GAS LEAKAGE ALARM

LPG gas is supplied in pressurised 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. 1 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. 

  

   Fig. 1: Circuit for gas leakage alarm

 

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.

    

Fig. 2: Pin details of gas sensor module

Whenever 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 multivibrator. The multivibrator 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 sounds 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.

MOTION SENSOR FOR SECURITY LIGHT

Here is a system based on PIR motion detector module BS1600 (or BS1700) that can be used for security or corridor lighting in power-saving mode. The 12V DC power supply required for the motion detector and the relay driver is derived from 230V, 50Hz mains using a transformerless circuit as shown in Fig. 1.

Fig. 1: Circuit of motion sensor for security light

The working of the circuit is simple. When you power-on the circuit after assembling all the components including the CFL, the CFL will glow for 10 seconds, turn off for 30 seconds, glow for 10 seconds and then turn off. Now the circuit is ready to work.

When any movement is detected, around 3.3V appears on the base of relay-driver transistor T1 and it conducts to energise relay RL1. As a result, Triac1 (BT136) fires to provide full 230V and light up the CFL. Another normally-opened contact of the relay (N/O2) is used here to hold the output until reset. If the switch is not in 'hold' position, the light will remain 'on' for about ten seconds (as programmed in the motion sensor). In short, when there is a movement near the sensor, the CFL glows for about ten seconds. It will remain 'on' if switch S1 is in 'hold' position.

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Use a three-pin connector for connecting the PIR sensor in the circuit with correct polarity. The motion detector is embedded onto the transparent cover of the light assembly as shown in Fig. 2

Fig. 2: PIR motion detector module (BS1600 or BS1700

An arrangment of CFL assembly in the author's prototype (Fig. 3) is shown in Fig. 4. In this arrangement, a PIR sensor and 23W, 230V AC CFL are used. Seal all four sides with Blue Tac for water-tightness. Insulate the track side of the PCB using an insulating foam and glue to the base. 

Fig. 3: Author's prototype

Fig. 4: CFL assembly

Intercom Using LM386

Keep in touch with your family members from one room to another and also from outside areas such as the garage, using this intercom circuit for bidirectional communication. The advantage of this circuit is that there is no talk/listen switch as many intercoms have. The circuit is built around two low-power LM386 audio amplifiers.

Fig. 1 shows the block diagram of the intercom system. Fig. 2 shows the circuit. It has two simple and identical channels—unit 1 and unit 2. As shown in Fig. 2, the gain of both the amplifiers (built around IC1 and IC2) is about 200, which is usually enough to work with the condenser microphones. The circuit also works with carbon microphones or other high-level and low-impedance microphones.

As both units are identical, working of only the first unit is described here. When you speak in front of the microphone (MIC1), the low-level signal is amplified by the amplifier built around transistor T1. Transistor T1 (BC547) is low-noise and high-gain type.

The value of resistor R11 is selected such that the voltage between the collector and emitter of T1 is approximately half the power supply voltage. Resistor R12 should have a minimal value. For example, if the emitter of T1 is connected to the ground, the value of R12 could be as low as 1 kilo-ohm (usually, the range is 680 ohms to 4.3 kilo-ohms). Use of capacitor C18 is optional. The preamplifier may exhibit instability at very high frequencies. So minimal appropriate value is chosen for resistor R1 in order to make the input circuit less vulnerable to electromagnetic noises.

Signal from the microphone is filtered by the combination of resistor R2 and capacitor C1. Capacitor C2 blocks the DC component but allows AC signal to pass. The volume is adjusted by potentiometer VR1. The potentiometer can be replaced with a preset of the same value because the volume need not be adjusted frequently. The signal is amplified by IC1 and fed to the loudspeaker (LSP2).

The second unit works in the same way as the first unit.

Assemble the circuits for units 1 and 2 on separate general-purpose PCBs and enclose in suitable cabinets. As shown in the block diagram, place microphone MIC1 and loudspeaker LSP1 in the first room, and microphone MIC2, loudspeaker LSP2 and the entire electronics block plus the power supply in the second room. Keep the interconnection cables as short as possible—preferably shorter than 10 metres. These should be kept away from the cables of the mains power supply and other sources of electromagnetic interference. Use a shielded cable for connecting the microphone to the circuit.

It is important that there is no acoustical feedback between the microphone and the loudspeaker on both the sides. So the microphones and the loudspeakers should not face each other. As the gain of each channel is not very high, the probability of acoustical feedback is low. In case of acoustical feedback, lower the volume of the amplifier and change the positions of the microphones and loudspeakers.

The preferred power supply for the circuit is 6V or above but the circuit also works with regulated 5V from IC 7805 (not shown in the diagram). If a higher dynamic range is needed, the power supply should be 9V or even 12V. For power supply, you can use an AC-DC wall adaptor, dry batteries or rechargeable batteries.