Slave Flash Trigger
Posted on May 3, 2012 5350
The SCR is wired across the trigger circuit of the flash gun. Normally, the SCR is off, so the flash gun is able to cha
rge to its trigger voltage. Photo transistor Q1 is used to monitor the light level. When a high-intensity flash occurs, Q1 briefly conducts and supplies gate current to the SCR. That causes the SCR to turn on, which then triggers the slave flash gun via the hot-shoe adapter terminals. Once the flash gun has triggered, the SCR quickly turns off again. That happens because the current in circuit quickly falls below the SCR"s holding current. The resistor at the base of Q1 (RI) determines the sensitivity of the circuit. If you wish, you can reduce the sensitivity, simply by reducing the value of the resistor from that shown. The l-KOhmhm resistor between the gate and cathode of the SCR (R3) prevents the SCR from false triggering if high voltages are applied between the anode and the cathode. Q1 can also be a GEL14G2.