Battery Voltage Indicator

Monitors battery voltage, Three-LED Display

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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.

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Circuit diagram:

Battery Voltage Indicator Circuit Diagram

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Parts:

R1 = 1k

R2 = 100K

R3 = 1k

R4 = 3.3K

R5 = 3.3K

R6 = 1k

R7 = 3.3K

R8 = 3.3K

Q1 = BC547

Q2 = BC547

Q3 = BC557

D1 = Red Led

D2 = Amber Led

D3 = 1N4148

D4 = 1N4148

D5 = 1N4148

D6 = BZX79C10

D7 = Green Led

D8 = 1N4148

D9 = 1N4148

D10 = BZX79C12

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Notes:

• 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 is charging.