extend 50ms pulse with cap

[jdc4357]

pulse extend circuit

Hi,
I have a electric submeter that puts out a 12v 50ms pulse for each .01 kwh. Unfortunately my daq/datalogger needs at least a 62.5ms pulse to increment the counter. I was thinking of using a capacitor to extend the pulse just long enough for the daq to see the pulse. What value of capacitor should I use? This pulse may happen once every 5 seconds probably if I have the dryer on etc...
Any help is appreciated.
thanks,
jamieC

 

[kender]

pulse using cap

Your prblem is typically solved with a 555 timer in a monostable (one shot) circuit. Google 555 monostable, and you'll find a dozed schematics.

 

[betwixt]

creating 50ms pulse with 555 timer

Using a capacitor isn't the best way to do it as you will lose the pulse shape and some of it's amplitude. A monostable is the real solution.

The value depends on several factors, primarily, how much current is available from the pulse and how much is being drawn from it by your counter.

You cant try this but I give no guarantees:

Feed your pulse through a 10K resistor and on the counter side, connect a 100nF capacitor to ground. Across the resistor, connect a small signal diode (1N4148 for example) with the cathode end toward the counter.

The idea is that the capacitor charges quickly through the resistor and diode so the rising edge of the pulse is still quick. The diode will not conduct backwards so the capacitor discharges slower with only the resistor as its leakage path.

As I said though, without specifications of the counter and meter it is impossible to give optimal values. 10K and 100nF would be a good starting point as a guess though.

Brian.