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Smoke, alcohol or LPG alarm This circuit raises an alarm if it detects smoke or LPG cooking gas leakage, or even alcohol vapours. This is achieved by using the same circuit with one of several sensors designed to detect smoke, LPG or alcohol. So different alarms can be made by simply changing the sensor. For a smoke alarm, use the MQ2 sensor; to detect alcohol, use the MQ3; or use the MQ6 sensor for LPG. The MQx sensors each have six pins. The heater filament, between pins H-H, is powered from the 5V rail. Two pairs of A-B pins connect across the sensing element; it doesn’t matter which pair you use. Half of the LM358 dual op amp the lug using a screw in a hole already drilled for the purpose. The wiring is hidden under foam acoustic pads inside the ear cups. The cable connecting to the phone has two wires, pink and white, plus a screen braid, with a three-way jack plug on one end. Do not mix up the microphone’s white return wire with the white wire in the cable back to the phone. If necessary, put a label on the microphone's white wire, change its colour with a marking pen, or solder it to the common lug first. Circuit Ideas Wanted 98  Silicon Chip (IC1) is wired as a comparator. A reference voltage set using potentiometer VR1 is applied to the inverting input (pin 2) while the sensor voltage goes to the non-inverting input (pin 3). The sensor produces a current that is converted to a voltage by the 10kW resistor between the B pin and ground. Output pin 1 of the op amp swings high whenever the sensor voltage goes above the reference voltage. A small amount of hysteresis is provided via positive feedback using a 10MW resistor so that the output doesn’t vacillate when the sensor voltage hovers around the trigger threshold. When pin 1 of IC1 goes high, The centre (ring) contact on the jack plug connects to one side of the string of two induction coils and the 150W resistor. The opposite end of that string goes to the common solder lug. The third and longest contact (sleeve) at the base of the jack plug is returned via the screening braid, which you should also connect to the common lug. When finished, check for around 157W between the jack plug ring and sleeve contacts. Depending on the probe polarity, there should be around 1.2-2.5kW between the tip transistor Q1 releases the reset signal on 555 timer IC2, which is configured as an astable multivibrator, so it starts oscillating. The frequency depends on the value of the capacitor connected to pin 6 and the two resistors connected to pin 7. The resulting square wave is AC-coupled to a small 8W speaker to produce the alarm tone. After switching on the 5V supply, wait about ten seconds for the filament to heat up. Then adjust VR1 until the alarm just stops sounding. Enclose the PCB in a suitable box with vents so that fumes can circulate near the sensor. Raj K. Gorkhali, Hetadu, Nepal. ($75) and sleeve. If all indications are OK, plug your induction headset into a suitable phone and press talk to hear a dial tone. Using my Silicon Chip Hearing Loop Tester/Level Meter (November & December 2010; siliconchip.au/ Series/15), I found that the dial tone was smack bang on 0dB or 0.1A/m. The phone also has a volume control that can reduce the volume below that level. Anthony Leo, Cecil Park, NSW. ($90) Got an interesting original circuit that you have cleverly devised? We will pay good money to feature it in Circuit Notebook. We can pay you by electronic funds transfer, cheque or direct to your PayPal account. Or you can use the funds to purchase anything from the SILICON CHIP Online Store, including PCBs and components, back issues, subscriptions or whatever. Email your circuit and descriptive text to editor<at>siliconchip.com.au Australia's electronics magazine siliconchip.com.au