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<h1>The 'Kitsilano' Oscillator Circuit</h1>


<p><strong>Kitsilano</strong> is an oscillator circuit based on two NPN transistors
and a single capacitor.  It  is so named because it is the most interesting circuit
(really, the only significant circuit) that I
designed while I was living in the Kitsilano neighbourhood
of Vancouver, British Columbia, Canada.
Its design was a byproduct of a quest which I have since recognized as futile,
and abandoned: the design of a single-transistor, inductor-less oscillator.
The pursuit itself was somewhat interesting though, so I'll tell you about it
in the next section.  In the third section, I'll describe the Kitsilano
oscillator itself.</p>


<div id="figure1">
<p><img src="1Q0L_oscillator.png" alt="The 'Kitsilano' Oscillator Circuit"
title="Figure 1.  A single-transistor (and apparently inductor-less) oscillator."/></p>
<p class="caption">Figure 1.  A single-transistor (and apparently inductor-less) oscillator.
(From <a href="#1">Sessions 1975</a>).</p>

<p>The notion of a single-transistor oscillator, built without an inductor,
tantalized me for a while after I came across what looked like such a circuit
in a book of somewhat older circuits that I checked out from the public library<sup><a href="#1">[1]</a></sup>.
It is shown in Figure 1.  It clearly contains only one transistor and no coils, and
the caption claims that the tone it produces, though not loud,
is adequate for keying (that is, Morse code) practice.
Further, the circuit is organized in a way that coincides with
my understanding of how one might go about eliminating the inductor
from a Colpitts oscillator: by replacing it with a capacitor and
a resistor in parallel and in series with another resistor, an arrangement which
can be thought of as a very rough equivalent to an inductor.</p>

<p>However, every attempt I made at building it failed to produce
any results.
It was not until much later that I came up with a plausible theory for
why it didn't work.  The audio
output in this circuit was shown as a pair of headphones labelled
"Hi-Z".  This means "high-resistance", and indicates old-style
<em>piezoelectric</em> headphones rather than the more modern
magnetic-coil speakers.
What took me so long to realize is that piezoelectric elements are crystals, and
<em>crystals provide inductance</em> (which is why they can be
used in crystal oscillators!)  In this circuit, the headphones are
apparently a critical component which acts as an inductor in the
oscillator.  I haven't been able to hunt down a genuine
crystal-element earpiece yet, so I haven't been able to test this theory,
but it's the best idea I've come up with yet for why it doesn't work
without one.</p>

<h2>The Design of the Kitsilano Oscillator</h2>

<div id="figure2">
<img src="kitsilano.png" alt="The 'Kitsilano' Oscillator Circuit"
title="Figure 2.  The 'Kitsilano' Oscillator Circuit."/>
<p class="caption">Figure 2.  The 'Kitsilano' Oscillator Circuit.</p>

<p>Having given up on a single-transistor, inductorless oscillator,
and discovering in other library books several single-transistor, one-inductor
designs (such as the Colpitts oscillator), I concentrated my efforts on
designing a two-transistor, no-inductor oscillator.</p>

<p>I had encountered several two-transistor designs previously.
One is the "multistable multivibrator", which uses two transistors
of the same type, and two capacitors.  Each transistor-capacitor pair
acts as a timer which triggers the other pair when it has discharged.
Another design involves only a single capacitor, but two transistors of
complimentary type (NPN and PNP.)  Many circuits based on both
of these oscillator designs can be found in Mims<sup><a href="#1">[2]</a></sup>.</p>

<p>Well, what I wanted was an oscillator built from two transistors
of the <em>same</em> type, but incorporating only <em>one</em>
capacitor.  This effort resulted in Kitsilano.</p>

<p>The theory of Kitsilano's design was adapted from a fairly standard
oscillator design that utilizes two CMOS inverters.
This is usually implemented with half of a 4001 chip (tying the inputs of
each NAND together to form an inverter.).  One of the inverters is fed
its own output through an RC circuit, and the
other inverter is used to stabilize the feedback and "square off" the output.
Circuits incorporating this oscillator design can also be found in
Mims<sup><a href="#1">[2]</a></sup>.
<!--An article containing some sage
advice about their design can be found in ETI<sup><a href="#1">[3]</a></sup>.

<p>(In fact, it's not required that such an oscillator be constructed from
CMOS gates.  <a href="2NOR_oscillator.png">This figure</a> shows a
circuit along the same lines that I built from LSTTL NOR gates, driving
a series-resistor-less LED via a transistor.  Measuring
the current usage shows why CMOS is preferable: LSTTL uses a lot.)</p>

<p>Kitsilano uses the fact that <strong>an inverter can be built with a single
transistor</strong> to replace the two CMOS inverters with two transistors of the
same type.  The remainder of the circuit is officially a hack, since it was
designed "by dint of sheer building."  The circuit is depicted in Figure 2.</p>

<p>For the construction itself, I chose two 2N4124 transistors — they're NPN
and they're about as cheap as they come.  The requisite task of an oscillator,
as far as I'm concerned, is to blink an LED, so I chose C1 large enough to
make this action visible to the unaided eye.</p>

<p>R1 was not originally part of the circuit: there was no connection between
Q1's base and +5V.  This oscillator would oscillate sometimes, while at
other times would fail to oscillate.  I eventually discovered that it was very
sensitive to where my hands were placed above or around the circuit, so I
added to the high-resistance path to +5V to make sure there was always
some voltage at Q1's base, making its behaviour more stable.</p>

<p>D1 was originally a resistor (I forget the ohmage.)
The circuit worked fine with a resistor there, but I wanted something
stranger, so I experimented with replacing it with a diode.  This worked
too, although I cannot quite tell you why (does the voltage drop across
the diode serve the same function as the resistance?) so I kept it in.</p>


<li><a name="1">Kendall Webster Sessions, ed.
<i>Master handbook of 1001 practical electronic circuits.</i>
Blue Ridge Summit, Pa. : G/L Tab Books, 1975.</a></li>
<li><a name="2">Forrest Mims III.
<i>Getting Started in Electronics</i>.
Master Publishing, Inc., 2003.  ISBN 0945053282.</a></li>