22 June 2015
Radio frequency identification (RFID) devices are ubiquitous in
electronic controlled-access systems. Although they’re considered to be
secure devices, they’re a little too expensive for hobby applications. So,
here’s a design for an electronic lock using an inductance coil.
By Devlin Gualtieri, Tikalon LLC
BUILD AN ELECTRONIC
BUILD IT YOURSELF
n FIGURE 1. The four
fundamental electrical circuit
elements: the resistor, capacitor,
inductor, and memristor. The
resistance (R), capacitance (C),
inductance (L), and memristance
(M) are defined by derivatives
of the voltage (v), current (I),
charge (q), and magnetic flux (f).
n FIGURE 2. An inductance coil, mating key, and some
ferrite core pieces used to make the key. The coil was
wound with AWG 38 wire-wrap copper wire, but enameled
copper magnet wire could be used.
22 June 2015
People have been securing their homes and
possessions with lock and key devices for millennia.
Our electronic age has seen the appearance of non-mechanical locks, such as the wireless entry systems
for automobiles. Many of us have employer-issued
access cards that serve as door keys, and they also
serve as a way to monitor who opens a certain door,
and when that happened.
The early access cards used a magnetic stripe.
Later, more reliable cards used magnetized wires
embedded in the plastic. Presently, RFID devices
are used in most access cards. RFID devices are
small computer chips that harvest operating power
from an electromagnetic field generated by the card
reader. They then use this power to transmit a coded
signal, sometimes in response to a “challenge”
signal. These RFID chips possess a lot of computing
power, and high level cryptographic protocols can
be employed to make for very secure access. The
access cards themselves can be quite inexpensive,
but an RFID reader is somewhat costly. In a typical
access system, this cost is shared among many
access cards, so the cost is warranted for the security
If you’re interested in implementing an electronic
key for a hobby project, RFID is definitely too costly.
Wireless access systems are a little less expensive,
but not by much. This article presents a simple and
inexpensive electronic key system. It has the possible
advantage that the key can be made to actually look
like a key.
If we go back to basics, nothing in electronics is more
simple than the three principal electrical circuit elements.
These are the resistor, the capacitor, and the inductor.