Transmitter
Any 433 MHz transmitter that is compatible with ASK / OOK should work with OpenShock.
Logic level compatibility
Please note that all ESP32s operate at 3.3V logic levels. To avoid overvolting your ESP's IO pins, it is recommended to either: connect your transmitter's power input to a 3V supply pin on the ESP's board, or use a logic-level shifter if your transmitter really requires more than 3V power to operate.
Below are transmitters that have been tested to work.