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08-03-2009

Automotive Voltage Indicator

Monitors battery voltage
Three-LED Display

Circuit diagram:

Automotive Voltage Indicator

Parts:
 

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

Comments:

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.

LED indication:

  • 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.

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