Clever Rain-Alarm

Usually rain-alarms employ a single sensor. A serious draw-back of this type of sensor is that even if a single drop of water falls on the sensor, the alarm would sound. There is a probability that the alarm may be false. To overcome this draw-back, here we make use of four sensors, each placed well away from the other at suitable spots on the roof. The rain alarm would sound only if all the four sensors get wet. This reduces the probability of false alarm to a very great extent. The four rain-sensors SR1 to SR4, along with pull-up resistors R1 to R4 (connected to positive rails) and inverters N1 to N4, form the rain-sensor monitor stage. The sensor wires are brought to the PCB input points E1 to E5 using a 5-core cable. The four outputs of Schmitt inverter gates N1 to N4 go to the four inputs of Schmitt NAND gate N7, that makes the alarm driver stage.

Circuit diagram :

Clever Rain-Alarm Circuit Diagram

When all four sensors sense the rain, all four inputs to gates N1 through N4 go low and their outputs go high. Thus all four in-puts to NAND gate N7 also go high and its output at pin 6 goes to logic 0. The out-put of gate N7 is high if any one or more of the rain-sensor plates SR1 through SR4 remain dry. The output of gate N7 is coupled to inverter gates N5 and N6. The output from gate N5 (logic 1 when rain is sensed) is brought to  ‘EXT’ output connector, which may be used to control other external devices.

The output from the other inverter gate N6 is used as enable input for NAND gate N8, which is configured as a low-frequency oscillator to drive/modulate the piezo buzzer via transistor T1. The frequency of the oscillator/modulator stage is variable between 10 Hz and 200 Hz with the help of preset VR1. The buzzer is of piezo-electric type having a continuous tone that is inter-rupted by the low-frequency output of N8. The buzzer will sound whenever rain is sensed (by all four sensors). 6V power supply (100mA) is used here to enable proper interfacing of the CMOS and TTL ICs used in the circuit. The power supply requirement is quite low and a 6-volt battery pack can be easily used. During quiscent-state, only a negligible cur-rent is consumed by the circuit.

 

 

Even during active state, not more than 20mA current is needed for driving a good-quality piezo-buzzer. Please note that IC2, being of TTL type, needs a 5V regulated supply. There-fore zener D1, along with capacitor C2 and resistor R5, are used for this purpose.A parallel-track, general-purpose PCB or a veroboard is enough to hold all the components. The rain-sensors SR1 to SR4 can be fabricated as shown in the construction guide in Fig. 2. They can be made simply by connecting alternate parallel tracks using jumpers on the component side.

Use some epoxy cement on and around the wire joints at A and B to avoid corrosion. Also, the sensors can be cemented in place with epoxy cement. If the number of sensors is to be increased, just add another set of CD40106 and 7413 ICs along with the associated discrete components. Another good utility of the rain-alarm is in agriculture. When drip-irrigation is employed, fix the four sensors at four corners of the tree-pits, at a suit-able height from the ground. Then, as soon as the water rises to the sensor’s level, the circuit can be used to switch off the water pump.

Author : M.K. Chandra Mouleeswaran - Copyright: EFY

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Protection For Your Electrical Appliances

Here is a very low-cost circuit to save your electrically operated appliances, such as tv, tape recorder, refrigerator, and other instruments during sudden tripping and resumption of mains supply. Appliances like refrigerators and air-conditioners are more prone to damage due to such conditions. The simple circuit given here switches off the mains supply to the load as soon as the power trips. The supply can be resumed only by manual intervention. Thus, the supply may be switched on only after it has stabilised.

Circuit diagram :

Protection For Your Electrical Appliances

The circuit comprises a step-down transformer followed by a full-wave rectifier and smoothing capacitor C1 which acts as a supply source for relay rl1. Initially, when the circuit is switched on, the power supply path to the step-down transformer X1 as well as the load is incomplete, as the relay is in de-energised state. To energise the relay, press switch S1 for a short duration. This completes the path for the supply to transformer X1 as also the load via closed contacts of switch S1. Meanwhile, the supply to relay becomes available and it gets energised to provide a parallel path for the supply to the transformer as well as the load.

