LED Flasher using Transistors only
LED Flasher Using BC548 and BC558 Transistors
Here is an LED flasher, otherwise known as an LED blinking
circuit that any GCSE student can make very easily and cheaply. Usually,
students use a 555 timer IC to build a simple LED blinker, which is fine for
temporary experimentation; however, it is not cost effective to dedicate an IC
permanently for such a mundane task. Instead, we can use cheaper components to
blink LEDs. The great thing about this circuit is that since it is cheap, you
could make permanent blinkers on strip board, after you have built a prototype
on breadboard.
Circuit Operation
This circuit uses a BC558 PNP transistor connected to a
BC548 NPN transistor in such a way so that the BC548 conducts for a very brief
time making the LED light. The resistor R and Capacitor C determines the timing
of the flasher, and the period is proportional to R × C. The capacitor C
charges through the resistor R, and since its value is 100 K MΩ it does so very
Quickly. When the capacitor has sufficient charge, it discharges through the
emitter junction of the BC558 transistor. This charge, then flows through the
base junction of BC548 making it conduct and thereby powering the LED.
Providing you use the general-purpose low power LEDs, the
great thing about this circuit is that it consumes very little power and a 9 V
battery should last almost for a month.
Experiment
The first step is to build this on breadboard so you can
experiment with it and tailor the timing and LED for your taste. Try changing
the resistor values to see how the blinking rate changes. Also, have a go
changing the capacitor values to see how the timing changes. You can also
connect a buzzer in series with the LED to see if it makes a sound periodically
Just to give you the heads up on this, installing the
transistors will be easy if you use this diagram. I actually had to look it up
in the documentation to make sure. The BC557 and BC547 have very similar
looking packages, and their pin outs are identical as well, which makes
building this circuit simple. You need to pay special attention to the
collector (c), base (b), and emitter (e) junctions of the transistors when
installing.
Installing the LED
When installing the LED, you need to pay special attention
to the anode and cathode terminals. If you install it incorrectly, then the LED
will not light. I have a special LED Resistor Calculator article that shows how
to determine which is which.
Capacitor C
When installing the capacitor, you need to pay special
attention to the polarity, because it is an electrolytic one.
Cost
This little circuit uses the cheapest parts possible so it
should be affordable for anyone including me! Firstly, we are using the common
as muck general purpose BC558 and BC548 transistors, which I got online. They
cost as little as Rs. 5/-, and if you order from an honest seller, then you can
get 50 transistors for Rs. 100/-.
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