> From:         Phil[SMTP:phil@SPIDERWEB.COM.AU]
> Sent:         Sunday, February 08, 2004 2:33 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE:]Pulse counting

> Hello everyone,

> I'm trying to accurately count reed switch closures on the end of a cable that
> is 20 metres in length. The pulses occur at a rate of about 4 per second and
> are feed to the input port of a PIC for processing.

> The best that I have been able to achieve is a variation of 2.5% in the count
> and that was through the use of a Schmitt Trigger (74LS13 with the inputs
> tied together).

> Should I expect an accuracy better than 2.5%? How do commercial satellite dish
> trackers, for example, achieve an accurate count?

> --
> Regards,
> Phil.

At that speed, there should be no error. It is unlikely that a pulse from a reed switch
would be too short to be counted, so the most likely problem would be spurious
extra pulses caused by contact bounce or noise. A Schmitt trigger is a good
beginning, but you may need a more sophisticated signal conditioning circuit
preceding it. A low pass filter made from an op amp, resistors, and capacitors
could be useful for reducing contact bounce. Also, you could provide a similar
function in software. After detecting the appropriate edge at the port, your firmware
could wait a few milliseconds before reading the port again. If the line is still at
the appropriate level for a contact closure, only then count the edge as a switch
activation. Similarly, when the level changes after that, check that the line remains at
the "off" level before considering the switch open.

John Power

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