If there is any interruption in the power supply, the supply to the transformer is not available and the relay de-energises. Thus, once the supply is interrupted even for a brief period, the relay is de-energised and you have to press switch S1 momentarily (when the supply resumes) to make it available to the load. Very short duration (say, 1 to 5 milliseconds) interruptions or fluctuations will not affect the circuit because of presence of large value capacitor which has to discharge via the relay coil. Thus the circuit provides suitable safety against erratic power supply conditions.

Author : MALAY BANERJEE - Copyright : EFY

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Telephone line Based Audio Muting and Light-On Circuit

Very often when enjoying music or watching TV at high audio level, we may not be able to hear a telephone ring and thus miss an important incoming phone call. To overcome this situation, the circuit presented here can be used. The circuit would automatically light a bulb on arrival of a telephone ring and simultaneously mute the music system/TV audio for the duration the telephone handset is off-hook. Lighting of the bulb would not only indicate an incoming call but also help in locating the telephone during darkness.

Circuit diagram :

Telephone line Based Audio Muting and Light-On Circuit Diagram

On arrival of a ring, or when the handset is off-hook, the inbuilt transistor of IC1 (opto-coupler) conducts and capacitor C1 gets charged and, in turn, transistor T1 gets forward biased. As a result, transistor T1 conducts, causing energisation of relays RL1, RL2, and RL3. Diode D1 connected in antiparallel to inbuilt diode of IC1, in shunt with resistor R1, provides an easy path for AC current and helps in limiting the voltage across inbuilt diode to a safe value during the ringing. (The RMS value of ring voltage lies between 70 and 90 volts RMS.) Capacitor C1 maintains necessary voltage for continuously forward biasing  transistor T1 so that the relays are not energised during the negative half cycles and off-period of ring signal. Once the handset is picked up, the relays will still remain energised because of low impedance DC path available (via cradle switch and handset) for the in-built diode of IC1.

After completion of call when handset is placed back on its cradle, the low-impedance path through handset is no more available and thus relays RL1 through RL3 are deactivated. As shown in the figure, the energised relay RL1 switches on the light, while energisation of relay RL2 causes the path of TV speaker lead to be opened. (For dual-speaker TV, replace relay RL2 with a DPDT relay of 6V, 200 ohm.) Similarly, energisation of DPDT relay RL3 opens the leads going to the speakers and thus mutes both audio speakers. Use ‘NC’ contacts of relay RL3 in series with speakers of music system and ‘NC’ contacts of RL2 in series with TV speaker. Use  ‘NO’ con-tact of relay RL1 in series with a bulb to get the visual indication.

Author : Dhurjati Sinha - Copyright : EFYmag

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Power On Indicator

Some types of electronic equipment do  not provide any indication that they are  actually on when they are switched on.  This situation can occur when the back-light of a display is switched off. In addition, the otherwise mandatory mains  power  indicator  is  not  required  with  equipment  that  consumes  less  than  10 watts. As a result, you can easily forget  to switch off such equipment. If you want  to know whether equipment is still drawing power from the mains, or if you want  to have an indication that the equipment  is switched on without having to modify the equipment, this circuit provides a solution.

One way to detect AC power current and  generate a reasonably constant voltage  independent of the load is to connect a  string of diodes wired in reverse parallel in series with one of the AC supply  leads. Here we selected diodes rated  at 6 A that can handle a non-repetitive  peak current of 200 A. The peak current  rating is important in connection with  switch-on  currents.  An  advantage  of  the selected diodes is that their voltage  drop increases at high currents (to 1.2 V  at 6 A). This means that you can roughly  estimate the power consumption from  the brightness of the LED (at very low  power levels). The voltage across the diodes serves as  the supply voltage for the LED driver. To  increase the sensitivity of the circuit, a  cascade circuit (voltage doubler) consisting of C1, D7, D8 and C2 is used to double  the voltage from D1–D6. Another benefit  of this arrangement is that both halve- waves of the AC current are used. We use  Schottky diodes in the cascade circuit to  minimise the voltage losses.

