Hello fellow PICers! I have been working on a fustrating project now for
quite some time... I inhereted someone else's design for a punching bag
sports trainer. An electret microphone (Mode #72-160) is used to sense the
relative intensity of punches to the bag by the pressure difference between
inside and outside the bag.

Now I know that there's a whole lot of sources of error in such a sensing
method, but the biggest problem is that the microphone is being used for
something it was'nt designed for. Variation from one microphone to the next
is tremendous, resulting in a large number of rejected units (even though
there is a calibration adjustment provided). I believe its due to variations
in the microphone's IC as opposed to the mechanics itself.

To save on battery drain the original designer is only providing a few 10uAs
of load current to the microphone, which causes the internal IC to perform
erratically. Increasing load current improves, but does not cure, the problem.

The product, in 10K+ production right now, is very cost sensitive as it is a
consumer item. Therefore, the sensor has to cost something like $0.50USD in
large quantities.

I've been exploring other sensing technologies, such as piezo and dynamic.
I'd like a sensor that provides a signal proportional to pressure difference
instead of one that provides the differential (rate of pressure change).

Does anyone have any ideas on the matter? Is there an inexpensive pressure
transducer available (the lowest cost one i've seen is $5.00)? Can one get
the electret microphone without the intergral IC? What exactly is inside the
microphone's IC? Is there a datasheet somewhere for study?

Regards, Dana Frank Raymond
dfr@icom.ca