Project
by Tom Napier
What the L is it?
This Month’s
Projects
What the L is it? . . . 34
The MIDI-Nator . . . 42
USB Interface . . . . . 50
Utility Meter . . . . . . 57
The Fuzzball
Rating System
To find out the level
of difficulty for
each of these
projects, turn to
Fuzzball for
the answers.
The scale is from
1-4, with four
Fuzzballs being
the more difficult
or advanced
projects. Just look
for the Fuzzballs in
the opening header.
You’ll also find
information included
in each article on
any special tools
or skills you’ll
need to complete
the project.
Let the
soldering begin!
Check Inductors Quickly With a PIC-Based
Meter That Also Measures Frequency
After doing the same thing many
times, I’ll sometimes stop and ask
myself why I’m doing it that way.
A while back, I designed some DC-DC converters so I wound and measured a lot of
inductors. Each time, I soldered a 5,000 pF
1% mica capacitor across the inductor. Then I
hooked up a signal generator and checked
the combination’s resonant frequency on an
oscilloscope. Five seconds on my HP-11C
calculator gave me the inductance.
Was there a simpler solution? My existing
procedure couldn’t be easily automated, but
there was an alternative. I could plug an
unknown inductor into an LC oscillator and
measure the resulting frequency. I’d have to
design an oscillator that would work with a
wide range of inductors and devise something
to measure the frequency and display the
corresponding inductance.
The latter was no problem. Small PIC
microcontrollers can be programmed to
measure and display frequencies to tens of
MHz. Knowing the tuning capacitance, a PIC
could easily calculate and display the inductance. Put a wide-range oscillator in front and
I’d have a handy device that would measure
both inductance and frequency.
Which Oscillator?
As this circuit must work with almost any
inductor, I chose a Colpitts oscillator. This resonates an untapped inductor with two capacitors in series and uses an emitter-follower
as its amplifying device. The slight voltage
step-up provided by the capacitors balances
the below-unity gain of the emitter-follower.
In Figure 1, C1 and C2 are the tuning
capacitors and Q1 is the emitter-follower. (All
NPN transistors are type 2N3904, PNP transistors are 2N3906s. Close equivalents will
suffice in both cases.) The rest of the circuit
provides a sine and a pulse output and
Figure 1. This oscillator, with its output buffer and gain control circuit, runs well with almost any inductor.
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34
AUGUST 2005