24 June 2015
of 35 turns each.
Using that coil, the oscillator had a frequency of 516
kHz when no key was inserted. Inserting a key containing
two ferrite cores decreased this to 387 kHz. A key with
three ferrite cores caused oscillation at 286 kHz.
The easiest way to make the reference coil is to wind a
coil identical to the key coil, and then attach an assembly
that allows a controlled insertion of a tube filled with
ferrite. In my case, I used a nylon bolt and nut for this
purpose since conductive pieces (such as metal) are not
The schematic diagram in Figure 5 shows the mixer,
low pass filter, and rectifier. The mixer uses a CD4013 “D”
type flip-flop as a digital mixer. A precaution observed in
using CMOS logic is to ground unused inputs since these
might float “high” and draw excessive device current. Such
digital mixers do produce a difference signal, but they
also respond to harmonics of the input frequencies. This
“feature” doesn’t matter in our circuit.
The low pass filter is a two-pole Butterworth with
a corner frequency of about 2 kHz. This circuit is
characterized by one capacitor (C9) being twice as large
as the other (C10). The easiest way to implement this is
to just buy three capacitors of the lower value, and make
the higher value using two parallel capacitors. This makes
a lot of sense in this circuit since there are many 0.01 µF
The choice of operational
amplifier is not critical. You can use
any “rail-to-rail” type operable on
a five volt supply. Rectification is
accomplished with diode D1, the
rectified signal is filtered by C12,
and then amplified. There’s a logic
level output you can use to drive
other circuitry; for example, an
optically-isolated solid-state relay, or
just a power transistor for driving a
The circuit is calibrated by
inserting the key and then adjusting
the reference inductor to give a
slightly lower frequency within the passband of the low
pass filter. This setting is aided by the LED, which lights
when the frequency condition is matched. Note that
setting the frequencies exactly equal is not desirable since
the low difference frequency will not filter well.
As shown in Figure 6, the mixer responds to frequency
differences on both sides of an exact frequency match.
That means you can also set the reference oscillator for a
higher frequency than the key oscillator. This will work, too,
but there will be a blip in the output as the key is inserted
or extracted as it passes through the zero point.
The circuit shouldn’t generate any radio frequency
interference; but, as a precaution, it should be built inside
a grounded metal enclosure. You can ensure that radio
frequency interference is not being emitted by placing
n FIGURE 5. Schematic diagram of the mixer, low pass filter, and rectifier of the electronic key.
n FIGURE 6. Performance of the mixer circuit. The LED will
light when the key oscillator frequency is within a range of the
reference oscillator frequency.
n FIGURE 8. Photos of the circuit boards of the two versions of the electronic
key circuit. The microcontroller version (right) uses fewer components, but it does
require a programmed microcontroller chip.