also capable of operating at higher voltage rails of ± 24V (max.). Also its input bias current (100 pA) is many orders lower than its bipolar counterpart’s. This ensures a multifold reduction in noise.

A channel seperation of 142 dB exists between In the circuit, buffer is essential for the proper working of the subsequent blocks. A nominal input impedance of 47k is offered by this stage which prevents overloading of the preamplifier. The tone control is a baxandall type filter circuit.The bandwidth limiter is basically a low-pass filter with an upper cut-off ceiling at the end of the useful audio spectrum. The gain at 10 kHz is approximately 17 dB. The design is essentially 3-pole type and the upper frequency is set at 25 kHz. This lSetting the unit is fairly simple. Check the power leads feeding the IC for symmetrical voltages. High quality audio output from the line output socket is to be fed as the input signal to this preamplifier. Output of the preamplifier is fed to the power a The whole circuit consumes about 10 mA when the above-mentioned ICs are used. Power supply requirements are not critical as the circuit works on 7.5V to 15V DC..

Source : electronicsforu

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Four-in-One Burglar Alarm

In this circuit, the alarm will be switched on under the following four different conditions: 1. When light falls on LDR1 (at the entry to the premises). 2. When light falling on LDR2 is obstructed. 3. When door switches are opened or a wire is broken. 4. When a handle is touched. The light dependent resistor LDR1 should be placed in darkness near the door lock or handle etc. If an intruder flashes his torch, its light will fall on LDR1, reducing the voltage drop across it and so also the voltage applied to trigger 1 (pin 6) of IC1. Thus transistor T2 will get forward biased and relay RL1 energise and operate the alarm. Sensitivity of LDR1 can be adjusted by varying preset VR1. LDR2 may be placed on one side of a corridor such that the beam of light from a light source always falls on it. When an intruder passes through the corridor, his shadow falls on LDR2. As a result voltage drop across LDR2 increases and pin 8 of IC1 goes low while output pin 9 of IC1 goes high. Transistor T2 gets switched on and the relay operates to set the alarm.

 

Circuit diagram:

Four-in-One Burglar Alarm Circuit Diagram

 

The sensitivity of LDR2 can be adjusted by varying potentiometer VR2. A long but very thin wire may be connected between the points A and B or C and D across a window or a door. This long wire may even be used to lock or tie something. If anyone cuts or breaks this wire, the alarm will be switched on as pin 8 or 6 will go low. In place of the wire between points A and B or C and D door switches can be connected. These switches should be fixed on the door in such a way that when the door is closed the switch gets closed and when the door is open the switch remains open. If the switches or wire, are not used between these points, the points should be shorted. With the help of a wire, connect the touch point (P) with the handle of a door or some other suitable object made of conducting material. When one touches this handle or the other connected object, pin 6 of IC1 goes ‘low’.

 

So the alarm and the relay gets switched on. Remember that the object connected to this touch point should be well insulated from ground. For good touch action, potentiometer VR3 should be properly adjusted. If potentiometer VR3 tapping is held more towards ground, the alarm will get switched on even without touching. In such a situation, the tapping should be raised. But the tapping point should not be raised too much as the touch action would then vanish. When you vary potentiometer VR1, re-adjust the sensitivity of the touch point with the help of potentiometer VR3 properly. If the alarm has a voltage rating of other than 6V (more than 6V), or if it draws a high current (more than 150 mA), connect it through the relay points as shown by the dotted lines. As a burglar alarm, battery backup is necessary for this circuit. Note: Electric sparking in the vicinity of this circuit may cause false triggering of the circuit. To avoid this adjust potentiometer VR3 properly.

 

Source : EFY

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Power Supply for USB Devices

More and more equipment is sold that runs off internal rechargeable batteries. Although a matching charger is usually supplied in the package, there are also devices that can only be charged via a USB port. That is not surprising in the case of USB MP3 players, which have to ‘dock’ in the PC anyway for some time for the purpose of file transferring. Still, the same ‘feature’ can be a serious disadvantage, for example, on ‘computer-free’ holidays. Sometimes it makes you wonder how simple the solutions to such problems actually turn out to be. After all, if it’s just a supply volt- age we’re after, then a USB port is easily imitated.

Circuit diagram :

Power Supply for USB Devices Circuit Diagram

The circuit shown here is nothing but a 7805 in a dead standard configuration. The innovation, if any, might be USB connector to which the MP3 player can be connected. The 7805 comes in different flavours — most devices can sup- ply 1 A, but there are also more advanced variants that achieve up to 1.5 A. Because a USB device is never allowed to draw more than 500 mA from the port t is plugged into, the circuit shown here should be able to supply charging and/or operating current to up to two (or three) USB devices at the same time. The input voltage may be a direct voltage of anything between 7 and 24 volts, so for use at home or abroad a simple wall cube with DC output is sufficient.

Another useful bit to make your-self might be a cable with an in-line fuse and a cigarette lighter plug so you can tap into a vehicle supply (note that this may be up to 14.4 V with a running engine). At an output current of 1 A and an input voltage of just 7 V, the 7805 already dissipates 2 watts. Assuming you’re using the most commonly seen version of the 7805, the TO-220 case with its metal tab will have a thermal resistance of about 50 °C/W. Also assuming that the ambient temperature is 20 °C, the 7805’s internal (chip) temperature will be around 120 °C. In most cases, 150 °C is the specified maximum, so ample cooling must be provided especially in a car and with relatively high input voltages.

Author: Roman Mittermayr - Copyright : Elektor Electronics

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Simple Audio Peak Detector

This audio peak detector allows a pair of stereo channels to be monitored on a sin-gle LED. Identical circuitry is used in the left and right channels. Use is made of the switch-ing levels of Schmitt trigger NAND gates inside the familiar 4093 IC. The threshold level for gate IC1.A (IC1.B) is set with the aid of preset P1, which supplies a high-impedance bias level via R2 (R1).

Circuit diagram :

Simple Audio Peak Detector Circuit Diagram

When, owing to the instantaneous level of the audio signal superimposed on the bias voltage by C3 (C2), the dc level at pins 1 and 2 (5 and 6) of the Schmitt trigger gate drops below a certain level, the output of IC1.A (IC1.B) will go High. This level is copied to the input of IC1.C via D2 (D1) and due to the inverting action of IC1.C, LED D3 will light. Network R3-C1 provides some delay to enable very short audio peaks to be reliably indicated. Initially turn the wiper of P1 to the +12 V extreme — LED D3 should remain out. Then apply ‘line’ level audio to K1 and K3, preferably music with lots of peaks (for example, drum ‘n bass). Carefully adjust P1 until the peaks in the music are indicated by D3. The circuit has double RCA connectors for the left and right channels to obviate the use of those rare and expensive audio splitter (‘Y’) cables.

Author : Flemming Jensen – Copyright : Elektor Electronic

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General-Purpose Alarm

The alarm may be used for a variety of applications, such as frost monitor, room temperature monitor, and so on. In the quiescent state, the circuit draws a current of only a few microamperes, so that, in theory at least, a 9 V dry battery (PP3, 6AM6, MN1604, 6LR61) should last for up to ten years. Such a tiny current is not possible when ICs are used, and the circuit is therefore a discrete design.  Every four seconds a measuring bridge, which actuates a Schmitt trigger, is switched on for 150 ms by a clock generator. In that period of 150 ms, the resistance of an NTC thermistor, R11, is compared with that of a fixed resistor. If the former is less than the latter, the alarm is set off.  When the circuit is switched on, capacitor C1 is not charged and transistors T1–T3 are off.

Circuit diagram :

General-Purpose Alarm Circuit Diagram

 

After switch-on, C1 is charged gradually via R1, R7, and R8, until the base voltage of T1 exceeds the threshold bias.  transistor T1 then comes on and causes T2 and T3 to conduct also. Thereupon, C1 is charged via current source T1-T2-D1, until the current from the source becomes smaller than that flowing through R3 and T3 (about 3 µA). This results in T1 switching off, so that, owing to the coupling with C1, the entire circuit is disabled.  Capacitor C1 is (almost) fully charged, so that the anode potential of D1 drops well below 0 V. Only when C1 is charged again can a new cycle begin. It is obvious that the larger part of the current is used for charging C1. Gate IC1a functions as impedance inverter and feed-back stage, and regularly switches on measurement bridge R9–R12-C2-P1 briefly. The bridge is terminated in a differential amplifier, which, in spite of the tiny current (and the consequent small transconductance of the transistors) provides a large amplification and, therefore, a high sensitivity.

 

esistors R13 and R15 pro-vide through a kind of hysteresis a Schmitt trigger input for the differential amplifier, which results in unambiguous and fast measurement results. Capacitor C2 compensates for the capacitive effect of long cables between sensor and circuit and so prevents false alarms. If the sensor (R11) is built in the same enclosure as the remainder of the circuit (as, for instance, in a room temperature monitor), C2 and R13 may be omitted. In that case,C3 will absorb any interference signals and so prevent false alarms. To prevent any residual charge in C3 causing a false alarm when the bridge is in equilibrium, the capacitor is discharged rapidly via D2 when this happens. Gates IC1c and IC1d form an oscillator to drive the buzzer (an a.c. type). Owing to the very high impedance of the clock, an epoxy resin (not pertinax) board must be used for building the alarm.

 

For the same reason, C1 should be a type with very low leakage current. If operation of the alarm is required when the resistance of R11 is  higher than that of the fixed resistor, reverse the connections of the elements of the bridge and thus effectively the inverting and non-inverting inputs of the differential amplifier. An NTC thermistor such as R11 has a resistance at –18 °C that is about ten times as high as that at room temperature. It is, therefore, advisable, if not a must, when precise operation is required, to consult the data sheet of the device or take a number of test readings.  For the present circuit, the resistance at –18 °C must be 300–400 kΩ. The value of R12 should be the same. Preset P1 provides fine adjustment of the response threshold.  Note that although the proto-type uses an NTC thermistor, a different kind of sensor may also be used, provided its electrical specification is known and suits the present circuit.

Design By: K. Syttkus – Copyright : Elektor Electronics

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1.5-V White LED

Thanks to their high light output and long lifetimes, a white LED is an excellent choice as a replacement for the incandescent bulb in a penlight torch. However, there is a ‘but’. Depending on the current level, white LEDs need a voltage of 3 to 4 V. You thus need a penlight with at least three batteries, which is not exactly what you can call compact. Fortunately, this problem can be remedied using a simple adapter circuit.

 

The design described here allows a white LED to be operated from a single 1.5-V battery. It consists of a simple step-up converter and an oscillator. If the circuit is built using SMD components as much as possible, it will not be difficult to fit everything into the torch.  The actual step-up converter consists of L1 and T1. The coil is wound on an EP7 core, which consists of a spool, two core halves (T-38 core material) and a clip/screen. It is available from Farnell, among others. Wind 17 turns of 0.5-mm enamelled copper wire on the spool. If you make the windings neat and tight, the core halves will just pass over the wound coil. Handle the spool carefully, since it breaks easily.

 

Circuit diagram :

1.5-V White LED Circuit Diagram

The inductance of the coil made in this manner is around 360 µH, and it has a Q of 50 (at 1 kHz.). A Zetex SMD transistor (ZXM61N02F) was used for the prototype. This miniscule MOSFET has a very low RDS(ON) and a low threshold voltage. The driver oscillator for T1 is a classical R–C oscillator using a Schmitt-trigger inverter (IC1a, a Texas Instruments 74HC14). This proved to still work at 1.5 V. The frequency has been made adjustable so that the brightness can be increased when the battery is low by changing the frequency.