Circuit diagram :

Power On Indicator Circuit Diagram

The LED driver is designed to operate the LED  in blinking mode. This increases the amount  of current that can flow though the LED when  it is on, so the brightness is adequate even  with small loads. We chose a duty cycle of pproximately 5 seconds off and 0.5 second  on. If we assume a current of 2 mA for good  brightness with a low-current LED and we can  tolerate a 1-V drop in the supply voltage, the  smoothing capacitor (C2) must have a value of  1000 µF. We use an astable multivibrator built around two transistors to implement a  high-efficiency LED flasher. It is dimensioned to minimise the drive current of  the transistors. The average current consumption is approximately 0.5 mA with a  supply voltage of 3 V (2.7 mA when the  LED is on; 0.2 mA when it is off). C4 and  R4 determine the on time of the LED (0.5  to 0.6 s, depending on the supply volt-age). The LED off time is determined by  C3 and R3 and is slightly less than 5 seconds. The theoretical value is R × C × ln2,  but the actual value differs slightly due to  the low supply voltage and the selected  component values.

 

Diodes D1-D6 do not have to be special  high-voltage diodes; the reverse volt-age is only a couple of volts here due  the reverse-parallel arrangement. This  voltage drop is negligible compared to  the value of the mains voltage. The only  thing you have to pay attention to is the  maximum load. Diodes with a higher  current rating must be used above 1 kW.  In addition, the diodes may require cool-ing at such high power levels.  Measurements on D1–D6 indicate that  the voltage drop across each diode is  approximately 0.4 V at a current of 1 mA.  Our aim was to have the circuit give a  reasonable indication at current levels  of 1 mA and higher, and we succeeded  nicely. However, it is essential to use a  good low-current LED.

 

Caution: the entire circuit is at AC power potential. Never work on the circuit with the mains cable plugged in. The  best enclosure for the circuit is a small,  translucent box with the same colour as  the LED. Use reliable strain reliefs for the  mains cables entering and leaving the  box (connected to a junction box, for  example). The LED insulation does not  meet the requirements of any defined insulation class, so it must be fitted such that it  cannot be touched, which means it cannot  protrude from the enclosure.

By : Ton Giesberts (Elektor Labs) – Copyright : Elektor

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Preamplifier for RF Sweep Generator

The RF sweep frequency generator (‘wobbu-lator’) published in the October 2008 issue of Elektor has a receiver option that allows the instrument to be used as a direct conversion receiver. This receiver does however have a noise floor of only –80 dBm, which really should have been –-107 dBm to obtain a sensitivity of 1 µV. So, for a good receiver sommore gain is required. A wideband amplifie however, generates a lot of additional noisas well and as a consequence will not resuin much of an improvement.  As an experiment, the author developed a selective receiver with a bandwidth of about 4 MHz. Because a gain of at least 35 dB is required, the preamplifier consists of two amplifying elements.

The input amplifier is designed around a dual gate MOSFET, type BF982. This component produces relatively little noise but pro-vides a lot of gain. The output stage uses a BFR91A for some additional gain. Preamplifiers where both the gate and the drain are tuned often struggle with feedback via their  internal capacitance. Here, the drain circuit has a relatively low impedance, which prevents this from happening. In the prototype that was tested, the input and output are located at right angles with respect to each other to prevent inductive coupling (see photo). Despite the high gain, the amplifier was perfectly stable even without any shielding.  The two air-cored coils in the circuit both consist of 4 turns and have an internal  diameter  of  6 mm,  made from 1-mm diameter silvered copper wire and with a tap after 1 turn.

Circuit diagram :

Preamplifier for RF Sweep Generator Circuit Diagram

The amplifier is mainly intended for the 144 MHz amateur band, but with other coils can also be used for the FM broadcast band, for example. FM detection is achieved by tuning near the edge of the IF filter. At an offset of 15 kHz this is only a few dB lower than at the centre of the pass-band, so that damping is not noticeable. The measured sensitivity in the 2 m band was about 1 µV (6 dB).A good antenna always contributes to the reception, of course. A wideband (scanner) outdoor antenna will give good results. Adding this wobbulator/receiver option results in a nice monitor receiver. By setting the scan frequencies of the spectrum analyser to 144 and 146 MHz (or 148 MHz where applicable), any signal within this range is directly visible. When a signal is detected it is merely a case of clicking the scan stop button and then clicking on the signal in the display window using the right mouse button.

