Non-standard 555 Astable Multivibrator

Posted on Feb 7, 2014

I need to make a square-wave with variable frequency, but 50% duty cycle. The 555 timer seemed like a good idea, but with the normal astable multivibrator circuit you see everywhere, changing the frequency always ends up changing the duty cycle a bit. However, In my 20 year old `IC 555 Projects` book I found a circuit which i

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t specifically states has a variable frequency but 50% duty cycle. Not shown on the diagram is my decoupling of the power rails, and my decoupling of pin 5 (the control pin) to ground with a 0. 1uf capacitor (this is often recommended if you are not using it). The ultimate plan is a stroboscope that can go well into audio frequencies with one pot controlling frequency and another controlling duty cycle. I will send the square wave from the 555 through an integrator, producing a triangle wave, then stick that through a comparator so I can vary the duty cycle completely independently of the frequency (which doesn`t seem possible with the 555 by itself). You can further investigate what is happening by probing the voltage on the capacitor, at pins 2 & 6. You should see a sawtooth moving between 1/3 and 2/3 of the supply voltage. (Make sure the capacitor value is large enough that the probe capacitance does not distort the circuit. ) You may also have a problem with the LED loading the output down too much. Remember the output is providing current to charge up the timing capacitor, and if you make it also drive the LED you might overburden it. Put a high value resistor in series with the LED to limit the current, or put a high impedance buffer like an op amp between the 555 and the LED. However, from the scope trace, it comes out as about 6 KHz. The timebase is 50us, and the period of the waveform is about 3. 2 divisions 1/(3....

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