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I'd like to convert a hall-effect flow meter to operate as on-off switch. the output is in pulses, which arrive at a rate proportional to the flow rate. what I would like to do is have a circuit that would turn on and off like a conventional flow switch. i.e., a circuit that you can tell: "if X pulse rate, turn switch on."

Thanks! edit: what I'm thinking about is basically a delay on make timer with the interval set to some timing that would give me the toggle function. e.g., we are getting 5 pulses per second, with the timer set to something just over 200ms, such that the on state is maintained as long as that many pulses continue to register.

but if it goes to 4 pulses, the frequency will be too low, and rejected by a Schmitt trigger or comparator.

There, I think I just told everyone how little I know. :)

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  • \$\begingroup\$ Welcome to EE.SE and thanks for sharing your little project. This isn't a free design service so you need to show some effort and ask for help where you are stuck. You have given no details on voltage and frequency so it would be difficult to give a useful answer. \$\endgroup\$ – Transistor Aug 19 '17 at 19:26
  • \$\begingroup\$ thanks Transistor~ I'll see what I can do and get back later. \$\endgroup\$ – cfarley Aug 20 '17 at 20:08
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Your basic approach is correct. This is a common requirement in a fan failure alarm. The stream of pulses is integrated into a voltage level, and that is compared against a reference. The more the pulses are filtered, the longer it takes to detect either the on or off states, But, more filtering makes for less ripple in the signal voltage, and this makes it possible to reduce the amount of hysteresis around the trip point. Search for 'missing-pulse detector' and 'fan fail circuit' schematics.

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I would do this by hooking the signal to a microcontroller interrupt pin. Once I saw that interrupts were coming in at >200ms, I would take an action. You could avoid the need for a Schmitt trigger by using a Hall sensor with a digital output. I would also build some hysteresis into the program by requing N pulses in a row to be >(200+h)ms to declare a low rate and M (or maybe N==M) pulses in a row to be <(200-h)ms to qualify as the high rate (to keep it from oscillating). Note that the uC chip is going to be very small; program using ISP.

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