Gas Leakage Alarm

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.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 data sheet, it has high sensitivity to propane, butane, isobutane, 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. 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.

         

Fig-1, Circuit for Gas leakage Alarm.                         

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VERSATILE POWER SUPPLY

 Using this circuit, you can obtain the following voltages(approx.) at a current limited to one ampere: 3.3V, 5V, 6V, 9V, 12V and 15V. The AC mains is stepped down by transformer X1 to deliver the secondary output of 18V AC at a maximum current of 1A dependent upon the load. The transformer output is rectified by the bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and fed to regulator IC LM317, which is a 3-terminal positive regulator capable of providing 1.2V to 37 volts at 1.5A current to theload. Resistor R13 and selected combinations of resistors R1 through R12 are used to produce approximately 3.3V, 5V, 6V, 9V, 12V and 15V at the output. The desired resistors are selected by switching into conduction one of the six pnp transistors T1 through T6 by grounding the corresponding transistor base using rotary switch S1.For example, to get regulated 3.3V, simply rotate the knob of rotary switch to 3.3V position. Consequently, transistor T1 is forward biased to switch resistors R1 and R2 (in series) across Adj pin of LM317 and ground to produce 3.3V. Other voltages can be produced in the same way by using rotary switch S1. Capacitor C2 bypasses any ripple in the output. Diode D5 is used as the protection diode. Use a heat-sink for dissipation of heat from IC LM317. The fuse-rated lamp provides protection against short circuit. This 1A rated power supply can be used for testing of various circuit ideas as well as construction projects.

 By-Sunil Kumar.

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MEDIUM-POWER FM TRANSMITTER

The range of this FM transmitteris around 100 metres at 9V DC supply.The circuit comprises three stages.The first stage is a microphone preamplifier built around BC548 transistor.The next stage is a VHF oscillator wired around another BC548. (BC series transistors are generally used in low-frequency stages. But these also work fine in RF stages as oscillator.) The third stage is a class-A tuned amplifier that boosts signals from the oscillator. Use of the additional RF amplifier increases the range of the transmitter.Coil L1 comprises four turns of 20SWG enamelled copper wire wound to 1.5cm length of a 4mm dia. air core.Coil L2 comprises six turns of 20SWG enamelled copper wire wound on a 4mm dia. air core. Use a 75cm long wire as the antenna. For the maximum range, use a sensitive receiver.VC1 is a frequency-adjusting trimpot. VC2 should be adjusted for the maximum range. The transmitter unit is powered. by a 9V PP3 battery. It can be combined with a readily available FM receiver kit to make a walkie-talkie set as shown in Fig. 3.

 Fig-1, FM transmitter.

                                     

Fig-2, Transistor pin details.                        Fig-3, Walkie-talkie arrangement.

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