A TTL-operated switch
for coaxial cables

This is a little gadget I find handy sometimes. It lets me use a TTL line to select between two routes for a coaxial cable. It’s dirt simple– just an SPDT relay– and I’ve coded the inputs by their location on the enclosure: the switch-operating input is by itself on the front, and the common pole is on one side, while the two throws are next to each other on another side. I use this to quickly switch between different signals for the oscilloscope, different synchronization schemes depending on my experiment protocol, and fast changes between different amplifier outputs. I put a magnet on the back of it, so that I can stick it to an equipment rack, Faraday cage, or optical table.

Wire all the grounds together, then wire the center pins of the coax connectors to the relay pins as indicated in the datasheet. I used a 5v relay from Omron. (datasheet)

It couldn’t be simpler. A different choice of relay and/or additions to the circuit could allow it to operate at lower voltages (5v is at the top end of TTL specs) and/or draw less current. The version I show here has worked with National Instruments DAQ digital outs and is clean enough for analog signals from PMTs and electrophysiology amplifiers. Keep all the wires short and use a metal enclosure.