okay, so I have a device I want to turn on for 10 to 20 seconds via the output going to a relay, off to be a few seconds.

I just downloaded pspice student version to see if I can mimic the circuit which I am not sure if the student version can do so.....

using a kit from Ramsey

http://www.ramseyelectronics.com/downloads/manuals/UT5.pdf

other res. I have on hand are 1M (variable), a couple of 100K and a bunch of 1K....for extra caps, 100uF is the max

Mueller,
If I remember right you find time in seconds by multiplying capacitance X resistance. Using mico farads and meg ohms the numbers work easy. 100 micofarads X 1 meg.the micro farads and megs numbers cancel out ( .000001 X 1000000) =1. So with a 100 micro F capacitor and 1 Meg resistor you would get 100 sec. If you want 10 sec use the 100K res. Ie one 10 the resistance of the 1 meg.
Hope this helps. I experminted with a 555 and 556 to get a simple alarm circuit to work and as long as the out put was way very low in current , I had success. As soon as I attempted to have the output set or unset a relay , the circuit would fail.
Tom

thats why it may have helped to put the output into a NAND chip to output to a 5V relay

Hey Mike,

The 555 datasheet has a reference circuit for an astable oscillator and it will give you formulas to solve for the R's and C's. The on and off times can be set independently using a network of two resistors and one cap.

If you haven't figured it out by tonight I can take a look when I get home. I just build a simple 555 circuit the other day so the datasheet is sitting on my desk.

Hey Mike,

Here's a datasheet for the LM555; whenever I need a datasheet, I just look it up at digikey.com. This sheet has some formulas for choosing R and C values for astable operation on pp 7-8. As Tom found out, the outputs on this chip are kinda flaky : you have to buffer them somehow, or the circuit won't work right. That kit from Ramsey looks like a good way to go : by the time you hunt around for a the chip, a board, etc, you'll be up close to $10 anyway. And I'm sure they have more user-friendly directions. In the photo, you can see a transistor that they use to buffer the output; they claim 100 mA, which should be plenty to power a relay. Since the circuit is really all analog, it's gonna change its behavior with changing temperature. I would guess that the ratio of off to on time will stay pretty constant, but that the frequency will drift some. If you're just blinking lights or something, though, it shouldn't matter.

Oops, can't attach the pdf file. try :

LM555

google and ye shall find

555 timer calculator...great for stupid/lazy people like me....


thanks for the info peoples.....

Some of the datasheets are flakey also. I forget which one, but a BIG name supplier didn't have a resistor in the discharge to ground that would limit the current to a reasonable value. You use a big cap and it would fry the chip since the current was unlimited; ie: a direct short to ground. The CMOS versions are very current limited. Don't forget the diode across the relay solenoid to kill the turn-off spike, that would cause flakey behavior if not included.