There is an optimum setting, since the battery volt-age drops when the battery is nearly empty and a large cur-rent is drawn. With a full battery, the lowest frequency gives the largest current. With the indicated component values, the frequency can be set between 50 kHz and 300 kHz. The brightness is greatest at the lowest frequency with a full battery; in this situation the current consumption is 16 mA and the efficiency is 84%. The working principle of the converter is simple. When T1 conducts, the current through L steadily increases; at 50 kHz and a duty cycle of 50%, it will reach a value of 40 mA. When T1 stops conducting, the current in the coil continues to flow through D1. The inductive voltage across T1 is limited by D1. The current through the white LED may be as high as 20 mA (in our case). Although the current peaks rise as high as 40 mA, the average value is significantly lower.

Author : unknown  - Copyright : Elektor Electronics

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Speech Filter

In communications receivers and microphone amplifiers for transmitting equipment, there is frequently a need for a narrow, low-frequency band-pass filter that lets only the voice band through. This band is usually defined to be the portion of the audio frequency spectrum between approximately 300 Hz and 3300 Hz. In order to implement such a filter, we have calculated the values for two fifth-order Butterworth filters having these corner frequencies and connected them in series. The result is a band-pass filter for the desired pass-band with a skirt steepness of 100 dB/decade.  The first opamp (IC1) acts as a buffer.

Speech Filter Image :

The circuit can be powered by a unipolar supply voltage between 5 V and 18 V, which is a broad enough range that it should always be possible to find a suitable voltage when building the filter into existing equipment. The current consumption of the filter is only a few milliampères, which should rarely pose a problem. There is fairly wide selection of suitable candidates for the opamps, since the circuit is not critical in this regard. In addition to the indicated OP27A, you could consider using a TL081N or even an old-fashioned 741.

Circuit diagram : 

Speech Filter Circuit Diagram

Due to unavoidable spreads in component values, the pass-band curve of the filter will never be completely perfect in actual practice. However, the deviations will be very small and in any case inaudible. In the pass-band region, the gain is approximately unity. The printed circuit board design shown here allows the speech filter to be built in a very compact form, which can be an important factor if it must be fitted into existing equipment. You can quickly check the fully assembled circuit by momentarily measuring the voltages at the inputs and out-puts of the three opamps. Half of the supply voltage should be present at all of these locations.

PCB Layout :

Parts LIST:

Resistors:
R1.R2 = 22kΩ
R3,R11,R12,R18,R19 = 100kΩ
R4 = 470Ω
R5 = 150Ω
R6 = 10kΩ
R7 = 18kΩ
R8 = 15kΩ
R9 = 33kΩ
R10 = 82kΩ
R13-R17 = 3kΩ3

Capacitors:

C1,C8,C14,C15 = 100nF
C2 = 1µF MKT
C3-C7,C11 = 22nF
C9 = 33nF
C10 = 18nF
C12 = 10nF
C13 = 4nF7
C16,C17 = 10µF 16V

Semiconductors:

IC1,IC2,IC3 = OP27A, TL081CN

Miscellaneous:

Bt1 = 9-V battery

Author: G.Baars  - Copyright : Elektor Electronics

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Small Circuit Card Radio

Among some of our modern contemporaries, ‘musical’ post-cards evoke strong reactions of astonishment about hyper modern microcontroller technology. However, such flat melody memories would only have elicited a weary smile from our forefathers.

As early as 1928, there are reports that radio cards with the dimensions of a regular postcard and a thickness of only a few millimeters were being made. These cards concealed a basketwork coil with a sliding tap for tuning the frequency of the received signal, a fixed capacitor and a miniscule detector device consisting of a small crystal with a ‘whisker’ contact. A similarly simple circuit can also be implemented using cur-rent resources. For this, you will need an interesting local medium-wave transmitter and a high-impedance headphone (1–2 kΩ), as well as a good aerial (such as a metal downpipe or an earthed radiator). The aerial is connected to an LC resonant circuit tuned to the frequency of the local transmitter, and a diode provides the demodulation. The necessary capacitance following the diode is provided by the cable to the headphone or amplifier.

