I created an arduino project for a tally light. The design plans are available here.
The software (mac only) is available at the App Store:
You can connect to a Blackmagic ATEM switcher by IP address.
You can setup IP addresses for the tally lights. If an IP address is provided, the software will send a http request in the form of:
to the tally light.
It should respond with 'OK' (two bytes) only if it received the GET request correctly.
How to build the Tally :
DISCLAIMER: I am not a professional electronics designer, I'm only self taught and I builded this project with trial and error. I don't give any warranty on the information below.
I used a UNO+WiFi R3 , actually only the ESP8266 processor is used (the arduino is bypassed), so maybe it can be done cheaper and with a smaller chip.
Tallylight ESP8266 code:
(At line 27 add your Wifi Access point credentials).
3d printer files:
JVTallyBox stl files.zip
Electronic scheme (made with circuitlab.com):
2 green leds
2 red leds
UNO+Wifi R3 (+ antenna)
2 resistors 30 Ohm (100 is probably to much, you could experiment with a little higher or lower, depending on how much light comes of the resistor.