The purpose of a tally-light is a simple one. It tells the camera operator when his camera is on-air. Of course this only makes sense during a live-production. As professional tally-light solutions are expensive, I built one by myself using parts for only a few bucks.
Here is what you need:
I decided to use standard XLR-cable to control the tallys, because they are robust, you can get them nearly everywhere and many stages have laid them already. The basic principle is very easy: The computer tells the Arduino which cam is on-air, the Arduino powers the corresponding LED (and turns of the others).
- Cut the XLR-cables in the middle. The part with the male connector is for the tally-side, the female part for the Arduino-side.
- Strip the isolation of the cables. You just need two of the three strands, but don't mess it up which one you used.
- Drill two holes in the upper plate of the hot shoe adapter and insert the strands from below.
- The LED consist of 6 small LEDs in one housing (this is why they have 12 leads). In order to prevent my tallies to get too bright I just connected 3 of them to the strands. Solder the strands to the leads of the LEDs and use the epoxy resin to fix it on the upper side of the plate.
- Solder each anode to a PWM-output of the Arduino and all cathodes to the ground.
- Basically, you're done. You can test it by using StandardFirmata for Arduino and the Firmata Test Program. Caution: Consider the max voltage of the LEDs (3V for the ones linked above).
The next challenge is to control the tallies. For live-production I use the ATEM video-switcher and my atemOSC-software to select the cameras via OSC. I extended this software to send serial messages to the Arduino. As described on the github-page, this software exposes the controls of the ATEM switcher via OSC. The tally controls are optional and require an Arduino connected to the computer.