I had some empty phiales of an alimentary integrator and i decided to obtain unidirectional valves.
1) 1 Empty integrator Phiales(plastic).
2) 1 EuroCent (the important is that the diameter fits exactly the bottleneck).
3) Insulating tape.
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Step 1: Hole in the Cap
make a hole in the cap
Step 2: Coin on the Bottleneck
Cut a piece of tape(narrower than tape)
Attach the piece of tape to the coin.
(for better sealing you can also cover the other face with tape as a gasket)
Attach the other part of the tape to the bottleneck.
Step 3: Cut the Bottom of the Bottle
Cut the bottom of the bottle with saw or cutter.
If you want the valve to be more compact you can cut also much closer to the cap reducing the length.
Step 4: Conclusion
Of course you can use also different kind of bottles, but the space between the cap and bottleneck is critical because the coin must have the room to open freely. For example usual soft drinks bottles-caps do not have this empty room.
This valve is usefull as an extremely cheep(and poor) component for many different projects like pumps(air and other fluids), breathing masks( it has also a nice Darth-Vader-like noise).