Sent: Saturday, March 11, 2000 5:23 PM
Subject: [noparse][[/noparse]STAMPS] Light/Dark battery protection?
> I've been working on this circuit for some time and it just isn't
> working the way I want it to. I have a battery operated circuit that I
> to switch off in total darkness. At anything less than that (even v. low
> light levels) I want the stamp to run. Right now I have the power line of
> the stamp switched by a NPN darlington with C to +5v, E to stamp and Base
> connected to +5v through a photocell. This lets power to the stamp at
> moderate light levels and turns it "off" in darkness but the threshold is
> wrong. The problem is that I need it to run even at low light levels and
> shutdown completely in darkness. The stamp is being switched on by an
> external reset and then runs it's program and turns itself off via another
> NPN darlington that switches the ground (-) line. Even at moderate light
> levels, the resistance of the photocell (~90K) is too great to pullup the
> base of the transistor...it only drops to a low enough value when flooded
> with light.
******* Duncan: *******
Darlingtons are great for current gain, but they are bad for voltage drop.
I suspect that part of the problem is too little voltage getting to the
stamp because of the darlington drop. Try connecting a PNP; E to +5, C to
stamp, and drive the base with your NPN darlington as follows: Ground the
darlington E, connect the darlington B to +5 thru the photocell, and connect
the darlington C to the PNP base thru 1k. A 2N2907 or 2905 would be a good
>"via another NPN darlington that switches the ground (-) line." <<---
This is another concern. If the ground line of the stamp is grounded thru
another darlington, you are losing ~1V here. If this is the case, and you
must switch the gnd, use a single transistor like a 2N2222 or 2219.