On Thu, March 6, 2014 6:33 pm, James Cameron wrote:
> G'day,
>
> I've got this hall effect fluid flow sensor, ZD1200 from Jaycar, with
> a mostly square wave output, at around 9 pulses per second at a flow
> of 20 litres per minute.
>
> The sensor has a specified output rise time of maximum 10?S, typical
> 1.0?S, and a falling time of maximum 1.5?S, typical 0.3?S.
>
> Looking at the rise and fall using an instrument at 80k samples per
> second, and 20cm of cable, gives the picture linked below; the X axis
> is milliseconds.  The test flow was air only.
>
> http://dev.laptop.org/~quozl/zd1200-max-air-flow.png
>
> The sensor is powered from the micro, at 5V, at about 5mA.
>
> The micro is using a falling edge interrupt to increment a counter,
> and supervises an RF transmitter module, and it turns out the best
> position is about 6m away from the fluid sensor.
>
> I may need to condition the signal somewhat before it passes through
> the cable, otherwise I may see effects from cable capacitance.
>
> Any suggestions?

Do you need to go a lot faster than 9 pulses per second? I would think
that cable capacitance would only help you in this case. When the signal
is slow, there is definitely a thing as 'too much bandwidth.' At 9 pps,
the only factor I would really be concerned about in the cabling is DC
resistance.

I'd expect that this would be able to go directly into a Schmitt Trigger
pin on a (5V) PIC, with no real signal conditioning other than transient
protection- maybe even something like a debounce code to protect from
false readings.

Matt Bennett
Just outside of Austin, TX
30.51,-97.91

The views I express are my own, not that of my employer, a large
multinational corporation that you are familiar with.
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