In final form, a Bluetooth module and Arduino Pro Micro are stuffed inside the stereo along with a DPDT relay to switch the I2C bus.

The DPDT relay (Double Pole Double Throw), also knows as the “2 pole relay”, is quite interesting and can be used in various scenarios, including for changing the direction of a motor as you can see ...

It’s got more click and clack than the Tappet family, and is a work of art rendered in DPDT relays. The biggest consideration in designing a relay computer is the memory.