> 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. Work out your RC time constant. At 10E3 Ohms * 10E-6 uF you will have a 63% charge in 100ms. If your supply is 5V, that's over 3V in 1/10 of a second. With a non zener diode firing your "SCR", your circuit is firing somewhere (probably at the start) of your LED's knee voltage (red LEDs are around 1.8V, IIRC). I would guess you are firing at > 10Hz. This probably looks like a faint glow, I'm not sure what frequency the human eye's persistence of vision kicks in. > > 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= .. > If you are leaving the 10uF cap in the circuit, the flash you are seeing is 10uF charged at your source voltage dumped directly into your LED. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .