Flow Hall Effect Pulse Train
I have a circuit that will energize a relay if hall effect is true
or hall effect pulses are fast. As long as pulse rate is high relay stays energized
However if flow stops and hall effect lands high relay will stay true
Any ideas on what modification I would need to make, so if pulsing stops and hall effect is true
relay will drop out.

 

This might work depending on the Flow rate.
Probably will need to experiment to get the best value for C2.

 

if hall effect is true or hall effect pulses are fast. As long as pulse rate is high relay stays energized
However if flow stops and hall effect lands high relay will stay true
Any ideas on what modification I would need to make, so if pulsing stops and hall effect is true
relay will drop out.

You could capacitively couple the output from the Hall Effect sensor, so it only responds to pulses.
How fast is "fast", and how slow before you want the relay to drop out?

LTspice sim of example circuit below:

 

This might work depending on the Flow rate.

That circuit will turn the relay off when pulses are received.

Edit: Oops. That circuit will indeed pulse the transistor.

 

That circuit will turn the relay off when pulses are received.

when pulses are below 3PPS relay off and number of pulses above 3PPS relay on constand pluse meaning hall effect in high for more that 1 sec relay off

 

when pulses are below 3PPS relay off and number of pulses above 3PPS relay on constand pluse meaning hall effect in high for more that 1 sec relay off

right now any pulse turns on relay and if hall goes true relay stays true

 

when pulses are below 3PPS relay off and number of pulses above 3PPS relay on constand pluse meaning hall effect in high for more that 1 sec relay off

Okay.
Below is the sim of the first and second circuit stage modified to do that:

The output to Q2's base (red trace) goes low (relay off) about 1 second after the pulses stop low or high.

 

Im sorry I am not really understanding what you are saying. Are you saying take the output of my circuit and apply it to the 4n25 ? If so where does the relay go Q2b ?

 

Im sorry I am not really understanding what you are saying. Are you saying take the output of my circuit and apply it to the 4n25 ? If so where does the relay go Q2b ?

No, sorry I didn't make it more clear.
U1 and Q1 are just to simulate your part of the circuit.
You add C1, C2, D1, D1, and R2 between your opto output and your Q2's input.

 

You add C1, C2, D1, D1, and R2 between your opto output and your Q2's input.

I breadboarded that circuit and the relay is always ON regardless if there is a High or Low pulse.
This circuit works but relay stays on until flow rate is zero.
Cap values can be changed to activate the relay above 3pps but then the relay will pulse below 3pps until flow rate is zero.

 

I breadboarded that circuit and the relay is always ON regardless if there is a High or Low pulse.
This circuit works but relay stays on until flow rate is zero.
Cap values can be changed to activate the relay above 3pps but then the relay will pulse below 3pps until flow rate is zero.
View attachment 339898

On my circuit which is on a PCB the relay will toggle as the hall sensor is triggered it follows the led. Not sure why your breadboard would energize the relay all the time, NM i read that wrong your new circuit stays on

 

Correct, but only if the flow rate is 1pps or higher.

 

Correct, but only if the flow rate is 1pps or higher.

I will set this up on a PCB over the next few days will let you know how it goes. Thanks

 

I will set this up on a PCB over the next few days will let you know how it goes. Thanks

Added 100 resistor R5.

 

Lets go old school
Use a 555 IC in monostable mode (I think)
Set the time period to be greater than the minimum interval between pulses

 

Lets go old school
Use a 555 IC in monostable mode (I think)
Set the time period to be greater than the minimum interval between pulses

Occam's Razor!

 

Lets go old school
Use a 555 IC in monostable mode (I think)
Set the time period to be greater than the minimum interval between pulses

Commonly called a "Missing Pulse Detector".

 

Basically, this is the same as a fan tach alarm circuit, one that takes tach pulses from a fan and produces an alarm signal when the fan speed drops below a setpoint. The classic circuit uses the two comparators in an LM393. This was published in the apps section of fan catalogs in the 70's and 80's.

As partly shown above, the secret to having the circuit produce an alarm if the sensor freezes either high or low is to have pull-up resistors on both ends of the input coupling capacitor. That is missing in schematic #1, but shown in #3 and #7. #2, #10, and #14 rely on a transistor's base current to discharge the coupling cap, making that current part of the detection window math. Better to separate the two functions.

Here is an old schematic that covers the idea. This is a slice from a larger schematic with four circuits. Pins 4 and 11 go off to a Vcc/2 reference voltage.

Buckeye game coming up. More later.

ak

 

Better to separate the two functions.

That's debatable.
Will be watching the game also. Go Longhorns?

 

It wasn't pretty, but the good guys won. I'm going to Atlanta.

ak