Dale --

> Input is via a series .02uF cap to the base of a 2N2222a, 330K resistor
> to Vcc, 56K resistor to ground, PIC input is from the collector with a
> 4.7K pullup, emitter to ground.  It does what it's supposed to
> (amplify). I get a nice pulse to ground at the primary frequency of the
> string...  but I'm also getting what looks like some harmonics (or maybe
> just noise) that are apparently getting amped enough to trigger the PIC
> input (Schmitt trigger, A0 on a 12CE674).

The input cap, along with the input resistance of the amplifier (bias
network in parallel with the transistor itself), is creating a high-pass
filter with a cutoff of a few hundred Hz, so you're already emphasizing
harmonics over the fundamental. First step would be to increase this
cap to to 0.1 uF or even 0.47 uF. It might also help to raise the input
impedance of the amplifier by putting about 100 ohms or so in series with
the emitter, although this will reduce the gain somewhat.

Another thing to try would be to add some higher-frequency rolloff to help
kill off harmonics and noise. This could be done by adding either a
capacitor from the collector to ground (about 0.15 uF for a cutoff
frequency of 225 Hz) or a capacitor from the collector to the base
(negative feedback - about 0.02 uF would do it here).

Hope this gets you going.

-- Dave Tweed

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