Here is a simple circuit using op-amp 741 called “Battery voltage state indicator” used to indicate whether the battery is charged or not. The circuit shows three state of battery voltage i.e. charged, discharged and normal using three different color LEDs (LED1, LED2 and LED3).
Circuit Description of Battery Voltage State Indicator
The circuit of battery voltage state indicator is build around very popular and low costs op-amp IC 741. This circuit uses two op-amps in comparator mode, which compare the battery voltage with references voltage. The battery voltage is connected to E (pin 2 of IC1 and pin 3 of IC2) as show in circuit diagram, which further compares with reference voltage E1 and E2.
If the battery voltage is less then 11V, LED2 glow, similarly if battery voltage is 11V to 13V LED3 glow and if the battery voltage is greater than 13V LED1 glow.
The variable resistors VR1 and VR2 is so adjusted that the voltage between pin 2 of IC1 and earth is 13V and voltage between pin 3 of IC3 and earth I 11V.
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 – R3 = 2.7 KΩ|
VR1, VR2 = 22 KΩ
|IC1, IC3 = 741|
D1, D2 = 1N4148
LED1 = Red LED
LED2 = Orange LED
LED3 = Green LED