Automotive Voltage Indicator
|Parts Name||Parts Symbol||Parts Description||Quantity|
|Resistor||R1, R3, R6||1K (1/4W)||3|
|Resistor||R4, R5, R7, R8||3K3 (1/4W)||4|
|Diode||D3, D4, D5, D8, D9||1N4148 (75V, 150mA)||5|
|Diode||D6||BZX79C10 (10V, 500mW, Zener Diode)||1|
|Diode||D10||BZX79C12 (12V, 500mW, Zener Diode)||1|
|Transistor||Q1, Q2||BC547 (45V, 100mA, NPN)||2|
|Transistor||Q3||BC557 (45V, 100mA, PNP)||1|
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.