Digital Garage

T
his project uses a single chip — an ATMEL AVR
ATtiny2313 microcontroller — to read a keypad and
trigger activation of up to two garage doors. The four digit
access code can be programmed using the keypad itself,
allowing one to easily change the code as desired. A
built-in safety feature will stop the door with the single
press of any key following door activation. Using only
a handful of components, this becomes a truly useful
weekend project. Photo 1 shows the project assembled
and ready for installation.
Garage Door Interfacing
Most garage doors can be activated via two methods:
a doorbell type pushbutton switch or wirelessly. This
project mimics the doorbell switch, so it keeps costs
down and eliminates the need to know your garage door's
wireless frequency and encoding. On the back of most
garage door units are two terminals which are wired to the
pushbutton switch. The microcontroller activates a small
relay when the correct access code is entered. The relay
contacts are tied to the same terminals as the pushbutton
switch. Enter the code and the microcontroller effectively
pushes the button for you.
Reading a Matrix Keypad
The circuit for this project (shown in Figure 1) is
centered around the microcontroller. The selected
keypad has 16 keys, connected in a 4 x 4 matrix
arrangement. Four wires connect to the four rows of keys,
while four more wires connect to the four columns of
keys. Pressing a key shorts the associated row and column
wires together. For example, pressing the "6" key ties the
row 2 and column 3 wires together.
The microcontroller can scan the keyboard and tell
if any key has been pressed. To do this, the four column
lines are pulled high to +5 volts by resistors R4-R7. The
microcontroller outputs a 1, or +5V on each of the
four row lines. Then, one row at a time, the
microcontroller outputs a 0 on the row line, pulling it low.
The microcontroller then reads in the state of each of
the four column lines. If one of the keys in that row is
pushed, its column will be low. If there are no keys in
that row pushed, all four column inputs will read high,
being pulled high by the pull-up resistors. As an
example, if the "9" keypad is pressed, the row 3 and
column 4 lines are tied together. When the
microcontroller pulls row 3 low, columns 1, 2, and 3
will read high, while column 4 is now connected to the
low level and is read as low.
The microcontroller stores the sequentially pressed
keys in a first-in, first-out (FIFO) buffer. The access code
is four digits long, followed by the "Enter" key. If more
that four keys are pressed, the oldest key press is
overwritten, with the program always having the last four
keys in the buffer. When "Enter" is pressed, the program
checks the last four key presses against the access
code. If a valid code was entered, the microcontroller
activates a relay and closes its contacts, mimicking a
doorbell pushbutton switch being pressed.
Holding the relay closed for about 3/4 of a second
works well on my door openers, but the value can be
●●●●
GARAGE ACCESS GOES
DIGITAL
BY JAY CARTER, MD
An electric garage door opener is one of
the greatest convenience devices ever
created! Rain or snow, the weather
doesn't matter. You hit a button from
inside your car and the door instantly
performs as commanded. By adding a
digitally controlled keypad, one can
make keyless entry easy for children, too.
36 February 2009
■ PHOTO 1. The garage access keypad project shown
completed and ready for installation.
One chip — the ATtiny2313 — runs the show.
It scans the keypad, validates an access code,
and drives two relays, activating the garage doors.
The weatherproof enclosure includes a rubber gasket.
easily changed within the software.
Debouncing Switches
When one pushes a keypad or closes any switch for
that matter, the switch contacts tend to bounce, making
and breaking contact several times before assuming their
new state. This 'contact bounce' is highly variable between
switches, and even varies from activation to activation of
the same switch. When viewed with an oscilloscope, one
might see the switch open and close a half dozen times or
more during its activation. The duration of the individual
bounces is variable, and could be up to a millisecond or
two, although shorter pulses are common. It may take up
to 10 ms or even longer for the switch to assume a stable
state. This bouncing occurs with both opening and closing
mechanical contact switches.
When one turns a ceiling light on or off, the bounce
of the switch is imperceptible. When one is entering an
access code, however, switch bounce is very important.
If the circuitry and software do not account for switch
bounce, the microcontroller might register one keypad
press as multiple presses of the same key, making it
impossible to correctly enter the code. Entering 1, 2, 3, 4
might be read by the microcontroller as if one had
entered: 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 4, 4, 4, 4, 4.
Although one can eliminate the bounce in hardware
by placing a capacitor across the switch contacts, this
requires additional components and circuit board space.
For high voltage circuits — where each bounce is
associated with a small spark and the generation of
electromagnetic interference — this approach may be
needed. For this project, however, debouncing is done
in software.
Many techniques exist for debouncing a switch in
software. Perhaps the simplest technique — employed in
this case — is a simple timer. Here, when a key is pressed
its value is stored and the microcontroller just waits 75
ms before doing anything else. The switch may bounce
numerous times, but these will be ignored by the
microcontroller. After 75 ms, the microcontroller watches
to see when the key is released. It again waits 75 ms to
avoid detecting any switch bounce on the keypad release,
and then resumes its main loop, watching for key
presses. This may seem like a long time interval to wait to
insure that one has skipped over any keypad switch
bounces, but even taking 150 ms to read each key press
would still allow the user to enter the access code at
over six digits per second. This is much faster than the
normal individual could possibly press the keys. A more
efficient algorithm to detect key presses and eliminate the
■ FIGURE 1. The garage access keypad schematic.
The keypad connects to port B on the microcontroller via
a ribbon cable. The microcontroller interfaces with a
matrix keypad, relays, LEDs, and a piezoelectric beeper.
A generic five volt power supply rounds out the circuit.
