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 trigge=
r 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 transistor.

See http://www.wftelectronics.com/projects/DN20101003002/

For Z1 I used a 1N4148 which was forward biased.  I assumed this would prov=
ide about .7 volts.
I thought the capacitor would charge up to VLED + VZ1 + VT and then the SCR=
 would fire and sink current through the LED.
[ VLED is voltage drop of LED ; VZ1 is the voltage drop of the zener ; VT i=
s 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 turn=
 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 dim=
ly 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 f=
lash.
I checked the transistors with a DMM and they do not appear to be damaged.

Gus
--=20
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