Thermostat using with LM319

This first circuit is designed by me to replace the mechanical switches used in some thermal Electric I have heaters.The electrical contacts to these mechanical thermal switches are always stoned and “no longer be trusted.”They could easily be welded together, the maintenance of these heating on full. It is definitely not good! Coarse adjustment of the temperature is a trimmer on the track, set to give a nominal range When using the fine adjustment.

It is quite difficult in a D-53, manufactured by NEC. The plumb line that is not isolated to the body of the thermistor is arranged. Why is it when you use the wire must be insulated to withstand the high temperatures such as fiberglass insulation against pipe.

Thermostat  Circuit Diagram:

What is the size of the thermistor is the disk diameter 7 mm and a thickness of 2 mm. The end temperature control is a standard isolated potentiometer with a knob and / or good Protection against electric shock. Harter can be adjusted with this controller Auto-On and Off, “which automatically adjusts the room temperature.

The power control for the triac should be a 2 watt potentiometer with a knob isolated and / or Tree for protection against electric shock.  It can also be set up a 2-watt resistor on the circuit, an appropriate level of heat.“In general, full on,” how would the “Normal” setting when you use the contacts.On the chart, show me a 10 Meg resistance hysteresis. This may be of Lower Austria or higherdepending on how many degrees of difference between you and off cycles.Values between 100 K-ohms and 22 ohms, Meg are acceptable.

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Experimental Hall Sensor

Hall sensors can of course be purchased but making them yourself is far more interesting (and satisfying)!
According to the theory the crucial thing is to use a touch layer that’s as thin as possible; the length and width are unimportant. An ‘obvious’ starting point for our trials would be copper, which in the form of printed circuit board material is easy to find and handle. Copper-clad board may be obvious but not ideal, because it has a very weak Hall constant.

Nevertheless we should be able to use it to demonstrate the Hall effect by using very powerful magnets in our sensor.To achieve detection we need the highest possible level of amplification. In the circuit shown here the voltage amplification is set by the relationship of the two feedback resistors of the first op-amp. With the values given (2.2 MΩ and 330 Ω) produce a gain of 6,667.

Experimental Hall Sensor Circuit Diagram :

This also creates a convenient bridge connection for taking measurements. The trimmer potentiometer allows fine adjustment. With zero set ting that’s accurate to within millivolts we could use this test point to measure Hall voltages of well below a microvolt. Finally in this way we could also measure the flux density of a magnet.

Copper has a Hall constant of AH= –5.3·10-11m3/C. The thickness of the copper layer is d = 35 µm. The Hall voltage then amounts to:VH= AH× I × B / d

When the field B= 1 T and current I= 1 A a Hall voltage of VH= 1.5 µV is produced. The6,667-fold gain then achieves a figure of 10 mV. The circuit thus has a sensitivity of 10 mV per Tesla. That said, adjusting the zero point with P1 is not particularly easy. The amplifier has a separate power supply in the form of a 9 V battery (BT1). To take measurements we connect a lab power supply with adjustable output current (BT2) to the Hall sensor (the copper surface) and set the current flowing through the sensor to exactly 1 A. Then the zero point must be adjusted afresh.

Next we place a strong Neodymium magnet below the sensor. The output voltage of the circuit should now vary effectively by several millivolts. Note that there are several effects that can influence the measurements we take. Every displacement of the magnet will pro-duce an induction voltage in the power feed wires that is significantly greater than the Hall voltage itself. Every time you move the magnet you must wait a while to give the measurements time to stabilise. With such small voltage measurements problems can also arise with thermal voltages due to temperature variations. It’s best not to move and inch — and to hold your breath as long as possible!

Author : Burkhard Kainka - Copyright : Elektor

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Water Level Detector

To monitor the filling of a bath, a water-tank, or a swimming pool, or to warn when a gully is overflowing, here’s a very simple water level detector built around a CD4011 CMOS quad NAND chip. Gates IC1.A and IC1.B are wired as an astable multivibrator. The oscillator frequency is determined by C1, R2 and preset P1.

Water Level Detector Circuit Diagram :

When quiescent, resistor R1 pulls the input to gate IC1.A down to logic low, which there-fore by default blocks the operation of the oscillator in the absence of water. When water is present between the e+ an d e−electrodes, IC1. A is taken high, enabling the oscillator. The output signal from gate IC1.B is shaped by IC1.C to obtain a rectangular waveform. Gate IC1.D inverts the signal so that transistor T1 is held of f in the absence of water, which avoids current flowing in the primary of transformer TR1 when the system is at rest. TR1 is a 12 V 1.5 VA AC power transformer wired as a step-up trans-former i.e. with the low-volt age winding connected to T1. The transformer’s step up ratio affords ‘passive’ amplification of the signal present at the drain of T1. The trans-former’s high voltage winding is connected to piezo sounder BZ1 (e.g. Murata; the ‘28’indicates the diameter) which produces the audible warning.

In order to optimise the sound output of the unit, you’ll need to adjust P1 so as to set the oscillator frequency to the resonant frequency of the piezo transducer; this setting can be done by ear. The electronics and batteries can be housed into a salvaged case (for example, the kind of oval box found inside giant chocolate ‘surprise’ eggs). The electrodes, formed from simple rigid copper wires, pass out through the case; the join is made watertight using epoxy adhesive.

 

Author : André Thiriot  - Copyright : Elektor

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Dag-gerboard Position Detector

In sailing regattas it’s handy to have a dag-gerboard that can be raised and lowered vertically. As the winding handle or positioning motor needs to rotate the spindle of the lifting device some 100 to 150 times throughout its full range it would be extremely handy to have a quick idea of its current position. An electronic count of the number of revolutions would be ideal. Thank goodness most sailors now have a 12-V supply available!

To get this to work you need to apply white and black markings to the spindle, each covering half of the circumference. Next, mask off two electric eye devices (reflected light sensors) next to one another (approximately 10 mm apart). For secure detection both sensors should be positioned not more than 5 mm from the paint markings.

Dag-gerboard Position Detector Circuit Diagram :

The markings to be read by the sensor should be displaced laterally, so that the direction of rotation can be recognised in addition to the number of revolutions counted. At the heart of our circuit is a PIC16F628 from Microchip, which as usual can be bought ready programmed from Elektor or you can do this bit yourself by downloading free firmware (for details of both see [1]).

At pins 1 of the two reflected light sensors IC3 and IC4 we need to ‘see’ more than 2.0 V from the white segment and less than 0.8 V from the black mark (with an operating volt-age between 4.5 and 5.5 V). The two signals detected are taken to plug connector along with the operating voltage and ground. It’s convenient if you also provide a connector from the microcontroller as well, so that the sensor and the controller board can be linked by a test lead.

The multiplexing of the three seven-segment displays is programmed at a rate of 100 Hz.

Acceptable values for the revolution count are between 0 and 140. If the count exceeds or falls below these limits, then the counter is not incremented. The RESET key S2 sets the counter back to zero. Jumper K2 enables you to reverse the direction of counting. The count is retained if the operating voltage is removed and is loaded again when next pow-ered up.

The source code can also be downloaded from the website mentioned above, making it possible (for instance) to define alternative counter limit values (the maximum value is defined in the line #define max 140). For compiling the code you can use the CC5X compiler, of which there is a free version (www.bknd.com/cc5x).

Author : Hermann Sprenger - Copyright : Elektor

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Petrol/Diesel Level Sensor

This sensor is particularly suitable for use in small spaces, such as the petrol tank of a  motorbike. It has the advantage of not having any moving parts, unlike a conventional sensor with a float and float arm that make it difficult to fit in a tank.

The sensor circuit is made from standard, inexpensive components and can be put together for little money.

