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From: email@example.com (Tom Bruhns)
Subject: Re: Oscillator with small resistance change
Date: 16 Dec 2002 15:04:03 -0800
References: <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: 16 Dec 2002 23:04:03 GMT
Winfield Hill wrote in message news:...
> Tom Bruhns wrote...
> > I developed a circuit which will do this just fine...
> 'Course we're interested Tom, what do you think this forum is,
> a place where we boast about clever circuits while hiding them?
> Come-on, spill the beans! :>)
Thanks, Win. ;-) Actually, I'm really busy right now getting ready
to head off to see my daughter and some relatives I've never met, but
this is a good time for a plug for the site where I expect it will get
posted: http://www.techlib.com/ Charles Wenzel has some wonderful
little things on that page, and he agreed that the continuity beeper
might be a good companion for the "Curious C Beeper" he has there (at
http://www.techlib.com/electronics/c_beeper.html). I highly recommend
poking around that site, if you're into interesting things that can be
done economically with circuits.
I've been procrastinating (great lack of round tuits), but finally
just this morning I sent off a package including a copy of the tracer
and its documentation to him. I might be able to put it up on a
different site for a while if Charles won't be able to get to it, but
not till my return. (I'm not as facile with ASCII schematics as you
are, Win! ;-)
A preview, though: the current incarnation is a one-transistor
blocking oscillator using a 500CT:8CT ohm audio transformer. The
secondary couples to a tiny speaker. The current which charges the
blocking osc. capacitor comes from a current mirror, but the emitter
of one of those current mirror transistors goes not directly to ground
but through the path/resistance being tested. To get reasonable drop
across the tested path, there's a 1k resistor to the +1.5V supply...or
actually, to the base of the power switch transistor, which turns off
the current into the "input" side of the current mirror when the test
port isn't connected to a load. Since the current mirror is
essentially a differential pair, the output current swings a big
percentage with very few tens of mV change, so the nom. 1mA test port
bias current lets you hear the pitch change with an ohm or so change.
It's simple to adjust that test port current (since it comes from its
own resistor) to change the sensitivity.
The earlier incarnation used a "two-output" current mirror: three
transistors instead of the usual two. The two outputs set the current
to the bases of an astable flipflop using another couple transistors.
Easy to get an almost-square wave from that, but the blocking osc is
actually better for covering a wide range of freqs easy to hear from a
tiny speaker when the freq gets very low.
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