Price - Stock No 550

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P.C.Board * 2xBC548 Transistor

3 LEDs Red,Green,Yellow * 2- Eye Terminals

470R Resistor - yellow/violet/brown'* 4- Molex Pins

10OR Resistor - brown/black/brown * Hook-up Wire Red & Black

2xl0K Res. brown/black/orange * Solder

680R blue/grey/brown * 2x 6v8 Zener Diode(IN754)

IN914 Silicon Diode  (IN4148) * l1v Zener Diode  (IN962)


Schematic Diagram

PCB Layout


1. Mounting the components is relatively easy. The resistors will have to have the legs bent to match their holes, and can go in any way round.

2. The IN914 (little glassy diode) and the zener diodes all have bands on the body which must be put in the direction shown on the drawing.  The bands ar ' e at the K end of the diodes. A magnifying glass will help to locate the bands and also to check the values of the zeners.

To identify them, the following procedure will help : with a magnifying glass find the diode that' has the code IN4148 printed on the glassy surface.  This is the silicon diode usually referred to as IN914, NOT a zener.  The zeners may have their voltage printed on the side, or they may have numbers. Remember that there are two 6V8 zeners and one 11V, so
if you find two that have the same numbers they are the 6V8.

They can be soldered off as you go if you wish.

The  transistors have a flat on the body which must be in the direction shown on the drawing.

The three LEDs also must be polarised. Look for the small flat on the flange below the globe and put these as shown on the sheet.

4. The molex pins are to connect the wires to the circuit. Push them through in their holes and solder. Bare the Red and Black hook-up wires, tin them and solder to the pins. Red is Positive, Black Negative

Bare the other ends of the wires and solder and /or crimp the Eye terminals in place.

5. Connect the circuit to a power supply with a variable Voltage control, but make sure the polarity is co ect. Wind up the voltage and check the performance of your circuit against the quoted performance figures.

TROUBLE SHOOTING: Failure to perform will be due to one of the following: * diodes in wrong polarity

transistors in wrong way round

LEDs in-wrong way round

resistors incorrectly placed

poor soldering  IV

Check all the above- and make a visual check for bridges of solder.

Technical Notes

This circuit depends for its operation upon the different voltage drops across different colour LEDs.  At 20mA the voltage drops across red, yellow and green LEDs are typically 1.7, 3.0 and 2.3 volts respectively. 

When the vehicle battery voltage is too low to cause either ZDI/ZD2 or ZD3 to conduct, Q1 and 02 are held off by R3 and R5. Under these conditions the yellow LED is forward biased and conducts 'via D1 producing a potential of about 3.7 volts at the "A legs" of the LED's .

When the supply rises above about 11.6 volts ZD3 conducts, biasing Q2 on. By virtue of its lower voltage recuirements the green LED conducts, reducing the voltage at the "A legs" of the LED's to approximately 2.6 volts. This is not enough to bias DI/LED2 on, so the yellow LED goes off. The green LED 'steals' the bias from the yellow LED. 

When the supply rises above about 14.2 volts, Q1 is biased on and the red LED 'steals' the bias from the green. The potential at the "A legs" of the LED's falls to two volts and only the red LED conducts.

R1 limits the current through the LEDs.

R2 and R4 limit the base currents into Q1 and Q2.

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