On 07 Sep 2013 20:21:35 GMT, Jim Mueller wrote:

On Fri, 06 Sep 2013 20:59:46 -0700, Guv Bob wrote:

"Jim Mueller" wrote in message
b.com...
On Fri, 06 Sep 2013 09:57:57 -0700, Guv Bob wrote:

snip


Thanks Jim. I have no experience with 555 timers. Where would I
find a basic circuit for this application using a 555?

Here's a data sheet: http://www.ti.com/lit/ds/symlink/lm555.pdf. It
has a number of generic circuits. What you want is a monostable
circuit. It can be set up to trigger from power-on or from a separate
signal. Can you modify the example circuit yourself or do you need
help?

--
Jim Mueller

To get my real email address, replace wrongname with dadoheadman.
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Thanks, Jim, that's a big help. I'm afraid I do need more help.
Someone also mentioned needing a diode. I'm pretty lost at the
component level.

If it helps, the application is to energize a small LED for approx 5
seconds when 12VDC is applied to the input. After 5 seconds the LED
should go off even though 12VDC is still on the input.

I don't have the LED current draw, but it's very small - probably on the
order of 10ma.

Many thanks for this help!

Bob

The circuit in the link from Jim Thompson, http://www.analog-
innovations.com/SED/5-Second_Timer.pdf, turns the LED on after 5 seconds
which is the opposite of what you want. To reverse this (one way of
several) is to disconnect R1 from +12V and connect it to pin 3 of the
555. Then, the cathode of the LED is disconnected from pin 3 and
connected to ground. The 555 is somewhat unique for an older part in
that it can source a substantial current as well as sink.

Huh?

The way I read Bob's OP...

http://img839.imageshack.us/img839/4650/uh3.gif

the relay closes after 5 seconds, activating the LED.

Did I miss a post reversing that?


Manufacturer's data sheets show pin 5 bypassed to ground with 0.01uF
although I have seen many circuits without it. As usual, be sure the
power supply is bypassed.

D1 on the schematic is to quickly discharge C1 when the power is turned
off so that the circuit resets promptly. This depends on having enough
additional load to accomplish this. If you don't have another load, a
resistor from +12V to ground (on the timer side of the switch) will do
it. 47K would be a good value but it isn't at all critical.

Have fun,

...Jim Thompson
--
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