Help with a timer circuit...

[BKSwindell]

Hello, I work at radio shack so I have some circuit design backgrond but believe me that it is not too much. I've designed a circuit that is basicly a delay timer, the way it was going to work is that the limit switch has got to be held for 3sec or so, to set of the relay. The 100k pot. is the delay setting. Except that the circuit worked a little differently. If you hold the limit switch down then the SCR never tiggers to let the the current pass through. I need to modify the circuit to only tigger the SCR after the switch has been held down for a few seconds. And as far as the SCR, I know I sould use a gate but ive never done any work with the before. If some could give me a shove in the right direction I would really appricate it. Thanks for the help. The circuit is below. Thanks again.

Brad

 

[betkro]

BKSwindell, There are some errors in your schematic. The '555 is not intended to work with a floating ground (pin 1) as seen in the schematic. This is tricky and its behavior can be unexpected. The pin 2 is permanently grounded making R6k8 useless. Why the use of a SCR? . Once triggered, there is no way to latch off the relay. I don't see the purpose for the Diode in series with R 3K3. A normal use of a diode is in parallel with the relay’s coil.

I would suggest to remake the schematic by using the 555’s standard polarization and to trigger through pin 2 or 5.

 

[Paulcet]

Brad,

Look around the store there for a little book by Forrest Mims on 555 timer circuits. Radio Shack used to carry them.

 

[nbucska]

Brad:
How did you draw the schematics ?
If you use a pot as var. resistor, always connect the free end to the wiper otherwise you may have open circuit time
to time during adjustment causing much more noise and
perhaps circuit failure.This is what you described:

____----______----------__________ _= sw. open -= pushed
_______________________------------- -= relay closed _=open

..............<-- 3s ->

What will open the relay?

<nbucska@pcperipherals DOT com> subj: eng-tips