TIRPOR - The Irrational Pursuit Of Reason



Electronics for photography

Disclaimer: these circuits have been designed and tested by myself. They work for me. Use them at your own peril - if you break yourself or your equipment don't blame me !


Nikon Flash Duration Control

Nikon Speedlights use low voltage control signals to control the duration of the flash discharge. The signals basically consist of a "fire" and a "quench" command which sequentially short a 5V signal at the appropriate flash connection to ground. Most Nikon TTL capable speedlights can run in a manual mode where (I assume) this sequential connection is controlled internally. I wanted to control a bank of Speedlights and so designed an external circuit to accomplish this.

The circuit gets its power from the "Ready" signal from the first flash unit in the sequence. It is inserted into the middle of a Nikon SC-17 in order to easily make the flash and camera connections. There are two other options for firing the circuit - a manual test button and a flash PC socket.

The circuit diagram is here: flash duration control circuit. Note: this diagram does not show the "test" and "PC " input connections.

It is built around an HCF4538 which is basically two monostable timers. These are cascaded together and both are set to be trailing edge triggered. The output of the first switches the flash fire to ground. As it finishes its cycle it triggers the second timer whose output switches quench to ground. The second timer's inverted output is also used to reset timer one to help prevent retriggering. The flash duration can be controlled with Pot1. With the component values shown flash pulses of around 1/45,000" to 1/15,000" should be possible. The quench pulse is about 47microseconds. This could have been longer but I wanted the capability for a rapid retriggering.

I've tested the circuit on a bank of 5 Nikon SB-26/SB-800 Speedlights, all set in TTL mode. Seems to work !