Automotive Voltage Indicator

Monitors battery voltage
Three-LED Display

Circuit diagram:

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.