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<blockquote data-quote="WayneF" data-source="post: 261560" data-attributes="member: 12496">PC sync (and also the large center pin of the hot shoe) is just a switch that grounds that pin to the metal frame.&nbsp; It works at any shutter speed (but actual flash only works well up to maximum sync speed, which is only time the full picture frame area is fully open to pass flash light).Your circuit would be a low voltage source, typically ballpark of about 5 volts today, through a protective current limiting resistor (lots of R to protect the switch), into the cable to the PC sync switch.&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; +5V&nbsp; -----------/\/\/\/\/---- A -------------------------------------------------&gt;&nbsp;&nbsp; Camera PC or hot shoe&lt;-----------------------------------------------ground wire ------------------&gt;When that camera shutter switch is open, there is no current, and so no voltage drop, and your point A also reads 5V.But when the shutter sync switch is closed (an SCR today), point A drops to near zero volts (grounded to the metal frame, probably 0.6V semiconductor voltage still remains).&nbsp; &nbsp;&nbsp; This voltage drop is the pulse signal detected by the flash, which triggers.&nbsp; It is NOT about the current (so use high R to protect the camera switch).&nbsp; It merely detects that there was a voltage drop, comparable to transistor TTL 0 - 1 logic.You can short that flash pin with a paper clip to see it flash, to verify this.&nbsp; Don't flash it into your eye, and do not lay it face down on anything the flash tube heat can scorch (because it can).</blockquote>

[QUOTE="WayneF, post: 261560, member: 12496"] PC sync (and also the large center pin of the hot shoe) is just a switch that grounds that pin to the metal frame. It works at any shutter speed (but actual flash only works well up to maximum sync speed, which is only time the full picture frame area is fully open to pass flash light). Your circuit would be a low voltage source, typically ballpark of about 5 volts today, through a protective current limiting resistor (lots of R to protect the switch), into the cable to the PC sync switch. +5V -----------/\/\/\/\/---- A -------------------------------------------------> Camera PC or hot shoe <-----------------------------------------------ground wire ------------------> When that camera shutter switch is open, there is no current, and so no voltage drop, and your point A also reads 5V. But when the shutter sync switch is closed (an SCR today), point A drops to near zero volts (grounded to the metal frame, probably 0.6V semiconductor voltage still remains). This voltage drop is the [B]pulse signal[/B] detected by the flash, which triggers. It is NOT about the current (so use high R to protect the camera switch). It merely detects that there was a voltage drop, comparable to transistor TTL 0 - 1 logic. You can short that flash pin with a paper clip to see it flash, to verify this. Don't flash it into your eye, and do not lay it face down on anything the flash tube heat can scorch (because it can). [/QUOTE]

Verification