I have just built the flash gun trigger from the hiviz website, when I block the light between the two IR leds, I get a nice pulse at pin 6 of the ne555, resulting in a nice trigger pulse out of pin 9, monitoring pin 9 the pulse is strong on the output pin, it is strong on the SCR side of the 22k resistor until I solder the SCR in circuit then the pulse drops to about 1 volt, is this correct? is this enough to fire the SCR because i'm getting nothing happening.

Where do I attach the flashgun ? does it fit between the output of the SCR and ground? is that my problem?

Neil.

 

ok so I took out the 22K resistor in the gate lead of the thyristor and now everything works fine with the trigger pulse going unhindered from the 555 to the gate terminal of the thyristor, can I run the circuit safely in this configuration?

Neil

 

can I run the circuit safely in this configuration?

That depends on the SCR specification. You might get away with it. What is the absolute/recommended maximum gate current?

 

800 mA....is max trigger current?

Neil.

 

the pulse is strong on the output pin, it is strong on the SCR side of the 22k resistor until I solder the SCR in circuit then the pulse drops to about 1 volt

Are you certain the device is an NE556 as opposed to a TS/TLC 556?
Note that the latter are CMOS devices with limited drive capability...

800 mA....is max trigger current?

Neil.

Well then, clearly, a resistance as low as Ca. 12 Ohms will limit the current to a safe value for the thyristor -- Though it would be advisable to start with, for instance, 1K then 'work your way' down or up until a satisfactory result (i.e. the greatest resistor value consistent with reliable triggering) is obtained --- You are further advised to perform this procedure at a supply EMF equal to that of the lowest anticipated battery level...

Good luck
HP

 

For modern flashes the SCR does not switch the flash current directly, but the circuit has historical compatibility. 800 mA is the SCR forward current carrying capability; the gate current is much lower. The idea here is that the flash has a control input that expects to see a switch closure to ground (or close to ground), and the SCR provides this when triggered. It is also a latch, again from back when two-transistor latches were difficult and expensive. Since the 556 circuit has a nice fat output pulse, the latching aspect of an SCR is not needed, but it is what flashes expect to see.

The SCR anodes (1 or 2) go to the center conductor of a sync cable or the center contact of a hot shoe. The other cable/shoe contact goes to ground. Note that the circuit really should be battery powered to prevent potential ground (or ground potential) conflicts.

ak

 

800 mA is the SCR forward current carrying capability

Ah! Ha! I thought 800mA was a rather 'buff' Igt (or, even, Igm) for a thyristor in the described application!

TTFN
HP

 

TO-92 package, 800 mA, about 50-200 uA trigger current, sensitive enough to act as a touch switch with nothing more than a 1 M pulldown, 50 to 400 volts or more across the product line. Fun low-power part a while back.

ak