Clap switch

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THE circuit shown in Fig.4.1 is designed to switch on a relay, RLA, when microphone MIC1, senses a single clap of the hands, and switch off the relay on the next hand-clap, and so on.

How it works

The circuit comprises four main building blocks. The first is centered on NPN transistor TR1, which switches on when the sound of a handclap is sensed by the microphone. Current fl owing through the collector (C) and emitter

(E) terminals of TR1 causes a fall in the voltage at its collector, which triggers the second building block.

The second building block is based on IC1, a 555 timer wired as a monostable. On receiving the sharply falling voltage from the collector of TR1, IC1 produces a short pulse of about 20ms, which is fed into the clock terminal of IC2, a JK fl ip-fl op, which makes up the third building block.

If IC2 pin 14, the Q output, is logic low, transistor TR2 is off and the relay is not energized. The pulse from the monostable changes the state of the fl ip-flop and the Q pin goes logic high, switching on TR2, hence energising the relay. Thus TR2 switches on or off, energising or de-energising the relay on subsequent claps, each clap ‘toggling’ the fl ip-flop. LED1 is optional, but is useful in monitoring the change of the Q output of the fl ip-fl op.

The relay connected in parallel with resistor R8 and LED1 is energized when LED1 lights and its normally-open con­tact is used to switch external circuits on and off. Note that this circuit must not be used to control mains-powered circuits.

Breadboard

The Protobloc component layout for the Clap Switch is shown in Fig. 4.2.

When soldering the leads to the electret microphone insert connecting pads be as quick as possible as it does not take kindly to excessive heat. Note that one pad is also connected to the case of the mic, so make sure this lead is connected to the board 0V line.

Notes

! Do not use the relay to control power from the mains supply. If you want to control mains-operated devices you should seek the help of a qualified electrician.

! Use the Circuit Tester described in Project 1 to identify the base leads of TR1 and TR2 to confirm that they are both NPN transistors.

! Once assembled on Protobloc, you will find that LED1 is either on or off, but a sharp clap of the hands will change this by either switching the relay on (LED1 lit) or off (LED1 out).

! Diode D1 is used to protect the semiconductors, ie the transistors and integrated circuits, from possible dam­age by the sharp surge of voltage known as back EMF, which is generated as the relay switches off.

! No adjustments are necessary to the circuit, but you might like to fashion a small paper cup, a curved reflector, and fit it around the microphone to enhance the directional sensitivity of the circuit to the sound of a clap.

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Dr: Malcolm Plant

Everyday Practical Electronics, January 2009

Gas Leakage Alarm

 

LPG gas is supplied in pressurized steel cylinders. As this gas is heavier than air, when it leaks from a cylinder it flows along floor and tends to settle in low spots such as a basement. This can cause fire or suffocation if not dealt with. Here is a circuit that detects the leakage of LPG gas and alerts the user through audio-visual indications. Fig shows the circuit of the gas leakage alarm. The circuit operates off a 9V PP3 battery. Zener diode ZD1 is used to convert 9V into 5V DC to drive the gas sensor module. The SEN-1327 gas sensor module from RhydoLABZ is used in this circuit. Its output goes high when the gas level reaches or exceeds certain point. A preset in the module is used to set the threshold. Interfacing with the sensor module is done through a 4-pin SIP header. Pin details of the gas sensor module are shown in Fig. 2. An MQ-6 gas sensor is used in the gas sensor module. As per its datasheet, it has high sensitivity to propane, butane, isobutene, LPG and natural gas. The sensor can also be used to detect combustible gases, especially methane. This circuit has been tested with LPG gas and was found to work satisfactorily. W h e n e v e r there is LPG concentration of 1000 ppm (parts per million) in the area, the OUT pin of the sensor module goes high. This signal drives timer IC 555, which is wired as an astable multi-vibrator. The multi-vibrator basically works as a tone generator. Output pin 3 of IC 555 is connected to LED1 and speaker-driver transistor SL100 through current-limiting resistors R5 and R4, respectively. LED1 glows and the alarm sound to alert the user of gas leakage. The pitch of the tone can be changed by varying preset VR1. Use a suitable heat-sink for transistor SL100

