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Schematic Diagram


PCB Layout



  1.  Turn the PCBoard over so that you are looking at the PLAIN side. From the drawing, find the locations of the components. You will need to turn the PCB around so that it coincides with the placement sheet. Remember that you are seeing the copper track through the PCB.
  2.  Identify the values of the three resistors by their colour bands and/or an ohm-meter. Bend the legs of the resistors to a shape that will fit into their location holes. Push them through and firm the resistor down against the PCB.
  3. The two transistors look the same but are really quite different. Look for the identifying codes on the body of the transistors. Offer the transistors into their positions with the flat on the body the way round that the drawing indicates. Bend the legs to fit into the holes. Push them in, but allow to stand high off the board.
  4. Spread the legs of the LED carefully so they spring into its correct holes. Push the legs just through the PCB so the LED stands high off the board. You should check that the small flat on the flange at the base of the globe is the same way round as the drawing shows. The LED is polarised - a semiconductor - and will not work if placed the wrong way round. The flat is adjacent to the Negative (K) leg.
  5.  The electrolytic capacitor is polarised. An arrow is printed on the body pointing down the negative leg. Check the placement sheet for its polarity and location, push the legs through and solder.
  6. The disc ceramic capacitor is not polarised so it can go in either way. Notice that the value of this capacitor is coded on its surface. This one reads 103. The first and second numbers are value digits and the third (3 in this case) is a multiplier, viz. indicates the number of zeros following. This shows a value of 10000 -(1 + 0 + 000). This value is always expressed in picafarads. Reference to a table will show that this converts to 10 nanofarads, or 0.01 uF.
  7. These components may be soldered in place.
  8. Insert and solder the PCB pins.
  9. The battery snap is sildered with the red wire to the positive rail, black to the negative rail.
  10. Connect a battery and the flasher will commence. The current draw is very small and the voltage is not critical, so the light will flash for a very long time from the battery even when the battery is almost completely flat. 
  11. Trouble shooting will consist of checking locations, polarity, and soldering. 

The cycle - how it works

For detailed information on individual components in this circuit see resistors,  capacitors and  transistors.

Stage 1 – The 10uF electrolytic capacitor charges. 

Both the transistors are off and the 10uF electrolytic capacitor charges via the 330K and the 22R resistor.  As this capacitor charges, the voltage at the junction of the 330K resistor, the 10uF capacitor and the base of the BC548 transistor rises.  When the voltage rises to about 6V the transistor begins to conduct from the collector to the emitter.

Stage 2 – The BC558 transistor turns on. 

With the BC548 transistor conducting, the BC558 transistor is turned on via the 1K resistor with the 0.01uF capacitor limiting the effectiveness of the 1K resistor (or makes it sluggish).  This illuminates the LED via the 22R resistor which limits the current through the LED to a safe level.

Stage 3 – The beginning of the ‘turn off’

As a result of the BC558 turning on, the voltage at the collector of this transistor jumps up – let’s call it V1.  The electrolytic capacitor is still charged and let’s call it’s voltage V2.  So now the junction of the 10uF capacitor, 330K resistor and the base of the BC548 rises sharply from “V2” to “V1+V2”.  With this higher voltage on the Base of the BC548, it turns on harder and the 10uF capacitor begins to discharge rapidly (through the B to E path in the BC548).  In a short time the 10uF capacitor has discharged and the BC548 turns off.

Stage 4 - The BC558 turn off.

With the 0.01uF capacitor again limiting the effectiveness of the 1K resistor, the BC558 begins to turn off.  And the LED begins to extinguish. This continues, the LED turns off and the voltage at the junction of the 22R, 10uF and the collector of the BC558 drops.  With both the transistors off, the cycle starts again.

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