(I explain why I made this machine at the end of the log)

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Today we are going to talk about solenoid valves, i.e. how to control compressed air using electricity.

They are usually very expensive and only used by professionals. Fortunately, I found these Airtac valves on Amazon for about 10 euros each (which is dirt cheap).

I will not try to explain in detail how solenoid valves work. But basically by sending electricity to a coil I can open the valve and release the compressed air. If you want more info, you can go here.

Here are some pictures of the valves I will use. The mechanical part:

The solenoid with terminal blocks to connect to the power supply:

I can chain them to save space and use less tubes and fittings:

To control the valves with a micro-controller, I use a simple circuit with a Mosfet transistor:

I then repeat the circuit six times on a proto board and connect the solenoids:

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Harmonica Machine

To test what I've made so far - the air tank, the solenoid valves and the circuit - I had the idea of using the compressed air to blow in an harmonica. After a few attempts, it seems that only 4 notes work: C, E, G and a lower c. That's good enough for a small melody, so I hook up my circuit, plug the compressor with the valves and the air tank, and program an STM32F4 with a small piece of software that triggers the solenoid valves to play music (sources). You can see and hear the result at the top of this project log.