# Odd circuit question

Discussion in 'Electronic Basics' started by Richie, Jul 16, 2003.

1. ### RichieGuest

I need a switching circuit that requires me to hold down a momentary
switch for a few seconds before it turns on and turns off when i let
go of the switch. I've been playing with a npn transistor, a
capacitor and some resistors but haven't had any luck. Does anyone
know what this type of circuit is called so a can research it or give
me some pointers to build something like this. Any help would be
appreciated.
Thanks
Rich

2. ### Jonathan KirwanGuest

How much power can the circuit consume when it isn't in use?
What voltage supplies do you have available? How many
transistors is okay to use? What voltage for the output when
the switch is not pressed and what voltage when it is pressed?
Can you use normally closed momentaries or only the normally
open variety? Is it okay to use a ramped voltage at the output
during the "few seconds" delay? Or must it be held fairly solid
until the delay is over?

I doubt this is what you are looking for, from the above, but
you might play with this so that you can say what it doesn't do
that you want:

+V
|
+V |
| \ R1
| / 82k
R2 \ \
330k / |
\ ,--------+
| c +-------> OUT
| |/ Q1 |
+---| NPN | +
| |\ ----- C1
| e -----
o | | 10uF
\ | |
S1 \ gnd gnd
o
|
|
gnd

The output voltage will rise along an exponential ramp when the
switch is closed. Otherwise, the output will be grounded.

Anyway, best of luck.

Jon

3. ### DaveGuest

I think you are looking for a delay circuit. You can buy solid state time
delay circuits at a good supply house. Some have a fixed delay, others
are variable by adding a few components.

4. ### ~^Johnny^~Guest

Use this one:
One chip (555), one transistor, two resistors, two caps, a diode
and a N.O.mom pb.

http://gyrogearloose.com/images/delay-on.png

It's basically a modified missing pulse detector.
Just adjust R1 - C1 for the desired turn-on delay.

-keep it simple; use junkbox parts
--
-john

~~~~~~~~
"The first step in intelligent tinkering is to
save all the parts." - Aldo Leopold
~~~~~~~~

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