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Triggering a flash circuit with a voltage pulse across a transistor Answered

Hi All,
       question from a newbie.

I'm working on a project for my (steampunk) daughter. For part of it I've taken the flash circuit from a disposable camera (PDF 1:Flash+MechanicalTrigger). This works just fine when I close the trigger mechanically. What I want to do is trigger it with the same pulse (from a 555 timer) that I'll be using to do other things at the same time. To this end I added a transistor as shown in PDF 2:Flash with transistorTrigger. My intention is to feed the 555 pulse to base, saturating it and triggering the flash.

Result, abject failure. Help!

The proposed triggering transistor is an MJE340(NPN). When set up as shown the indicator LED glows but voltage across Emitter-Base produces no result. When testing I noticed that if I mechanically make the collector-transformer connection it triggers a flash, even with no voltage across emitter-base.

I'm stumped.

Any ideas very gratefully accepted.





7 years ago

This is a flash circuit I have been planing to build someday.


Air Flash Unit.png

Reply 7 years ago

You'll note that A. has ALSO used a Thryistor to fire the trigger transformer.



Reply 7 years ago

Guys, well I'm assuming LittleHairyApe is a guy, thank you so much. My work on this one has been delayed by real life, yes boring but my partner and children were beginning to wonder who I am.

I've now obtained an SCA & as soon as time permits will commence experimenting & let you know the results. Later a full instructable on building the 'Reality Botherer', your weapon of choice when personal defence is more important than a minion who yells; "Dont cross the streams".

Fred, bespoke armourer to steampunk royalty

panic mode

7 years ago

LittleHairyApe is correct. Voltage across 80uF an 22nF is negative. for NPN to operate correctly, collector need to be at higher potential. technically you could still use it and swap emitter and collector but it will be awkward to trigger it.
next issue is if the transistor is suited for this job or not. It is possible to find transistor that will do the job but normally this is application for SCR (thyristor). the key here is allowing sudden discharge which means high current in a short pulse.

check this out (here using triac):


7 years ago

Reliable Flash triggering relies on extreme speed and high currents. The high current flows through a trigger coil which finally fires the tube. You need to use an SCR (aka Thyristor), not a transistor



7 years ago

Based on the polarity of the 80µF cap, I'd think an NPN driving a PNP would work a lot better for you. Have you tried that yet?