A somewhat oddball, and misnamed, type of gate circuit used by many vintage Moog synths. Instead of providing a high and low voltage output to indicate the pressing and releasing of keys, it uses what is known as an "open collector" output, which basically opens and closes a switch. It requires that the entity that is receiving the signal apply a voltage to the cable, in order to detect the switch opening and closing at the sender's end. Most Moog models that use the S-trigger circuit indicate its presence through the use of an unusual connector known as a Cinch-Jones connector, which resembles a smaller version of a U.S. two-prong power plug.

S-trigger is not compatible with conventional gate circuits. Trying to send a conventional gate signal to a synth that is expecting an S-trigger may cause circuit damage; however, a simple adapter circuit can be purchased or constructed. S-trigger should not be confused with the inverted gate signals used by some Korg and Yamaha synths.

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