On 03/10/2010 23:24, 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.  I assumed this would pr=
ovide 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
Try a 3.3V zener from  other side of LED at R/C  to "gate" of SCR, with=20
100k resistor or some such from gate to ground for leakage  ?

I don't think the circuit as drawn will work, not with a 1N4148 from LED=20
to gate.

Can you describe exactly what you want the probe to do?
What logic voltage level, frequency and duty cycle of logic pulse and=20
what the LED is supposed to do.



--=20
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