Petrol/Diesel Level Sensor Circuit Diagram :

The operating principle is  based on  measuring  the forward volt-ages of two identical diodes (check this  first by measuring  them).  The forward voltage of a diode decreases with increasing junction temperature. lf a resistor is placed close to one of the two diodes, it will be heated slightly if it extends above the surface of the  petrol. For best results,the other diode (used for reference) should be located at the same level. lf the diodes are covered by the petrol in the tank, the heating resistor will not have any effect because it will be cooled by the petrol. An opamp compares the voltage across the two diodes, with a slightly smaller current passing through the reference diode. When the petrol level drops, the output of the opamp goes high and the output transistor switches on. This causes a sense resistor to be connected in parallel with the sensor output. Several sensor circuits can be used together, each with its own switched sense resistor connected in parallel with the output, and the resulting output  signal can be used to drive a meter or the like.

Using this approach, the author built a petrol tank' sensors trip' tank consisting of five PCBs, each fitted with two sensor circuits. With this sensor strip installed at an angle in the tank, a resolution of approximately 1.5 litre per sensor is possible. Many tanks have an electrical fitting near the bottom for connection to a lamp on the instrument panel that indicates the reserve level. The sensor strip can be used in its place.

You will have to experiment a bit with the values of the sense resistors, but do not use values lower than around'100 O. It is also important to fit the diodes and heater resistor in a little tube with a small opening at the bottom so that splashing petrol does not cool the heater resistor, since this would result in false readings.

The circuit should be powered from a regulated supply voltage of 5 to 6 V to prevent the heating resistors from becoming too hot. After testing everything to be sure that it works properly, it's a good idea to coat the circuit board with epoxy glue to provide better protection against the petrol.

Tip: you can use the well-known 1M3914 to build a LED display with ten LEDs, which can serve as a level indicator. Several examples of suitable circuits can be found in back issues of Elektor.

Note: this sensor circuit is not suitable for use in conductive liquids.

Author : Paul de Ruijter - Copyright: Elektor

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A Simple Detector with Amplification

A simple shortwave radio detector is neither very sensitive nor very selective. However, with a little extra amplification we can improve the reception performance significantly.

The additional circuit is designed to compensate for the losses in the resonant circuit. A transistor is used to amplify the RF signal and feed it back into the resonant circuit. When the gain is set correctly we can make the amount of this feedback exactly equal to the losses. The resonant circuit is then critically damped and has a very high Qfactor. Now we can separate transmissions that are just 10 kHz apart, and we can tune in to very weak stations.

Detector with Amplification Circuit Diagram :

The tuning capacitor used has two gangs of vanes with capacitances of 240 pF and 80 pF. These two gangs are connected in parallel to make a 320 pF variable capacitance. The air-cored inductor has 25 turns on a diameter of 10 mm, with taps at 5-turn intervals. The resonant circuit so formed is capable of covering the full shortwave  band from 5 MHz to 25 MHz.

The short wave detector can be connected to a power amplifier, or, for exam-ple, amplified PC loudspeakers. The antenna does not have to  be very long: in experiments we used a one metre length of wire. Tuning the radio involves adjusting the variable capacitor to bring in the station and then adjusting the gain of the feed-back circuit for optimal output volume. If the potentiometer is turned up too far, the receiver will go into self-oscillation and become a mini-transmitter. At  the optimal setting the sound  quality is very pleasant and certainly no worse than many ordinary shortwave radios.

If you find shortwave detectors that use a battery and an amplifier a little new-fangled, you can get your fix of nostalgia by dispensing with the battery and connecting a crystal earpiece to the detector’s output. The radio will of course also work without the feedback circuit, but with rather poorer performance.

Author :Burkhard Kainka - Copyright : Elektor

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Simple Seismic Sensor

This circuit simulates a seismic sensor to detect vibrations/sounds. It is very sensitive and can detect vibrations caused by the movement of animals or human beings. So it can be used to monitor protected areas to restrict entry of unwanted persons or animals.

 

Circuit diagram :

Simple Seismic Sensor Circuit Diagram

The circuit uses readily available components and the design is straight forward. A standard piezo sensor is used to detect vibrations/sounds due to pressure changes. The piezo element acts as a small capacitor having a apacitance of a few nanofarads. Like a capacitor, it can store charge when a potential is applied to its terminals. It discharges through VR1, when it is disturbed.