CLOCK TIMER

D. MOHAN KUMAR

With this simple clock controlled timer, you will never again miss your favorite TV or radio program. The TV or radio will switch on automatically at the time preset by you and will remain ‘on’ until the power supply fails or is disconnected. The circuit uses the AC signals generated at the buzzer terminals of an alarm clock. The AC signals are amplified by transistors T1 and T2 and the amplified output from the emitter of T2 is fed to the inverting input of negative-voltage comparator IC LM311 (IC1). The non-inverting input of IC1 gets a presettable voltage through preset VR1. The inverting and non-inverting inputs of LM311 are different from other op-amps and it outputs sink current through pin 7 or source current through pin 1. When pin 3 of IC1 is at a higher voltage than pin 2, its output sinks as indicated by LED1. This gives a short negative pulse to the mono-stable wired around timer NE555. Resistor R5 keeps trigger pin 2 of IC2 high.

The short-interval monostable outputs a high signal for a brief period to the gate of SCR1 (BT169) and relay RL1 energizes. The latching action of SCR1 keeps the relay pulled even when the output of the monostable turns low. The relay can be de-energized by disconnecting the supply to the circuit The circuit works off a 9V battery.

Assemble it on a general-purpose PCB and enclose in a suitable cabinet. Provide an AC outlet in the cabinet to switch on the appliance using the circuit. As mentioned earlier, the input signal is obtained from the buzzer terminals of the clock. Remove the small buzzer of the clock and connect point ‘A’ to the positive terminal and point ‘B’ to the negative terminal of the buzzer. Connect the mains AC terminal outlet to the normally- opened (N/O) contact of relay RL1. So when the relay energizes, 230V AC operates the connected appliance. Set the desired time in the clock by adjusting the alarm set-up and switch on the circuit. When the set time reaches, the appliance will switch on automatically. The circuit can also be connected to digital clocks.

KNOCK ALARM

PRADEEP G.

This circuit used in conjunction with a thin piezoelectric plate, senses the vibration generated on knocking a surface (such as a door or a table) to activate the alarm. It uses readily available, low-cost components and can also be used to safeguard motor vehicles. The piezoelectric plate is used as the sensor. It is the same as used in ordinary piezobuzzers and is easily available in the market. The piezoelectric plate can convert any mechanical vibration into electrical variation. As it doesn’t sense sound from a distance like a microphone, it avoids false triggering.

The plate can be fixed on a door, cash box, cupboard, etc. using adhesive. A 1- 1.5m long, shielded wire is connected between the sensor plate and the input of the circuit. When someone knocks on the door, the piezoelectric sensor generates an electrical signal, which is amplified by transistors T1 through T3. The amplified signal is rectified and filtered to produce a low-level DC voltage, which is further amplified by the remaining transistors. The final output from the collector of pnp transistor T6 is applied to reset pin 4 of 555 (IC1) that is wired as an astable multi-vibrator. Whenever the collector of transistor T6 goes high, the astable multi-vibrator activates to sound an alarm through the speaker. The value of resistor R12 is chosen between 220 and 680 ohms such that IC1 remains inactive in the absence of any perceptible knock. When the circuit receives an input signal due to knocking, the alarm gets activated for about 10 seconds. This is the Fig. 1: The circuit of knock alarm Fig. 2: Proposed installation of knock alarm time that capacitor C5 connected between the emitter of transistor T4 and ground takes to discharge after a knock. The time delay can be changed by changing the value of capacitor C5. After about 10 seconds, the alarm is automatically reset. The circuit operates off a 9V or a 12V battery eliminator. The proposed installation of the knock alarm is shown.