The coil can be made using a circular piece of stiff cardboard with a diameter of a couple of centimetres. Cut an odd number of slots into the cardboard disc. Then wind enamelled copper wire (diameter 0.15–0.2 mm) back and forth through the slots. Forty turns will give an inductance of around 80 µH. The coil looks like the bottom of a reed basket,which explains its cryptic name in RF jargon. To tune the coil to the frequency of the local transmitter and determine the required frequency of the resonant circuit, connect a dual-gang or multiple-gang variable capacitor (500–1000 pF) to the coil, with the stator sections (the fixed portion of the capacitor plates) connected in parallel.

The rotor sections, which are connected to the shaft of the rotary capacitor, must without fail be connected to ground in order to pre-vent a ‘hand effect’ while tuning. Incidentally, the resonant-circuit formula cannot be used to determine the tuning capacitance, since it ignores the effect of the aerial. After the capacitor has been adjusted, estimate the value of the capacitance (or even better, measure it), dig out a suitable fixed capacitor from your parts box and solder it to the coil at the centre of the cardboard disc, along with a general-purpose germanium diode (AA119, AA112, OA95, etc.). Secure the capacitor and diode with glue. For terminals, you can use 4-mm tubular rivets for miniature plugs, as shown in the photo.

Card Radio Circuit Diagram

A suitable ‘enclosure’ can be made from ‘customer discount’ cards in credit-card format (you probably already have more than you really need). Use one card as the ‘circuit board’ for the receiver, and cut an opening in a second card to receive the circuitry. Ideally, this card should thick enough to fit the full height of the receiver. The cover is formed by a third card. After a final check, glue or rivet the cards together, and your card radio is finished. It’s not high-end, but it has astonishingly good performance for such a simple circuit.

One final glimpse into the past: already in the 1930s, such fixed-tuned detector receivers were available in the form of ‘Berlin plugs’, ‘Hamburg plugs’, and so on, for receiving local transmitter signals in various locations.

Author : G. Stabe - Copyright : Elektor

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Car Battery 12v Charger

The usual chargers of battery automotive, are simple and cheap appliances that charge continuously the battery, with a rythm of few amperes, for the time where the appliance is ON. If the holder do not close in time the charger, the battery will overcharge and her electrolytic faculty are lost with evaporation or likely exists destruction of her elements. The charger of circuit exceeds these faults. It checks electronic the situation of charge of battery and it has circuit of control with retroaction, that forces the battery charge with biggest rythm until charge completely.

Circuit diagram:

Car Battery 12v Charger Circuit Diaram

When charge completely, it turns on one RED led (LD2). The charger has been drawn in order to charge batteries of 12V, ONLY. What should watch it from what it manufactures the circuit, they are the cables that connect the transformer with the circuit and in the continuity the battery, should they are big cross-section, so that heat when it passes from in them the current of charge and also they do not cause fall of voltage at the way of current through them.

Adjustment

After assembling of the circuit, adjust TR1 to null value, power-up and make the following adjustments :-

1. Without connecting the battery check that the 2 LED?s are turned on.
2. Connect a car battery to the circuit and check that LD2 is OFF and a current (normally 2A to 4A) is flowing to the battery.
3. Adjust TR1 until LD2 turns ON and the charge current is cut.
4. Adjust TR1 to null value and charge the battery using the hydrometer technique (if you do not have or do not know how to use a hydrometer, then use a good condition battery and charge).

 

Carefully adjust TR1 so that LD2 begins to turn ON and the charge current falls to a few hundred milliamps (mA). If TR1 is set correctly then in the next round of charging you will noticed LD2 begin to flicker as the battery is being charged. When battery is completely charged, LD2 turns ON completely.TR1 does not need further adjustment anymore. Q1 is connected in line with the battery and is fired by R3, R4 and LD2. The R2, C1, TR1 and D2 sense the voltage of the battery terminal and activate Q2 when the voltage of the battery terminal exceeds the value predetermined by TR1.