In the circuit, IC TLO71 (IC1) is wired as a differential amplifier with both its inverting and non-inverting inputs tied to the negative rail through a resistive network comprising R1, R2 and R3. Under idle conditions (as adjusted by VR1), both the inputs receive almost equal voltages, which keeps the output low.

TLO71 is a low-noise JFET input op-amp with low input bias and offset current. The BIFET technology provides fast slew rates. Capacitor C1 is provided in the circuit to keep the differential input of IC1 for better performance.

When the piezo element is disturbed (by even a slight movement), it discharges the stored charge. This alters the voltage level at the inputs of IC1 and the output momentarily swings high as indicated by green LED1. This high output is used to trigger switching transistor T1, which triggers monostable IC2. The timing period of IC2 is determined by R7 and C5. With the shown values, it will be around two minutes. The high output from IC2 activates T2 and the buzzer starts beeping along with red light indication from LED2.

Assemble the circuit on a common PCB and enclose in a suitable cabinet. Connect the piezo element to the PCB using single-core shielded wire. Enclose the piezo element inside a rustproof, small aluminium box. The piezo element should be firmly glued to the enclosure facing the fine side towards the case. Fix the sensor assembly on the back side of a ceramic tile or granite tile with good adhesive. Fix the tile (or bury it in the earth) near the entrance with the sensor assembly facing downwards. Whenever a pressure change develops near the sensor, the circuit will be activated.

 

Author : D. Mohan Kumar - Copyright : EFY

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Cheapest Ever Motion Sensor

The RS-455-3671 sensor used in the Automatic Rear Bicycle Light project published in  the July/August 2010 edition can be replaced by a motion sensor that costs nothing instead of a fiver or thereabouts.

 

The replacement is a homemade device, built from components easily found in the workshop of any electronics enthusiast. Effectively it works as a variable resistor, depending on the acceleration force to which the device is  submitted. A prototype presented a resistance of 200 kΩ when not moving, and 190 kΩ when dropping about 1 cm.

Constructing is easy. Cut off a piece of about 10 mm of copper tubing. Take a piece of conductive foam, the kind used to protect integrated circuits. Cut a rectangular piece of 10  x 50 mm. Roll up firmly until it can be push-fitted securely into the copper cylinder. Then insert a conductive wire through the centre of the cylinder, bend it and (optionally) add protective plastic sleeving to each side. This is the first contact. Finally, solder a thin wire to the copper cylinder. This is the second contact. The foam resistance is pressure dependent. Consequently, when the device moves due to an external force, the inertia of the cylinder causes varying pressure in the foam, resulting in a small change of resistance between the inner conductor and  the cylinder. Because of that, it’s important to ensure the cylinder vibration is not restricted in any way by the connecting wire or the PCB.

The comparator circuit shown here is capable of resolving the resistance change of the proposed foam/wire/copper sensor, allowing it to detect the motion of a vehicle for alarm or other purposes.

 

Author : Antoni Gendrau – Copyright : Elektor

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Infrared Emitter & Detector

This circuit have applied to line detection of robot project, Good match between the transmitter and the detector is important for proper operation, especially if the hole is large.

Circuit diagram :

Infrared Emitter & Detector Circuit Diagram

Robot with a simple object or obstacle detection. Infrared Transmitter detector pair sensors are relatively easy to implement, although involved some degree of testing and calibration in order to make correct. They can for the impediment, motion detection, transmitters, encoders are used, and the color detection.

This can be done with a piece of rope stretched between and in accordance with LED and phototransistor. A length of stiff wire or plugs can be used to set the alignment. Another method that can be used for long distances is a laser pointer shone through a hole.

Source / detector alignment method The transmitter can be mounted above the track with the phototransistor placed between the rails in places like hidden deposits. Place the transmitter and the detector at an angle would again be useful.

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Room Noise Detector Schematic Circuit

This circuit is intended to signal, through a flashing LED, the exceeding of a fixed threshold in room noise, chosen from three fixed levels, namely 50, 70 & 85 dB. Two Op-amps provide the necessary circuit gain for sounds picked-up by a miniature electret microphone to drive a LED. With SW1 in the first position the circuit is off. Second, third and fourth positions power the circuit and set the input sensitivity threshold to 85, 70 & 50 dB respectively. Current drawing is 1mA with LED off and 12-15mA when the LED is steady on.

