Almost every one of us has fallen prey to a flat battery at one time or the other. On the lighter side, batteries some how have an uncanny sense of failing at our urgent moment. Here is circuit that gives a general indication of the health of the car battery and warns you to give it a boost. Do not blame me for another 555, after all it is so versatile. One more automobile circuit, but a very useful one!

The circuit is shown in Schematic Here again the 1.8 K oscillator is used as the alarm. However it is disabled by transistor Q1, whose base is driven by 10V Zener diode. As long as the battery voltage is above 10V, this diode conducts, transistor Q1 also conducts. It clamps the reset pin of IC 555 to ground and the oscillator cannot function. When the battery voltage falls below 10V, the transistor cannot hold reset pin to ground and the oscillator starts functioning. 555 then gives an alarm indicating low battery voltage. LED D2 will light up as long as the voltage is good.

Zener diodes are used to regulate the voltages. When forward-biased, zener diodes behave like normal rectifying diodes, but in reverse-bias, they do not conduct until the applied voltage reaches or exceeds the so-called zener voltage. Then it conducts substantial current, and while doing so, it will try to limit the voltage to that zener voltage. As long as the power dissipated now does not exceed the diode’s thermal limits, the diode will riot be harmed. Zener diodes are widely used to regulate the voltage and they are manufactured from a few volts to hundreds of volts.

Construction is similar to other 555 circuits and needless to say thlat the power supply is from the car battery. The circuit will be placed in an automobile and has to be necessarily rugged. Automobile body is grounded or connected to the negative terminal. Hence special care must be taken to protect the circuit from accidental short circuits. Suitable enclosure must be used. Power should be taken from the battery directly before the ignition switch and it is better to use 7555.

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