SLAVE FLASH TRIGGER

Posted on Feb 7, 2014

The SCR is wired across the trigger circuit of the flash gun. Normally, the SCR is off, so the flash gun is able to charge to its trigger voltage. Photo transistor Q1 is used to monitor the light level. When a high-intensity flash occurs, Q1 briefly con-ducts and supplies gate current to the SCR. That causes the SCR to turn on, which then triggers

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the slave flash gun via the hot-shoe adapter terminals. 0nce the flash gun has triggered, the SCR`s quickly tums off again. That happens because the current in circuit quickly falls below the SCR`s hold-ing current. The resistor at the base of Q1 (R1)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 1-k © resistor between the gate and cathode of the SCR (R3) prevents the SCR from false trigger-ing if high voltages are applied between the anode and the cathode. Q1 can also be a GEL14G2.

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