Flow sensor has two no flow states

[xray]

I am inquiring about some ideas on how to handle a turbine pickup flow sensor, cheap model about $12. The sensor has three states: 1. NO FLOW - gnd, 2. NO FLOW - +V, 3. FLOW - variable frequency square wave. The signal is low pass filtered and drives the gate of an nmos fet indicating flow or not.

Problem: the flow state and the +V no flow state drive the gate of the nmos fet to on. Any ideas on filtering this signal short of an MCU?

Much appreciated

Cy Drollinger

 

[VEBill]

Wouldn't one be looking at the frequency of the output?

 

[xray]

Yes, but Im only interested in flow not how much. So frequency indicates flow and yields a dc value turning on a fet, but so does one of the off states +V, when the turbine stops at the pickup.

 

[MikeHalloran]

So you need a circuit (or a commercial module) called a zero rotation detector, or shaft stop detector, or words like that. My point is, it's been done.

It's been done with 555s, of course, but you may find that a small PIC is cheaper than the resistors you need for any analog solution.

Mike Halloran
Pembroke Pines, FL, USA

 

[VEBill]

The 555 circuit is called a 'Missing Pulse Detector' in the 555 cook books. As soon as the applicable transition is late (as it would be with either stuck-high or stuck-low), it triggers the output.

 

[TurboXS]

Do you have a link to the datasheet?

 

[LionelHutz]

Something like this?

http://www.mod-tronic.com/mod_pdf/api_pdfs/api1700g1720g.pdf

 

[MikeHalloran]

Yes, that will work. Other solutions require more sweat equity.


Mike Halloran
Pembroke Pines, FL, USA

 

[xray]

Well, Im working on two circuits driven by the flow sensor: peak detect with final value at about 9V driving 100k resistor to the drain of FET_1 and in parallel a voltage divider consuming very little current driving the gate of the same FET_1.
The peak detect at approximately 9V at the drain of FET_1 drives the gate of FET_2 (flow on signal)while the voltage divider at the gate of FET_1 is not enough to turn FET_1 on.
When the sensor stops at a 12V output also seen by the drain of FET_1 a the voltage divider Vgs large enough to turn FET_1 on dumping the current of 12V through a 100k resistor.

Cy Drollinger

 

[MikeHalloran]

Why not write a spec for what you are actually trying to do, and hire an experienced EE to execute it?



Mike Halloran
Pembroke Pines, FL, USA

 

[xray]

Ouch!

 

[VEBill]

Sometimes one needs to tear-up what was badly implemented, and simply start again fresh. Interfacing from your choice of 'missing pulse detection' options (several options listed above) to the gates of FETs is relatively straightforward.

 

[OperaHouse]

It could be done with just an op amp. Switch is tied to a pull up resistor, pulsing signal feeds through a capacitor and rectified by two diodes. A resistor and capacitor provide smoothing and time constant. op-amp connected as a comparator. Simple enough for a neophyte to diagnose an issue.

Maybe it's just me but I cringe at the thought of using a 555.

 

[MikeHalloran]

Maybe it could be done with just an op amp, but we still don't know what 'it' actually is.


Mike Halloran
Pembroke Pines, FL, USA

 

[xray]

All is well, all you all are great, you all have fine suggestions. Have a great day!
CY