AUTOMATIC EMERGENCY LIGHT

By: PRIYANK MUDGAL

This emergency light has the following two advantages:

1. It turns on automatically when the mains power fails, so you need not search it in the dark.  2. Its battery starts charging as soon as the mains resumes. Operation of the circuit is quite straightforward. Mains supply is stepped down by transformer X1, rectified by a full-wave rectifier comprising diodes D1 and D2, filtered by capacitor C1 and fed to relay coil RL1. The relay energises to connect the battery to the charging circuit through its normally-opened (N/O) contacts. Freewheeling diode D3 acts as a spike buster for the relay. The charging circuit is built around npn transistor BD139 (T1). The transformer output is fed to the collector of transistor T1, which provides a fixed bias voltage of 6.8V to charge the battery. When the battery is fully charged, the battery voltage becomes equal to the breakdown voltage of the zener diode (ZD1). Zener diode ZD1 conducts to provide an alternative path for the current to ground and battery charging stops. When mains fails, relay RL1 de-energises. The battery now gets connected to the white LED array (comprising LED1 through LED6) through current-limiting resistor R2. The LEDs glow to light up the room. To increase the brightness in your room, you can increase the number of white LEDs after reducing the value of resistor R2 and also use a reflector assembly.

TELEPHONE RECEIVER

TELEPHONE RECEIVER

S.K. ROUSHON

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This simple telephone receiver without a dialing section can be connected in parallel to a telephone line. It can be easily assembled on a small Vero board or a PCB. A geometry box made in the shape of a telephone receiver will be an excellent cabinet for it. No external power supply is needed, which makes the circuit handy. The ringer section comprises R1, C1, and a buzzer. If your telephone has a loud ringer, this circuit can be avoided. A bridge rectifier consisting of diodes D1 through D4 protects the circuit from any polarity change in the telephone line. PNP transistor MPS-A92 (T1) is the main interface transistor. The output of T1 is regulated by zener diode ZD and capacitor C2 to get 6.8V for powering the amplifier section. This power is also used to bias the transmitter section.

The transmitter section comprises transistor BC548 (T2) together with a few discrete components and a condenser microphone. The transmit signal is fed to the base of interface transistor T1. The voice input for the amplifier comes directly from the positive end of the bridge rectifier.

The amplifier section is built around high-performance, low-wattage power amplifier IC LM386. This circuit is designed as a high-gain amplifier. A small 8-ohm speaker is good enough for the output. After all soldering is done, adjust presets VR1 and VR2 to their middle position and connect the circuit to the telephone line in parallel. Adjust VR1 and VR2 for optimum reception as well as transmission.

REMOTE CONTROL FOR HOME APPLIANCES

Connect this circuit to any of your home appliances (lamp, fan, radio, etc.) to make the appliance turn on/off from a TV, VCD or DVD remote control. The circuit can be activated from up to 10 meters. The 38 kHz infrared (IR) rays generated by the remote control are received by IR receiver module TSOP1738 of the circuit. Pin 1 of TSOP1738 is connected to ground, pin 2 is connected to the power supply through resistor R5 and the output is taken from pin 3. The output signal is amplified by transistor T1 (BC558).

The amplified signal is fed to clock pin 14 of decade counter IC CD4017 (IC1). Pin 8 of IC1 is grounded, pin 16 is connected to Vcc and pin 3 is connected to LED1 (red), which glows to indicate that the appliance is ‘off.’

The output of IC1 is taken from its pin 2. LED2 (green) connected to pin 2 is used to indicate the ‘on’ state of the appliance. Transistor T2 (BC548) connected to pin 2 of IC1 drives relay RL1. Diode 1N4007 (D1) acts as a freewheeling diode. The appliance to be controlled is connected between the pole of the relay and neutral terminal of mains.

It gets connected to live terminal of AC mains via normally opened (N/O) contact when the relay energies.