Price - Stock No 230

Schematic Diagram

PCB Layout

This project provides a convenient way to introduce students to concepts of experimentation and research in the field of Electronics by introducing a "SYSTEMS" approach.  The System involves separating the electronic circuit into three sections


A 555 IC (analogue timer) is used as the main component and is wired in an ASTABLE configuration. The OUTPUT, which consists of a LED that flashes regularly, depends on a CONTROL that consists of two resistors and a capacitor. The capacitor is charged and discharged between the two critical operating voltages for the 555 viz. 1/3 & 2/3 the input voltage. The time taken to charge/discharge can be varied by changing the value of the capacitor. The OUTPUT can be changed by changing the CONTROL, "in this case evidenced by different rates of flash.

The project 'includes two jumper leads to allow easy changing of the capacitor.


1. Check the PC Board for breaks 'in the tracks. Repair as needed. A continuity tester (Circuit Tester) is useful or a multimeter will test continuity.

2. Tum the board over to the plain side and push the IC socket into place, checking first that the notch is at the end where the locating DOT of the 555 will go. The legs may be soldered.

3. Identify the resisters RI & R2 by their colour bands AND a multimeter. Bend the legs with long nose pliers to fit the holes and push them into place. Spread the legs to hold them in place.

4. Identify the A leg (the longer one) of the LED and push it into the hole closer to the positive rail. The K leg goes to the middle hole - the output (leg 3) from the 555.

5. Clean the holes for Cl with a Imm drill and push the two pins in from the plain side so their flanges contact the board. Turn over and solder.

6. Solder the battery snap wires to the correct polarity.

7. The 555 will not fit into its socket until the legs have been closed a little. Push the legs on a flat surface, first one side then the other. Look for the locating DOT and push the 555 into place.

8. Make, up the two leads by soldering the croc. clips.

9. Connect the battery pack. Check the polarity of an electrolytic capacitor and connect it via the two leads to the stakes. The LED will flash. Change the capacitor and the flash rate will change.

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