After this, the receiver switches directly to this frequency and you can listen to the signal. You can subsequently resume the scanning so that you can continue to look for other signals. For narrowband FM detection you need to select the FMN button in the window for the receiver and this then provides the required offset for the edge detection at 25 kHz bandwidth. This value is adjustable via the ‘setting’ menu (default is 12,500 Hz) and can be changed experimentally for best results. To power the circuit you can use a 9-V battery. It is also possible to power the amplifier directly from the RF sweep generator, if output capacitor C6 is replaced with a link; in the ‘options’ menu you will then have to select the option ‘use probe’.

By : Gert Baars (The Netherlands) - Copyright : Elektor

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Solar Powered SLA Battery Maintenance

This circuit was designed to ‘baby-sit’ SLA (sealed lead-acid or ‘gel’) batteries using freely available solar power. SLA batteries suffer from relatively high internal energy loss which is not normally a problem until you go on holidays and disconnect them from their trickle current charger. In some cases, the absence of trickle charging current may cause SLA batteries to go completely flat within a few weeks. The circuit shown here is intended to prevent this from happening. Two 3-volt solar panels, each shunted by a diode to bypass them when no electricity is generated, power a MAX762 step-up voltage converter IC.

Circuit diagram:

Solar Powered SLA Battery Maintenance Circuit Diagram

The ‘762 is the 15-volt-out version of the perhaps more familiar MAX761 (12 V out) and is used here to boost 6 V to 15 V.C1 and C2 are decoupling capacitors that suppress high and low frequency spurious components produced by the switch-mode regulator IC. Using Schottky diode D3, energy is stored in inductor L1 in the form of a magnetic field. When pin 7 of IC1 is open-circuited by the internal switching signal, the stored energy is diverted to the 15-volt output of the circuit. The V+ (sense) input of the MAX762, pin 8, is used to maintain the output voltage at 15 V. C4 and C5 serve to keep the ripple on the output voltage as small as possible. R1, LED D4 and pushbutton S1 allow you to check the presence of the 15-V output voltage.

D5 and D6 reduce the 15-volts to about 13.6 V which is a frequently quoted nominal standby trickle charging voltage for SLA batteries. This corresponds well with the IC’s maximum, internally limited, output current of about 120 mA. The value of inductor L1 is not critical — 22 µH or 47 µH will also work fine. The coil has to be rated at 1 A though in view of the peak current through it. The switching frequency is about 300 kHz. A suggestion for a practical coil is type M from the WEPD series supplied by Würth (www.we-online.com). Remarkably, Würth supply one-off inductors to individual customers. At the time of writing, it was possible, under certain conditions, to obtain samples, or order small quantities, of the MAX762 IC through the Maxim website at www.maxim-ic.com.

Author : Myo Min - Copyright : Elektor

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Proximity Detector

This proximity detector is constructed using an infrared diode detector. Infrared detector can be used in various equipment such as burglar alarms, touch free proximity switches for turning on a light, and solenoid-controlled valves for operating a water tap. Briefly, the circuit consists of an infrared transmitter and an infra-red receiver (such as Siemens SFH506-38 used in TV sets).  The transmitter part consists of two 555 timers (IC1 and IC2) wired in astable mode, as shown in the figure, for driving an infrared LED. A burst output of 38 kHz, modulated at 100 Hz, is required for the infrared detector to sense the trans mission; hence the setup as shown is required.  To save power, the duty cycle of the 38kHz astable multivibrator is maintained at 10 per cent.  The receiver part has an infrared detector comprising IC 555 (IC3), wired for operation in monostable mode, followed by pnp transistor T1. Upon reception of infrared signals, the 555 timer (mono) is turned  ‘on’ and it re-mains  ‘on’ as long as the infrared signals are being received. 

