# Single Pulse Generator

Discussion in 'Electronic Basics' started by Brian, Jan 7, 2004.

1. ### BrianGuest

The problem might be due to contact bounce on your switch. You need a clean
transition from ground to + 12 volts on the input.

2. ### PhideauxGuest

I'm stumped. I know I'm not the sharpest tool in the shed, but while
I'm on vacation, I wanted to develop what I thought was a simple
circuit. I need a single pulse generator that will output one single
pulse each time a switch is thrown. Here is a link to the sketch of
what I thought should work.
http://im1.shutterfly.com/procserv/47b4df26b3127cce85a49627a37f0000001610
Can anyone enlighten me as to why this isn't working? Values for the
circuit are R1=1K R2=1M C=0.04uF I don't have an o-scope, so I've
sent the output to an r/s latch to see if it is indeed triggering an
output. The latch works just fine, the rest doesn't seem to be
psycological at this point.

3. ### Ian BellGuest

My first thought too. This is a common enough circuit and it does work.

Ian

4. ### Robert C MonsenGuest

^^^^^^^^^^^^

Take a deep, cleansing breath... Good.

Now, the problem is that you have the timing circuit a bit wrong. With the
circuit you are using, you could increase the value of R1 to 100k, and it
would work. However, its a bit strange to my eye. Try this one:

http://home.comcast.net/~rcmonsen/misc/pulse.gif

This one will more faithfully reproduce your desired waveforms. The length
of the pulse will depend on the RC constant of R1 and C1, and also on the
threshold of the 4001. The 1k resistor will waste some power, but is needed
to drain the circuit for the next firing. Note that you can use a larger
resistor, the value isn't very important.

Basically, the bottom input is delayed from coming up when the switch is
thrown by the need to charge C1, which is charging through R1. Because of
the NOR gate, and the inverter, the output is only high while the voltage
across C1 is less than the 'high' input level for the gate, which can vary a
bit.

Regards,
Bob Monsen

5. ### alsGuest

Try some of these:
http://people.txucom.net/fsmyth/pages/555apps.html
A bit more involved, but generally more useful.
<als>