Help Designing Flow Sensor Circuit

[Synreal]

Hi guys. I have been trying to figure out a circuit for awhile now and I have decided to turn to making a post because I am having a hard time. I have a hall effect flow sensor. I would like to close a relay when the flow sensor is spinning. If I were to wire a relay directly to the flow sensor output, it would cycle at the frequency of the spinning paddle inside the flow sensor. I have been trying to figure out ways to buffer the pulse given off by the flow sensor so that the missing voltages from the falling pulse will still allow my relay to stay closed.

Because the flow sensor is mechanical the pulse freq. varies based on flow speed.

I have researched 555 timer delay circuits and also OP amp differentiators but I am having a hard time applying them properly.

One problem I have had is that the flow sensor can either stop spinning with the hall chip outputting 5v or 0v depending on where the magnet on the paddle lands. Therefore, the circuit would only need to energize the relay if there were a pulse being output.

 

[(*steve*)]

Have a look at a 555 circuit for a monostable which has a capacitor on the inpit so that it only responds to signal edges.

Connect the hall sensor to that and you should have what you require.

Be aware that there will always be a flow rate slow enough that the signal goes on and off, but you can make this arbitrarily slow as long as you're happy with the output being asserted for some time after the sensor actually stops.

edit: see here.

 

[Arouse1973]

This might work. Choose R2 and C1 for your time constant.
Adam

 

[Synreal]

Hey guys thanks for the info. I got a circuit modeled in falstad online circuit simulator and it seems to accomplish exactly what I need. I am using a differentiator circuit leading into that 2nd time delay circuit.

 

[Arouse1973]

Put up a copy of the circuit so we can have a look out of interest.

 

[Synreal]

Here it is. It takes a 5v pulse wave, closes the relay and stays closed for a second or so. It will only open when there is no change in the pulse wave input. If the hall effect sensor paddle lands on 5v out the differentiator will convert the signal to 0v.