The SCR will not turn off until the current through it is 0 (for some values of "is" and "0"). While 10k drops the current down to 0.5mA, it's probably enough to keep the SCR turned on, and it's enough to keep the LED very dimly lit even when the trigger is not triggering. So the SCR need only trigger once, and then it will stay on. You will likely need a resistor going to ground from the trigger to prevent false triggering as well. I haven't analyzed the overall function of the circuit, so these may not be the only problems, but it appears to be the solution to the one problem you're trying to solve right now. -Adam On Sun, Oct 3, 2010 at 1:44 PM, YES NOPE9 wrote: > I was toying with the idea of making a self-powered logic probe. > I had the idea of collecting energy through a 10K ohm resistor into a 10 = uf capacitor. > When the voltage at the capacitor reached a selected level, it would trig= ger an SCR which would dump the capacitor energy through an LED. > I built the SCR out of a 2N3904 NPN transistor and a 2N3906 PNP transisto= r. > > See http://www.wftelectronics.com/projects/DN20101003002/ > > For Z1 I used a 1N4148 which was forward biased. =A0I assumed this would = provide about .7 volts. > I thought the capacitor would charge up to VLED + VZ1 + VT and then the S= CR would fire and sink current through the LED. > [ VLED is voltage drop of LED ; VZ1 is the voltage drop of the zener ; VT= is the trigger voltage of the SCR which I assumed would be .7 volts ] > Once the capacitor voltage decayed low enough, I thought the SCR would tu= rn off and the sequence would repeat. > I expected a series of light flashes from the LED. > > Instead, it appears the SCR stays turned on all the time and the LED is d= imly glowing with the current supplied through the 10K resistor. > > If I disconnect the SCR and short the LED to ground, I get a brief bright= flash. > I checked the transistors with a DMM and they do not appear to be damaged= .. > > Gus > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .