Battery Voltage Indicator Circuit


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.

Circuit diagram:

Battery Voltage Indicator Circuit-Circuit diagram


  • 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


  • 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.