A voice key or voice relay is a device that outputs a pulse when the subject speaks a sound.  This is used to trigger a computer timer when a person starts to speak, usefeful if we want to measure reaction time to some stimulus.

The method of the schematic can be used with TTL-level computer data ports.  Another method that can be used on the C-64 computer is to use a photocell connected to the analog input, optically coupled to a lightbul driven by an amplifier.  Although sluggish in response for absolute real-time measures, the photocell method was found to be reasonable for relative real-time measures where differences in reaction times, rather than absolute reaction times, are of interest.  For further details on the photocell method, see the technical details of the dichotic studies.  The method below is necessary if absolute reaction times are more important.