This article is about simple oscillators that you can build from blinking LEDs.

Circuit 1:

Circuit 2:

You can see the circuits working in those videos:

Circuit 1:

Circuit 2:

Those videos do not show the waveforms that the circuits are generating. You will see the waveforms in the testing section of this article.

Step 1: Design the Circuit

I have drawn the two circuits in PSpice Student Edition version 9.1.

I modelled the LED with three diodes because this old software does not have an LED component.

Circuit 1:

Maximum LED current:

Iled = (Vs - Vled) / Rled 

= (3 V - 2 V) / 100 ohms

= 10 mA

Circuit 2:

Maximum LED current:

Iled = (Vs - 2*Vled) / Rled 

= (5 V - 2*2V) / 100 ohms

= 10 mA

Step 2: Simulations

Simulations show a maximum current that is slightly smaller than 10 mA for both circuits because of inaccuracies in the diode model:

Circuit 1 Simulations (maximum current):

Circuit 2 Simulations (maximum current):

Step 3: Make the Circuit

I made the two circuits on a piece of cardboard to save money:

Circuit 1:

Circuit 2:

Step 4: Testing

In the photos of the circuits above, you see different colour wires connected to LED pins. Those wires are the recorded oscillator outputs.

I used the Hantek 6022BE USB Oscilloscope to record the signals:

Circuit 1 (Orange and Green):

(yellow waveform - orange wire, green waveform - green wire)

Circuit 1 (Blue and Green):

(yellow waveform - blue wire, green waveform - green wire)

Circuit 2 (Yellow and Green):

(yellow waveform - yellow wire, green waveform - green wire)

Circuit 2 (Blue):

Testing shows that not all LEDs generate a decent square wave (or pulse width modulation) signal.

Testing showed that bigger LEDs that consume more current generate higher amplitude waveform signals.

However, the signals from blue LEDs were a lot smaller than signals from green LEDs because the two LEDs were produced by different companies, although they are the same size.

Conclusion

The circuit with three single LEDs, generated waveforms that appeared to be square wave signals. The circuit with LED pairs generated waveforms that looked like PWM (Pulse Width Modulation) signals.

You can also make oscillators with buzzers:

https://www.instructables.com/Buzzer-Oscillators