Most of the Surveillance systems and CCTV camera uses rechargeable 9 volt battery. This simple Battery Monitor can be hooked to the 9 V battery so that it indicates the voltage in the battery through visual indications. When the Test switch is pressed, the Bicolour LED lights as Green if the battery voltage is above 9V (fully charged) , it lights Yellow when the voltage is between 7.5V and 9V, ( moderate charge ) and lights Red when the battery voltage is below 7.5 ( needs charging ).
You need the following components
1. 470 Ohms resistor – 2
2. 1K resistor – 2
3. 7.5V 400 mW Zener – 1
4. 5.1V 400 mW Zener – 1
5. Bicolour LED ( Red and Green – Common Cathode ) -1
6. BC547 Transistor – 1
7. Push switch – 1
How it works?
Two Zener diodes (ZD1 and ZD2) works like switches. When the battery voltage is above 9 volts (A fully charged 9V battery shows 9.2 – 9.5 terminal Voltage), Zener diode ZD1 conducts and give full base current to T1. T1 then conducts pulling its collector voltage to ground potential. So the Red half of the Bicolour LED turns off. Green LED still lights since ZD2 is conducting. This gives Green colour to LED to indicate that the battery is fully charged. When the battery voltage goes below 7.5V, ZD1 turns off. T1 also turns off at this time. So Red LED turns on with full brightness than Green LED and due to visual perception, the LED looks like red. But between 7.5 and 9 V, there is partial lighting of both Red and Green halves giving Yellow colour to LED.
Connect the circuit to the device battery. Press S1 to check the condition of the battery.For 12 volt battery, use 11V Zener as ZD1 and 8.2V Zener as ZD2.