Generally, the signal from a trigger source (e.i. sound card, optical sensor) must be converted to a binary signal to be accepted by a digital input on the recording equipment. This can be achieved with thresholding (preferably with hysteresis) of the analog signal. The equipment performing the thresholding is often referred to as a trigger box. 

Our trigger box design features galvanic isolation, battery power, low latency, and flexible thresholding with hysteresis. The design files for the PCB implementation of the trigger box is available below, along with 3D files for a casing.

Please refer the following paper if you use our design, additional information on temporal synchronization of data is available in the paper as well.
Kappel et al., “Temporal Synchronization of Multimodal Hyperscanning Recordings: Challenges, Methodologies, and Best Practices.”