The circuit makes use of characteristic of a diode. As already explained normally diode conducts only in one direction and it blocks voltage in the reverse direction. However there will be a little leakage current flowing through which is normally negligible. But if the temperature increases, this leakage current also increases and this effect is used here to create a fire alarm. This circuit as shown in Schematic 8.
Use any IN 4000 series diode as a fire sensor and connect the rest of the circuit also as shown in the diagram. Do not cut the leads of the diode short. It is better to use the full length of the leads as far as possible. For a trial, bring the tip of your hot soldering iron and touch the terminal wire of the diode and you hear the sound soon enough. Or bring a lighted matchstick. You will notice that the sound increases as the heat increases. If the diode is fixed the other way round, (or in other words, if it is reverse biased) the buzzer continues to sound, fire or no fire.There is nothing more thrilling than building an alarm and enjoying the response. Electronics is exciting. In the following pages you find a lot of alarms but before we build any of them, let us tackle one of the most common ICs.
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