 

When an uncharged battery is connected, the terminal voltage is low. Under this circumstance, Q2 is turned OFF and Q1 is fired in each half cycle by R3, R4 and LD2. The Q1 functions as a simple rectifier and charges the battery. If the battery terminal voltage is increased above the level that had been fixed by TR1, then Q2 shifts the control of Q1 gate. This deactivates Q1 and cuts off the current supply to the battery and turns LD2 ON indicating that the charge has been completed. Q1 and bridge rectifier GR1 should be mounted on heatsinks to prevent overheating. M1 is a 5A DC ammeter to measure the charge current.

Source :users.otenet.gr

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Sensitive Audio Power Meter

As a follow-up to the simple audio power  meter described in [1], the author has developed a more sensitive version. In practice,  you  rarely  use  more  than  1 watt  of  audio  power in a normal living-room environment.  The only time most people use more is at a  party when they want to show how loud their  stereo system is, in which case peaks of more  than 10 W are not uncommon. With this circuit, the dual LED starts to light up  green at around 0.1 watt into 8 ohms (0.2 watt  into 4 ohms). Naturally, this depends on the  specific type of LED that is used.

 

Circuit diagram:

Sensitive Audio Power Meter Circuit Diagram

 

Here it is  essential to use a low current type. The capacitor is first charged via D1 and then discharged via the green LED. This voltage-doubler effect  increases the sensitivity of the circuit. Above a level of 1 watt, the transistor limits the current through the green LED and the red LED con ducts enough to produce an orange hue.The red colour predominates above 5 watts. Of course, you can also use two separate ‘normal’ LEDs. However, this arrangement cannot generate an orange hue. For any testing that may be necessary, you should use  generator with a DC-coupled output. If there is a capacitor in the output path, it can cause misleading results.

Reference: Simple Audio Power Meter, Elektor July & August 2008.

Author : Michiel Ter Burg - Copyright : Elektor Electronic

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Tester for Inductive Sensors

This  tester  uses  a  LED  to  indicate whether an inductive sensor is generating a signal. It can be used to test the inductive sensors used in ABS and EBS systems in cars, with engine cam- shafts and flywheels, and so on. The circuit is built around an LM358 dual opamp IC. The weak signal coming from the sensor (when the wheel is turning slowly, for example) is an AC voltage. The first opamp, which is wired here as an inverting amplifier, amplifies the negative half cycles of this signal by a factor of 820. The second opamp is wired as a comparator and causes the red LED to blink regularly.

 

In order to judge the quality of the signal  from the sensor, you must turn the wheel  very slowly. If the red LED blinks, this means  that the sensor is generating a signal and the  distance between the sensor and the pole  wheel (gear wheel) is set correctly. If the distance (air gap) is too large, the sensor will not  generate a signal when the wheel is turned slowly, with the result that the LED will remain  dark, but it will generate a signal if the wheel  is turned faster and the LED will thus start  blinking. Irregularities in the blinking rate can  be caused by dirt on the sensor or damage to  the pole wheel (gear wheel).

 

Circuit diagram:

Tester for Inductive Sensors Circuit Diagram

If you connect an oscilloscope to the LED with  the engine running, you will see a square-wave signal with a pattern matching  the teeth of the gear wheel, with a frequency equal to the frequency of the  AC signal generated by the sensor. You can also use this tester to check  the polarity of the connecting leads.  To do this, first dismount the sensor  and then move it away from a metal-lic object. The LED will go on or off  while the sensor is moving. If you now  reverse the lead connections, the LED  should  do  exactly  the  opposite  as  before when the sensor is moved the  same way.

 

The circuit has been tested extensively  in several workshops on various vehicles, and it works faultlessly. The author has also connected the tester to  sensors on running engines, such as the cam-shaft and flywheel sensors of a Volvo truck  (D13 A engine). With the camshaft sensor, the  LED blinks when the engine is being cranked  for starting, but once the engine starts running you can’t see the LED blinking any more  due to the high blinking rate. 