Circuit diagram :

Room Noise Detector Circuit diagram

Parts List :

R1____________10K 1/4W Resistor
R2,R3_________22K 1/4W Resistors
R4___________100K 1/4W Resistor
R5,R9,R10_____56K 1/4W Resistors
R6_____________5K6 1/4W Resistor
R7___________560R 1/4W Resistor
R8_____________2K2 1/4W Resistor
R11____________1K 1/4W Resistor
R12___________33K 1/4W Resistor
R13__________330R 1/4W Resistor

C1___________100nF 63V Polyester Capacitor
C2____________10µF 25V Electrolytic Capacitor
C3___________470µF 25V Electrolytic Capacitor
C4____________47µF 25V Electrolytic Capacitor

D1_____________5mm. Red LED

IC1__________LM358 Low Power Dual Op-amp

Q1___________BC327 45V 800mA PNP Transistor

MIC1_________Miniature electret microphone

SW1__________2 poles 4 ways rotary switch

B1___________9V PP3 Battery

Clip for PP3 Battery

Use :

• Place the small box containing the circuit in the room where you intend to measure ambient noise.
• The 50 dB setting is provided to monitor the noise in the bedroom at night. If the LED is steady on, or flashes bright often, then your bedroom is inadequate and too noisy for sleep.
• The 70 dB setting is for living-rooms. If this level is often exceeded during the day, your apartment is rather uncomfortable.
• If noise level is constantly over 85 dB, 8 hours a day, then you are living in a dangerous environment.

Source : www.redcircuits.com/Page16.htm

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Shutter Guard

This sensitive vibration sensor is exclusively made for shops to protect against burglary. It will detect any mechanical or acoustic vibration in its vicinity when somebody tries to break the shutter and immediately switch on a lamp and sound a warning alarm. A 15-minute time delay after switch-on allows sufficient time for the shop owner to close the shutter.

Circuit diagram :

Shutter Guard Circuit Diagram

The front end of the circuit has a timer built around the popular binary counter IC CD4060 (IC1) to provide 15-minute time delay for the remaining circuitry to turn on. Resistors R3 and R4 and capacitor C2 will make Q9 output high after 15 minutes. Di-ode D1 inhibits the clock input (pin 11) to keep the output high till the power is switched off. Blinking LED1 indicates the oscillation of IC1.

The high output from IC1 is used to enable reset pin 4 of IC2 so that it can function freely. Transistor T1 amplifies the piezo-sensor signal and triggers monostable IC2. The base of transistor T1 is biased using a standard piezo element that acts as a small capacitor and flexes freely in response to mechanical vibrations so that the output of IC2 is high till the prefixed time period.  In the standby mode, the alarm circuit built around IC3 remains dormant as it does not get current. Timing components R8 and C6 make the output of IC2 high for a period of three minutes.

When any mechanical vibration (caused by even a slight movement) disturbs the piezo element, trigger pin 2 of IC2 momentarily changes its state and the output of IC2 goes high. This triggers triac 1 and the alarm circuit activates. Triac BT136 completes the lamp circuit by activating its gate through resistor R9. IC UM3561 (IC4) generates a tone simulating the police siren with R11 as its oscillation-controlling resistor. Zener diode ZD1 provides stable 3.1V DC for the tone-generating IC.

Assemble the circuit on a general-purpose PCB and enclose in a suit-able, shockproof case. Connect the piezo element to the circuit by using a single-core shielded wire. Glue a circular rubber washer on the fine side of the piezo element and fix it on the shutter frame with the washer facing the frame so that the piezo element is flexible to sense the vibrations. Fix the lamp and the speaker on the outer side and the remaining parts inside the case. Since triac is used in the circuit, most points in the PCB will be at mains lethal potential. So it is advised not to touch any part of the circuit while testing.

