Square Wave Oscillator

Posted on Feb 5, 2014

It`s most useful application is as a Signal Injector for radios and TV`s. A square wave is the most suitable for testing the IF (Intermediate Frequency) strip as the signal will pass through the IF transformers without any attenuation, no matter what the tuned frequency of the circuit. Normally only a sine wave of the correct frequency will get through but a square wave can be considered to be a composition of all of the

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multiples of a sine wave and no matter what the frequency of the tuned circuit, it will be processed. This project can also be used to test digital circuits by providing a clock pulse, using one of the 6 frequencies to determine if a circuit can be "clocked. " In fact it`s an invaluable piece of servicing equipment and I have used a similar device for testing the IF strip of TV`s on the odd occasion when a fault developed, and it located the trouble very quickly. The 6 frequencies are selected with a jumper lead and increment in decade values as is standard with test equipment. If you want the frequency to be variable, you can use a pot between pins 6 and 7 of the 555, instead of the capacitors, but keep in mind that you cannot get the same range by using a single pot - that`s why we have used capacitors. Make a short wander lead with hook-up wire and tin the end so that it fits into the hollow pins. Put the lead into the 1Hz position and connect the battery. You will see the LED flash on and off to prove the circuit is working. Change the lead to one of the higher frequencies and inject the signal into the IF strip of a radio. Start at the volume control end and work your way to the antenna. As you move through the strip, the signal strength will increase and you should compensate for this by adding resistors and capacitors on the output to reduce the signal. You can also check any of our FM transmitters by picking up the...

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