Circuit Diagram :

Proximity Detector Circuit Diagram

When no more signals are received, the mono goes  ‘off’ after a few seconds (the delay depends on timing resistor-capacitor combination of R7-C5). The de-lay obtained using 470kilo-ohm resistor and 4.7µF capacitor is about 3 seconds. Unlike an ordinary mono, the capacitor in this mono is allowed to charge only when the reception of the signal has stopped, because of the pnp transistor T1 that shorts the charging capacitor as long as the output from IR receiver module is available (active low).  This setup can be used to detect proximity of an object moving by. Both transmitter and receiver can be mounted on a single breadboard/PCB, but care should be taken that infrared receiver is behind the infrared LED, so that the problem due to infrared leak-age is obviated. 

An object moving nearby actually reflects the infrared rays from the infrared LED. As the infrared receiver has a sensitivity angle of 60o, the IR rays are sensed within this lobe and the mono in the receiver section is triggered. This principle can be used to turn ‘on’ the light, using a relay, when a person comes nearby. The same automatically turns  ‘off’ after some time, as the person moves away. The sensitivity depends on the current limiting resistor in series with the infrared LED. It is ob-served that with in circuit resistance of preset VR1 set at 20 ohms, the object at a distance of about 25 cms can be sensed.  This circuit can be used for burglar alarms based on beam interruption, with the added advantage that the transmitter and receiver are housed in the same enclosure, avoiding any wiring problems.

Author : K.S. Sankar Copyright: EFY

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Feather-Touch Switches For Main

An ordinary AC switchboard contains separate switches for switching ‘on’/’off’ electric bulbs, tube lights, fans, etc. A very simple, interesting circuit presented here describes a feather-touch switchboard which may be used for switching ‘on’/‘off’ four or even more devices. The membrane or micro-switches (push-to-on type) may be used with this circuit, which look very elegant. By momentary depression of a switch, the electrical appliance will be ‘on’/‘off’, independently. To understand the principle and de-sign of the circuit, let us consider an existing switchboard consisting of four switches. One live wire, one neutral wire, and four wires for four switches are connected to the switchboard, as shown in the illustration below the circuit diagram.

Circuit diagram:

Feather Touch Switches For Main Circuit Daigram

The switches are removed and the above-mentioned wires (live, neutral, L1, L2, L3, and L4) are connected to the circuit, as shown in the main diagram. The circuit comprises four commonly available ICs and four micro-relays, in addition to four micro-switches/membrane switches (push-to-on type) and a few other passive components. IC 7805 is a 5-volt regulator used for supplying 5V to IC2 and IC3 (7476 ICs). These ICs are dual J-K flip-flops. The four J-K flip-flops being used in toggle mode toggle with each clock pulse. The clock pulses are generated by the push-to-on switches S1 through S4 when these are momentarily depressed.

When a switch is momentarily depressed,its corresponding output changes its existing state (i.e. changes from ‘high’ to‘low’ or vice versa) . The outputs of flip-flops drive the corresponding relays, in conjunction with the four relay driver transistors SL100. The wires earlier removed are connected to this circuit. On the switch panel board, the micro-switches are connected, and under the board the connections are wired as suggested above. Relays RL1 though RL4 are 9V, SPST-type micro-relays of proper contact ratings.

The circuit may be expanded for six switches by using one more IC 7476, and an IC ULN 2004 which has an array of seven  Darling-tons for driving the relays. So two more micro-switches and relays may be connected in a similar fashion. This circuit can be assembled on a general-purpose PCB and the total cost should not exceed Rs 300. It is suggested that the circuit, after assembly on a PCB, may be housed in a box of proper size, which may be fitted on the wall in place of a normal switchboard.

Author :D.K Kaushil - Copyright : EFY mag

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USB Power Booster

The USB serial bus can be configured for connecting several peripheral devices to a single PC. It is more complex than RS232, but faster and simpler for PC expansion.Since a PC can supply only a limited power to the external devices connected through its USB port, when too many devices are connected simultaneously, there is a possibility of power shortage. Therefore an external power source has to be added to power the external devices.

