R1,R3,R6________1K 1/4W Resistors R2____________100K 1/4W Resistor R4,R5,R7,R8_____3K3 1/4W Resistors D1__________Red LED (Any dimension and shape) D2________Amber LED (Any dimension and shape) D3,D4,D5_____1N4148 75V 150mA Diodes D6_________BZX79C10 10V 500mW Zener Diode D7________Green LED (Any dimension and shape) D8,D9________1N4148 75V 150mA Diodes D10________BZX79C12 12V 500mW Zener Diode Q1,Q2_________BC547 45V 100mA NPN Transistors Q3____________BC557 45V 100mA PNP Transistor
Connecting this circuit to the battery of your vehicle, you will always know at a glance the approximate voltage available.
An indication of battery voltage is useful to the motorist for monitoring the battery's capacity to deliver current, and as a check on the efficiency of the dynamo or alternator.
Threshold voltages of the LEDs are set by means of two Zener Diodes (D6 & D10) plus two further Diodes wired in series (D4, D5 and D8, D9 respectively) adding a step of about 1.3V to the nominal Zener voltage.
- Red LED D1 is on when battery voltage is 11.5V or less. This indicates a low battery charge.
- Amber LED D2 is on when battery voltage is comprised in the 11.5 - 13.5V range. This indicates that the battery is good if the motor is off. When motor is running, this indicates no charge from dynamo or alternator.
- Green LED D7 is on when battery voltage is 13.5V or more. This indicates a normal condition when motor is running and dynamo or alternator are charging.