peg@psuecl.bitnet (09/30/89)
In article <4377@wpi.wpi.edu>, reynhout@wpi.wpi.edu (Andrew Reynhout) writes: > Help! This is an amusingly simple question, but I've been working on it > for some time now, and can't come up with an answer... > > I need a circuit that will, on application of power, cause a delay of 5-20 > seconds before allowing current to flow through (a different part of the circuit) > Basically, it should take, say, 12v, wait 10 seconds, then trip a relay. Ah, this really is a simple question! At least it is because I have such a circuit close at hand. I have a favorite circuit for microprocessor r resetting, suggested to me by a friend. It keeps the reset line low for about one second after the power is turned on. Hmmm... I was going to put it in here, but drawing these things is a drag. I'll explain it instead. It is easy. 1) Take a 555 timer IC. Radio Shack has them. 2) Connect Vcc (pin8) to power, and Gnd (1) to ground. 3) Connect Reset (4) to power, to disable it. 4) Leave Discharge (7) disconnected. 5) Connect a small cap, say 0.01uF from Bypass (5) to ground. I think this rejects noise to one of the comparators. 6) Connect a resistor from Threshold (6) to ground. 7) Connect a cap from Threshold (6) to power. 8) Connect Threshold (still 6) to Trigger (2). 9) Pin 3, output, is the output. Okay, sketch this thing and I'll explain it. When the power comes on, the timing cap (6 to power) is discharged. Thus, threshold is high, and the output (3) is low. The timing cap charges slowly through the timing resistor (6 to gnd). When it has charged, threshold is near ground, and the output goes high. The timing period is something like R * C, probably a little less, as threshold only has to drop to 1/3 (?) of the supply voltage for the output to go high. Anyway, I use 1uF and 1M, and get about 1 second delay. You should be able to use a very large cap and resistor and get quite a delay. Also, the CMOS version, the 7555, might give an even longer delay--not sure, I've never used it. If you want an inverted output, just add a TTL inverter or a transistor. I often add a TTL inverter to generate reset for those stupid Intel- type chips which use an active-high reset signal (yeeeeeech!). Well, hope this helps! The circuit always works very well for me, and the cost is negligible. Paul P.S. Credit for the idea goes to Bryan Jensen of PSUECL. Not sure where he got it, but THANKS AGAIN BRYAN!!