Author :D. Mohan Kumar  - Copyright : EFY

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Simple Acoustic Sensor

This acoustic sensor was originally developed for an industrial application (monitoring a siren), but will also find many domestic applications. Note that the sensor is designed with safety of operation as the top priority: this means that if it fails then in the worst-case scenario it will not itself generate a false indication that a sound is detected. Also, the sensor connections are protected against polarity reversal and short-circuits. The supply voltage of 24 V is suitable for industrial use, and the output of the sensor swings over the supply voltage range.

Circuit diagram :

Simple Acoustic Sensor Circuit Diagram

The circuit consists of an electret micro-phone, an amplifier, attenuator, rectifier and a switching stage. MIC1 is supplied with a current of 1 mA by R9. T1 amplifies the signal, decoupled from the supply by C1, to about 1 Vpp. R7 sets the collector current of T1 to a maximum of 0.5 mA. The operating point is set by feedback resistor R8. The sensitivity of the circuit can be adjusted using potentiometer P1 so that it does not respond to ambient noise levels. Diodes D1 and D2 recitfy the signal and C4 provides smoothing. As soon as the voltage across C4 rises above 0.5 V, T2 turns on and the LED connected to the collector of the transistor lights. T3 inverts this signal.

If the microphone receives no sound, T3 turns on and the output will be at ground. If a signal is detected, T3 turns off and the output is pulled to +24 V by R4 and R5. In order to allow for an output current of 10 mA, T3’s collector resistor needs to be 2.4 kΩ. If 0.25 W resistors are to be used, then to be on the safe side this should be made up of two 4.7 kΩ resistors wired in parallel. Diode D4 protects the circuit from reverse polarity connection, and D3 protects the output from damage if it is inadvertently connected to the supply.

 

Author:Engelbert Göpfert - Copyright : Elektor

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LED Photosensor

It is not always necessary to use special photoresistors or phototransistors to make light-sensitive switches. Although it is not well known, normal visible-light and infrared LEDs will also work. A voltage that depends on the intensity of the natural or artificial illumination falling on the LED can be taken from the anode of the LED. This behaviour can be easily verified by connecting a DVM or oscilloscope directly to the two leads of the LED.

Circuit Diagram:

LED Photosensor Circuit Diagram

Since the load on the photoelectric potential should be kept as small as possible, a JFET is used here as a buffer. The type used is not critical; similar transistors should work equally well. The buffered voltage is fed to the inverting input of comparator IC1. The threshold voltage can be adjusted to meet the desires of the user by means of the potentiometer. A pull-up resistor is connected to the com-parator output, since the LM393 has an open-collector out-put. The supply voltage may be chosen anywhere in the range of 5 to 9 V.

Copyright : Elektor

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Fog Lamp Sensor

For several years now, a rear fog lamp has been mandatory for trailers and caravans in order to improve visibility under foggy conditions.

Circuit diagram :

Fog Lamp Sensor Circuit Diagram

When this fog lamp is switched on, the fog lamp of the pulling vehicle must be switched off to avoid irritating reflections. For this purpose, a mechanical switch is now built into the 13-way female connector in order to switch off the fog lamp of the pulling vehicle and switch on the fog lamp of the trailer or caravan. For anyone who uses a 7-way connector, this switching can also be implemented electronically with the aid of the circuit illustrated here.

Here a type P521 optocoupler detects whether the fog lamp of the caravan or trailer is connected. If the fog lamp is switched on in the car, a current flows through the caravan fog lamp via diodes D1 and D2. This causes the LED in the optocoupler to light up, with the result that the phototransistor conducts and energises the relay via transistor T1. The relay switches off the fog lamp of the car.

For anyone who’s not all thumbs, this small circuit can easily be built on a small piece of perforated circuit board and then fitted somewhere close to the rear lamp fitting of the pulling vehicle.

Author :Harrie Dogge - Copyright : Elektor

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Colour Sensor

Colour sensor is an interesting project for hobbyists. The circuit can sense eight colours, i.e. blue,green and red (primary colours); magenta, yellow and cyan (secondary colours); and black and white. The circuit is based on the fundamentals of optics and digital electronics.

