I have a flow meter that puts out a 20Hz 50% 5V pulse when operating. I do not require flow rate monitoring, only weather it is flowing or not so I threw on a low pass filter and a relay.
The problem is as this is 20Hz only it needs massive smoothing (470k/1uf) which is to much of a load to run the relay anymore. Is there a simple solution to get this relay(5V, 30ma) to run reliably? (If it cycles in any way it can damage equipment)
Forget smoothing. Use a ‘pulse stretcher’. Google 555 one-shot.
Basically, get a pulse, 555 starts for, say 60ms. If you get another pulse, the 555 gets restarted, output stays high. No pulse within 60ms, 555 stops, output goes low.
Run the 555’s output thru an optoisolator, transistor, MOSFET, whatever. Bam…done…
Thanks. Examps in Mims notebooks for this even.
Any simple descrete circuits? I do not have any 555 on hand.
Google “transistor one shot”
You’ll want that one-shot to be retriggerable, otherwise your relay will be clicking on and off all the time.
Otherwise you could continue with your filtering idea, but use the output of the filter to drive a transistor that drives the relay. The problem is the same as filtering rectified AC voltage to a certain percentage of ripple.