Circuit

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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Same buzzer, the same power supply but with a little ingenuity this gives an alarm when the water falls below a certain level.This circuit as shown in Schematic 7, which can be used to indicate when the level falls low in overhead tank, a sump or even in a bore well.Now you will find a 100K resistance between the trigger terminal and positive supply which pulls the trigger terminal to the positive potential. As in the previous case both wires from trigger and ground are kept in the water. Even though pulled up, trigger will be at ground potential as long as there is water across the probes and hence the piezo buzzer can not go off. Once the water level goes down, the trigger terminal loses its ground potential and reaches the positive potential because of the pull up resistor and the buzzer will be on.

Digital Circuits

Incidentally in digital electronics parlance this is known as going high or pulling up. If the voltage at any terminal is close to the ground potential as dictated by system requirement, it is known as going low or pulling down. Conversely if the voltage at any terminal is close to the positive potential as dictated by system requirement, it is known as going high or pulling up. Digital circuits work usually with 0 andl. Obviously one is high level and zero is low level, which are known as logic states. Noise in the digital circuits can easily be eliminated. Follow the logic states depicted here.

Analogue Circuits

On the other hand analogue circuits process analogue signals which can output any value within a range of its power supply. Examples are audio, video amplifiers. Unlike digital circuits, noise here is difficult to control.Construction is similar to earlier project, except that the probes are now fixed at the required low level of water.

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This can be used as an alarm when filling overhead tanks. Quite some time ago, I used it as an indication when the municipal water used to start flowing to my pipeline. When I built this project, happiest one was my mother. Municipal water used to be pumped in the midnighl at unearthly hours and my mother used to wake up now and then to check if the water started pouring in.

With half wave power supply already built, connect the buzzer as shown in the schematic and fix the wires. As the water level rises to touch the wires, the buzzer gives out a loud alarm. I used to put these wires in a plastic bucket under the tap. When municipal water flows down, it indicates an alarm. It is one of those most useful and easiest projects I ever built.

You may use this as a rain alarm particularly if the lady of the house dries up clothes or grain in the open yard or even as bed wetting alarm for the baby ((Use only batter}/” power!!)).You can try this as a steady hand testing game. You may use this for a continuity tester but do not use this on live wires.

Bring two wires and keep them at the desired level in the tank with a little separating distance between them. You can use fairly long wires and run them along into the house. Needless to say scrap and expose the copper wires a little. Connect one of these wires to the trigger terminal (generally yellow) of the buzzer and the other to the positive terminal (red wire). Black wire goes to the negative.

If you want battery operation, use two battery cells (1.5V each). Use more cells for more sound. If you want to use mains supply, half wave circuit description is given above. Use a transformer with 12V or 6-0-6V at the secondary. Screw the secondary ends to the terminal strip; screw diode (Dl, IN 4003) CI is an electrolytic capacitor of 1000 mfd and 16V rating. Please follow the polarity marked on the can. Light emitting diode is optional. Use with resistor (Rl, IK). It gives an indication that the power supply is OK. You may use the mains plug mounted power supplies or eliminators and mount all the parts on it.

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