Reversing An Aquarium Air Pump


I recently bought a small aquarium air pump, not really realizing that “pump” in this case meant blowing air, not sucking. A quick search turned up almost no information online, but converting it into a vacuum pump turned out to be quite easy.

Why might you need a vacuum pump? I’m using it for picking up swarf while cutting vinyl records, but this would also be useful for getting rid of bubbles in mold-making or casting!

I’m using this pump which I got for $13 on Amazon. It’s very small and fairly quiet. To convert it to a vacuum pump, remove the four screws on the back. Inside is a small transformer and two arms. These arms have a magnet at each end, which is pulled by the transformer to activate their rubber bellows. Fortunately, to switch the pump, we just have to reverse the air inlet/outlets: suck air in, push air out vs suck air out, push air out.

Pry up the plastic bellows assembly with a small screwdriver, like shown above.


The inlet/outlet are above, sealed with a rubber O-ring. Pull the assembly out carefully and spin it 90º. Then just push it back into place. Be careful that the O-rings are seated properly, as they seem to want to pop out.

Close up the pump! You may need to seal it using caulk or hot glue if you’re not getting good suction, but mine seemed pretty good without.


Finally, attach the tubes. For more suction, the two tubes can be combined using a T-connector (my pump came with one).

Nothing earth-shattering, but hopefully this helps someone with a similar need!

RNG White Noise Tests



For the upcoming White Noise Boutique, I’ll be generating bespoke white noise for visitors using a variety of methods. These quick tests show that, while they sound essentially the same, different algorithms do in fact generate different white noise. For each generator, a spectrogram and frequency plot are shown for a ten-second sample. The plots were generated with Audacity.

Above: the AES_OFB algorithm from the Dieharder suite, one of two cryptographically-secure methods.



Linear congruential generator, an old-fashioned and non-secure generator.



The only “true” generator here, via the operating system’s built-in “dev/random” command, which creates random numbers from hard drive entropy.



The Threefish generator, also via the Dieharder suite and the other cryptographically-secure algorithm.

Racal MA-4204 Time Division Voice Scrambler


Another device using random acoustic noise for cryptographic purposes. This device from the early 1970s randomly rearranges the audio signal every half-second; a corresponding unit puts the audio back in the correct order on the other end. While not very secure (the three 8-position switches only provide 512 possible combinations), this device’s operation and style have a nice mix of of functionality and poetics.

Via Cryptography Museum.