Price - Stock No 80
Check your kit
- 1 - 330K
- 1 - 1K
- 1 - 22R
- 1 - LED
- 1 - BC548 Transistor
- 1 - BC558 Transistor
- 1 - 10uF Electrolytic Capacitor
- 1 - 0.01uF (10nF) PolyesterCeramic Capacitor (103)
- 1 - Battery snap
- 1 - Printed Circuit Board
- 2 - PCB Pins
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.
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.
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.
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.
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.
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.
These components may be soldered in place.
Insert and solder the PCB pins.
The battery snap is sildered with the red wire to the
positive rail, black to the negative rail.
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
- Trouble shooting will consist of checking locations, polarity, and
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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