This is a really useful instrument for your car. It uses three LED’s to give a visual indication of the state of charge of your car battery. When the red LED is on the battery voltage is low, green is the OK signal and orange is a warning that the battery is overcharging. The whole circuit is accommodated on a small p.c. board and can be fitted easily anywhere in the dashboard.
- Working voltage: 12 V DC
- Max. current: 40 mA
How it works
The principle behind the circuit is very simple. The first LED D1, is connected in series with R2 and D4 and will light when the battery voltage is below 11.5 V. If the voltage is above 12 V the zener Z3 will bias the transistor TR1 sufficiently to turn it on and this will light the second LED D2 which is green and indicates that the battery voltage is normal. As the collector to emitter voltage drop is less than the forward voltage drop of D4 the first LED does not get sufficient voltage across it to light and is turned off. If now the battery voltage rises above 13.5 volts then in a similar way the transistor TR2 is turned on and the third LED lights to indicate potential trouble. The zener diodes are used to provide the reference voltages for greater accuracy.
|Z1,Z2||6,8V zener diode (1/2W)|
|Z3||11V Zener diode (1/2W)|
|TR1, TR2||BC548 – BC547 NPN Transistor|