MFOS Square Wave Oscillator

After the 555 Timer Oscillator experiment, I decided to do some real audio oscillators. This time I was able to breadboard from a circuit diagram and not a video or tutorial. The design I chose was a simple square wave oscillator from the Make: Analog Synthesizers book by Ray Wilson.


This Oscillator requires a bipolar supply with a ground, and, unfortunately, the power supply I decided to buy did not have a ground, it instead has two leads, one for positive voltage, one for negative voltage. I decided to use two batteries connected in such a way that the 0V reference point was between the two batteries (you can see this from one of the images below). It also requires use of an op amp to generate the signal, and I decided to use the TL072.

To get the circuit correct, I decided to look up the datasheet on the TL072, and used the pinouts found there to model the diagram on the breadboard.

My one worry while wiring up the breadboard was that a 9V bipolar supply would not be enough to power this circuit (this was before I was able to calculate the load and power loss of a circuit), but it was a complete success.

As you can see, the oscillations were clean and musical (other than being relatively stuck at high frequencies due to the audio taper potentiometer I had), and the square wave kept its duty cycle all the way up into the high frequencies.

I was able to get the circuit to oscillate between  40Hz and 2.1kHz, which was surprising to me, given that the last oscillator I prototyped had a relatively low frequency range (probably due to my modifications and nothing else).

More than ever I am becoming aware that I need a bipolar power supply and proper potentiometers or trimpots.






555 Timer Oscillator Experiment




After the long wait for components to come in the mail, I decided to take a stab at making a 555 Timer Oscillator, one of the easier projects to do with very few components. I ended up modifying the circuit that was contained within the following tutorial:

During the video, you will hear him talking about the discharging capacitor used. He said that he picked an electrolytic capacitor that was 22µF so that it would blink the LED at a rate he could see.

Now, I knew this would be a square oscillator because without waveshaping, the 555 timer outputs digital pulses. Since I had to adjust the frequency of the oscillator in order for it to be audible, I decided to use a capacitor that discharges at a lower voltage. I also modified the design by adding in a potentiometer by replacing the second resistor with a 100k potentiometer, which adjusted the rate that the capacitor discharged, thereby adjusting the frequency of the oscillator.


I had trouble finding the right place to insert the potentiometer, but after I tried “replacing” one of the resistors with a variable resistor, I noticed changes. Apparently, after the voltage already went through two resistors, there wasn’t enough of a voltage to change with the potentiometer.

As you can see, the resulting wave looks and sounds “dirty”, meaning that as the oscillator gets higher in frequency the resulting wave gets distorted from a square shape to something else. The duty cycle of the higher pitched oscillations is more like 70/30 instead of the 50/50 of a traditional square wave. It is also pretty telling that the crest of the wave looks to be rising from what would be ‘100%’. This is probably due to the capacitor used and it’s tolerances. The duty cycle change is most likely due to the 555 Timer not having any biasing on the input pin.

Another weird thing is that the potentiometer I used was an “A” potentiometer, which has a logarithmic response curve and not a linear one, hence the twisting of the knob going from a gradual increase to a relatively quick increase to maximum frequency output.

The leads from the circuit to the oscilloscope went from the drain pin of the capacitor and the negative supply of the battery. This caused a DC signal. If a bipolar supply were used, this would have been an AC signal, but at the time I didn’t really know how to do that with batteries.

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