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From: horn@cbnewsi.cb.att.com (david.n.horn)
Newsgroups: sci.electronics,rec.photo
Subject: Re: How to trigger a photo flash?
Summary: An opto-isolated Triac is a safe way
Message-ID: <1991Jun27.230031.19352@cbnewsi.cb.att.com>
Date: 27 Jun 91 23:00:31 GMT
References: <4633@bnr-rsc.UUCP>
Distribution: na
Organization: AT&T Bell Laboratories
Lines: 28

Markus Wandel asks:

>I want to try some experiments which would require one or more photo flashes
>to be fired under control of a digital logic circuit.

I have successfully used several types of Silicon Controlled Rectifier (SCR)
or Triac to fire electronic flashes. Radio Shack used to sell a optically-
triggered SCR that made a very sensitive "slave" - just by itself, but they
have not been selling it for several years.
Another device that makes a good, safe, all-purpose interface to a flash is
a opto-isolated TRIAC/Driver combination, such as the Motorola MOC3009 -
MOC3021 family. I have recently bought one of these from Radia Shack, but I
do not have the part number. It is in a little white 6-pin DIP package.
Two pins are the photo-sensitive Triac. Connect them to the flash PC socket. 
The other two pins are an LED. Pass enough forward current through the LED
and the light from the LED will turn on the Triac and fire the flash.
The LED and the Triac are seperated by clear plastic, thus providing several
Kilovolts worth of insulation between the two sides for safety.
The MOC3009 requires 30ma though the LED, the MOC3010 15ma, the MOC3011 10ma.
You can drive the LED from a TTL device or anything else, but be careful to
limit the current or you could blow the LED.

I have also messed around building my own flashes. But let me warn anyone
delving into the inside of an electronic flash - they are deadly dangerous!
The capacitor holds a lethal charge at 200 - 500 volts, even hours after
being switched off.

	Dave Horn