Circuit diagram:

USB Power Booster Circuit Diagram

In USB, two different types of connectors are used: type A and type  B. The circuit presented here is an add-on unit, designed to add more power to a USB supply line (type-A). When power signal from the PC (+5V) is received through socket A, LED1 glows, opto-diac IC1 conducts and TRIAC1 is triggered, resulting in availability of mains supply from the primary of transformer X1. Now transformer X1 delivers 12V at its secondary, which is rectified by a bridge rectifier comprising diodes D1 through D4 and filtered by capacitor C2.

 Pin configurations of moc3021, bt136 and 5v regulator 7805

Regulator 7805 is used to stabilise the rectified DC. Capacitor C3 at the output of the regulator bypasses the ripples present in the rectified DC output. LED1 indicates the status of the USB power booster circuit. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Bring out the +5V, ground and data points in the type-A socket. Connect the data cables as assigned in the circuit and the USB power booster is ready to function.

Author :T.A. BABU  - Copyright:  EFYMAG

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Reflection Light Barrier with Delay

This circuit can be used to check, for example, whether the door of a refrigerator has been properly closed. An LED sends out a beam of light, which, if the door is closed, is reflected. An optical sensor (CNY70) then detects the amount of light. If the sensor does not receive the right amount of light, the buzzer will sound after about a minute. When the door is closed (and the CNY70 receives enough light again), the buzzer turns off.

Circuit diagram:

Reflection Light Barrier with Delay Circuit Diagram

The power supply for the circuit requires about 12 mA at 12 V. Potentiometer P1 adjusts the sensitivity of the sensor. The sensor works reliably from a distance of one centimetre. If the current through the LED is increased, the distance can be increased a little. The delay can be adjusted with C3. C4 provides extra filtering for the reference voltage. Thebuzzer would otherwise switch on with a ‘chirping’ sound. The well-known Ne555 is used to drive the buzzer. The buzzer is driven with a duty cycle of 2:1, which improves the audibility.

Author : Goswin Visschers - Copyright : elektor

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Automatic Plant Irrigator

The circuit presented here waters your plants regularly when you are out for a vacation. The circuit comprises a sensor part built using only one op-amp (N1) of quad op-amp IC LM324. Op-amp N1 is configured here as a comparator. Two stiff cop-per wires are inserted in the soil containing plants. As long as the soil is wet, conductivity is maintained and the circuit remains off. When the soil dries out, the resistance between the copper wires (sensor probes A and B) increases. If the resistance increases beyond a preset limit, output pin 1 of op-amp N1 goes ‘low’.

Circuit diagram :

Automatic Plant Irrigator Circuit Diagram

This triggers timer IC2 (NE 555) configured as a monostable multivibrator. As a result, relay RL1 is activated for a preset time. The water pump starts immediately to supply water to the plants. As soon as the soil becomes sufficiently wet, the resistance between sensor probes decreases rapidly. This causes pin 1 of op-amp N1 to go ‘high’. LED1 glows to indicate the presence of adequate water in the soil. The threshold point at which the output of op-amp N1 goes ‘low’ can be changed with the help of preset VR1. To arrange the circuit, insert copper wires in the soil to a depth of about 2 cm,keeping them 3 cm apart. When the soil the water. LED1 glows up as the water reaches the probes.

For small areas a small pump such as the one used in air coolers is able to pump enough water within 5 to 6 seconds. The timing components for IC2 are selected accordingly. The timing can be varied with the help of preset VR2. The circuit is more effective indoors if one intends to use it for long periods. This is because the water from reservoir (bucket, etc) evaporates rapidly if it is kept in the open. For regulating the flow of water, either a tap can be used or one end of a rubber pipe can be blocked using Mseal compound, with holes punc-gets dried, adjust VR1 towards ground rail until LED1 turns off and relay RL1 is energised. The motor starts pumping tured along its length to water several plants.

Author: Priyank MudgaL - Copyright: Efymag

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