Coin Sequencer

The Coin Sequencer is a physical interface for sequencing audio samples. It uses four IR transmitter/receiver pairs to detect reflective objects (such as coins) passing under one of four tracks. When such an object is detected, an audio sample is triggered on the computer.

In the video below you can see the Coin Sequencer in action.

For those who have contacted me asking for schematics, here they are. They are simple enough that I’m making them public domain. The usual disclaimers apply: I make no guarantees and I am not liable for any damages, and so forth.

Each IR transceiver unit was built as shown above, on the right. This configuration works, but is not exactly correct, since it doesn't give the phototransistor much “wiggle” room. The configuration on the left is how it should have been done, but I have not tried it (which is why I have the question mark in the resistor value). By the way, it is a good idea to use potentiometers for all resistors above to allow fine-tuning of the sensitivity.

For my first version, I placed four of these side-by-side, connecting the output of each to a digital buffer (the SN74HTC540N), which works as a 1-bit analog-to-digital converter. The outputs of the buffer where then sent directly to a parallel port connector. Later on, I modified the Coin Sequencer to work through USB by using an Arduino board. This means I could bypass the buffer and the parallel port, and just connect the IR transceiver outputs directly to the Arduino's pins. I then used ttymidi to produce MIDI events whenever the coin was detected.

The Coin Sequencer has appeared on the Make magazine blog, Hack-a-day, and of course Slashdot.

In addition, the Coin Sequencer was featured in the book “Makers: all kinds of people making amazing things in their backyard, basement or garage”, ISBN 13: 9780596101886, by Bob Parks, from O’Reilly Media. You should consider buying it :)