 

Circuit diagram :

 

Colour Sensor  Circuit Diagram

 

The object whose colour is required to be detected should be placed in front of the system. The light rays reflected from the object will fall on the three convex lenses which are fixed in front of the three LDRs. The convex lenses are used to converge light rays. This helps to increase the sensitivity of LDRs. Blue, green and red glass plates (filters) are fixed in front of LDR1, LDR2 and LDR3 respectively. When reflected light rays from the object fall on the gadget, the coloured filter glass plates determine which of the LDRs would get triggered. The circuit makes use of only ‘AND’ gates and ‘NOT’ gates.

When a primary coloured light ray falls on the system, the glass plate corresponding to that primary colour will allow that specific light to pass through. But the other two glass plates will not allow any light to pass through. Thus only one LDR will get triggered and the gate output corresponding to that LDR will become logic 1 to indicate which colour it is. Similarly, when a secondary coloured light ray falls on the system, the two primary glass plates corresponding to the mixed colour will allow that light to pass through while the remaining one will not allow any light ray to pass through it. As a result two of the LDRs get triggered and the gate output corresponding to these will become logic 1 and indicate which colour it is.

When all the LDRs get triggered or remain untriggered, you will observe white and black light indications respectively. Following points may be carefully noted:

•  

  1. Potmeters VR1, VR2 and VR3 may be used to adjust the sensitivity of the LDRs.
  2. Common ends of the LDRs should be connected to positive supply.
  3. Use good quality light filters.

 

The LDR is mounded in a tube, behind a lens, and aimed at the object. The coloured glass filter should be fixed in front of the LDR as shown in the figure. Make three of that kind and fix them in a suitable case. Adjustments are critical and the gadget performance would depend upon its proper fabrication and use of correct filters as well as light conditions.

 

Author : Tony Gladvin George - Copyright : EFY

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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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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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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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Snail Mail Detector

Since his letter-box is outdoors  and quite some way from the  house, the author was looking  for a simple means of knowing if  the postman had been without  having to go outside (contrary  to popular belief, the weather  isn’t always fine in the South of  France). Circuits for this kind of  ‘remote detection’ come up regularly, but always involve running cables between the letter- box and the detection circuit in  the house. Seeking to avoid running any extra cables, the author  had the idea of using the existing cables going to the doorbell,  conveniently located adjacent to  his letter-box.

 

The letter-box has two doors:  one  on  the  street  side  for  the  postman, and one on the gar-den side for collecting the post.  A  micro switch  is  fitted  to  the  street-side door, to light an indicator in the house showing that  the postman has been. A second  micro switch is fitted to the door  on the garden side, to turn off  the indicator once the post has  been collected. The only difficulty then remains to connect  these detectors to a remote circuit in the house that remembers  whether  the  postman’s  been or not.

 

The idea was to use the alternating half-cycles of the AC signal  on the cable going to the door-bell  to  transmit  the  information, according to the following logic:

• Both  half-cycles  present: no change in the status of the mail detector.
• An interruption (even brief) of one half-cycle: indicator lights permanently.
• An interruption (even brief) of the other half-cycle: the indicator goes out.

Note that the signal is tapped off  across the doorbell coil via R6  and the pair of diodes connected  in inverse-parallel (to limit the  signal,  par ticularly  when  the  bell is rung). The signal is then  filtered by R2/C1, before being  used by IC1, which is wired as a  comparator with hysteresis. The  trigger threshold is adjusted by  P1, using a pair of inverse parallel diodes as a voltage reference  (positive or negative according  to the output state):

 

For the detection to work, there  has to be continuity in the bell-push circuit this is generally  ensured by the little lamp illuminating the bell-push. Resistor R1  is added just in case the lamp is  blown or not present. To keep things simple, the circuit is powered directly from the  doorbell transformer itself (230 V  / 8 V). The author managed to fit  the little circuit within the door-bell unit, with the LED poking  through a hole in the casing so  it is readily visible in the hall of  his house.

Author : Philippe Temporelli (France) – Copyright : elektor electronics

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