 

Author : Hugo Stiers (Belgium) - Copyright: Elektor

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Guitar Amplifier PSU

Tubes (thermionic valves) have never departed from the amplified instrument scene and the majority of guitarists, including very young ones, wouldn’t use anything else. Some diehards think that the H.T. (high tension) rectifier should also be a piece of glass-ware and some manufacturers are still producing amplifiers incorporating one. The nett effect is really that a rectifier tube acts as a relatively effective heat-dissipating resistor, causing the HT rail to sag as output signal loading increases, generating a compressive characteristic which is fundamentally added distortion(‘crunch’). The traditional arrangement uses a centre-tapped HT winding on the power transformer but this has a number of drawbacks for an adequately rated core size including increased voltage stress, small wire size and a poor utilisation of the available winding window.

Circuit diagram:

Guitar Amplifier PSU Circuit Diagram

The example arrangement shown here reduces both of these problems and for a given core increases the current delivery capability of the winding by allowing the use of a heavier wire gauge. Normally some resistance is added in series to each anode to limit peak cathode current to minimise cathode-stripping during the high current pulses delivered to the input filter capacitor at each voltage peak.

Even if one includes such resistance (and a single resistor in series with the cathode or winding achieves the same end albeit with double the device dissipation) the benefits to the transformer of reduced voltage stress and increased wire insulation thickness (which scales with wire diameter) along with decreased heating in the windings, are obvious.

Alternatively, a smaller winding window (reduced core size) may be employed with-out diminishing power-handling capacity. The circuit shown here should is typically intended for the amplifier preamp and phase splitter stages. Due to the use of the EZ81 (6CA4) tube its maximum output current is about 100 mA. Higher currents call for a more powerful rectifier tube and diodes to match.

Author : Malcolm Watts (New Zealand) - Copyright : Elektor Electronics

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Simple Electronic Code Lock

The circuit diagram of a simple electronic code lock is shown in figure. A 9-digit code number is used to operate the code lock.When power supply to the circuit is turned on, a positive pulse is applied to the RESET pin (pin 15) through capacitor C1. Thus, the first output terminal Q1 (pin 3) of the decade counter IC (CD 4017) will be high and all other outputs (Q2 to Q10) will be low. To shift the high state from Q1 to Q2, a positive pulse must be applied at the clock input terminal (pin 14) of IC1. This is possible only by pressing the push-to-on switch S1 momentarily.

Circuit diagram:

Simple Electronic Code Lock Circuit Diagram

On pressing switch S1, the high state shifts from Q1 to Q2. Now, to change the high state from Q2 to Q3, apply another positive pulse at pin 14, which is possible only by pressing switch S2. Similarly, the high state can be shifted up to the tenth output (Q10) by pressing the switches S1 through S9 sequentially in that order. When Q10 (pin 11) is high, transistor T1 conducts and energises relay RL1. The relay can be used to switch ‘on’ power to any electrical appliance. Diodes D1 through D9 are provided to prevent damage/malfunctioning of the IC when two switches corresponding to ‘high’ and ‘low’ output terminals are pressed simultaneously.

Capacitor C2 and resistor R3 are provided to prevent noise during switching action. witch S10 is used to reset the circuit manually. Switches S1 to S10 can be mounted on a keyboard panel, and any number or letter can be used to mark them. Switch S10 is also placed together with other switches so that any stranger trying to operate the lock frequently presses the switch S10, thereby resetting the circuit many times. Thus, he is never able to turn the relay ‘on’. If necessary, two or three switches can be connected in parallel with S10 and placed on the key-board panel for more safety. A 12V power supply is used for the circuit. The circuit is very simple and can be easily assembled on a general-purpose PCB. The code number can be easily changed by changing the connections to switches (S1 to S9).

Author : Rejo G. Parekkattu – Copyright :EFYMag

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