February 2009 37
switch bounce could be used if the microcontroller had
other tasks to perform. In this case, no other tasks exist,
and this simplistic approach works well.
Beep, Beep, Beep
Whenever a key is pressed, the microcontroller
generates a short beep via a piezoelectric element. This
provides feedback to the user, letting them know that the
keypad press was detected. Piezoelectrics are powered
with a square wave. This is easily generated by a
microcontroller by toggling a digital output pin high and
low, generating a string of pulses, ideally at their resonant
frequency for clear tone generation. The volume of the
tone generated diminishes as one moves off the resonant
frequency. A 2 kHz tone played for 200 ms generates a
nice feedback for the user. The frequency and duration
can be easily changed in software to best match one's
specific element and preferences.
The piezoelectric element specified in the parts list
draws about 90 mA when powered with a 5V square
wave. The ATtiny2313 can source up to 40 mA per pin for
driving external devices, (200 mA chip total). The series
resistor — R11, 47 ohms — limits the current drawn to 10
mA. One could substitute a 22 ohm resistor, doubling the
current, and increasing the volume, if desired.
Relay Driver
The relays selected for this project draw about 40 mA
at 5V. This is right at the maximum current allowed per pin
on the microcontroller. The microcontroller could power
the relay directly, but for long term, reliable operation it is
best to avoid operating the chip right at its maximum
limits. Designing in a safety margin also protects the
chip in case a particular relay actually draws more current
than expected.
A 2N2222 transistor is used to drive each relay.
A high on the microcontroller pin turns on the transistor
via its base resistor. The microcontroller pin
now sources less than 5 mA when driving the
transistor — well within the 40 mA limit. The
transistor is operated in saturation mode as a
switch, being either fully turned on or off. When
on, the collector current activates the relay. This
general-purpose NPN transistor can handle 600
mA of collector current. It is coasting along at
40 mA, and does not require a heat sink to keep
it cool.
Each of the two relays have a reverse-biased
diode across their coils (D1 and D2). This is a crucial item
when driving an inductive load such as a relay coil. When
the relay is turned off, the energy stored in the coil can
generate a brief, high voltage spike across its terminals.
This spike can easily damage the driver transistor. The
diode clamps this spike, protecting the driver circuitry.
Flashing LEDs
The ATtiny2313 has far more digital input/output pins
than are needed for this project. Two spare pins were
used to drive two LEDs. The current software turns LED
#1 or #2 on whenever Relay #1 or #2 are activated. They
can be mounted on the project's front panel for additional
user feedback, left on the circuit board, or they can be
completely eliminated from the circuit. Being under
software control, their actual usage is at the discretion of
the programmer. The LEDs have a series resistor to limit
their current to about 10 mA. This is within the limits of
both the LEDs and the microcontroller pin source current
maximum of 40 mA.
Microcontroller Clock
All microcontrollers require a 'clock' to run. This
clock is actually a square wave which drives the internal
CPU and causes the microcontroller to execute
sequential instructions in its program. This particular
microcontroller can run on either an internal oscillator and
external crystal, or an external square wave clock signal.
As there was no need for any precise timing for this
project, the internal oscillator was selected. The
microcontroller is set to use its internal 8 MHz clock
source when it is programmed. If one were to modify the
project to automatically upload the date, time, door, and
access code to a computer logging program, using the
microcontroller's serial port one would want to utilize an
external crystal for precise timing and reliable serial
communications.
■ PHOTO 2. A close up view of the ExpressPCB
circuit board. The lower row of components
comprise a regulated five-volt power supply.
The two transistors seen below the
microcontroller drive the two orange relays to
their right. The programming header is located
on the top right corner of the board.
38 February 2009
Power Supply
The power supply voltage for the ATtiny2313 can
range from 2.7 to 5.5 VDC. Five volts was selected for
convenience, as 5V relays were used to interface to the
garage door controllers. The circuit draws less than 20 mA
in its usual state, awaiting someone to
enter a keypad code. With all of the
peripheral devices (two relays, two
software controlled LEDs, the power
supply's LED, and the piezoelectric
beeper) turned on, the circuit could
potentially draw a maximum of about
130 mA. In actual practice, the
maximum current drawn by the circuit is
less than one half of this value, about 65
mA when one relay is active. The
LM7805 is a three-terminal positive
voltage regulator which can supply up
to 1A of current at 5V. At these low
current levels, the regulator runs cool
without an attached heatsink.
A 9 VDC, 350 mA wall-wart type
power supply is used to power the
circuit. The 9V feeds the voltage
regulator which powers the remainder
of the circuit. Older wall-warts were
typically unregulated. Their usage
required a voltage regulator to provide a
fixed voltage to the microcontroller, and
to prevent exposing the microcontroller
to an over-voltage. Newer wall-wart
power supplies are available which
incorporate internal voltage regulators.
Using one of that type would eliminate
the need for the LM7805.
Outdoor Exposure
By its very nature, this project is
mounted outdoors. This allows one to
enter an access code and thereby raise
the garage door for easy entry. The
keypad and enclosure were both
selected due to their indoor/outdoor
weather rating. The plastic enclosure's
top and bottom halves incorporate a
rubber gasket to maintain a watertight
seal. Additionally, a bead of silicone was
placed around the keypad to keep the
enclosure watertight. This design utilizes
a plastic keypad, plastic enclosure, and
exposed mounting screws. A higher
level of security and vandalism
protection could be provided by using
an all metal keypad, metal enclosure,
and inaccessible mounting hardware.
This wasn’t necessary in my neighborhood.
Software and Features
The heart of any microcontroller project is its software
program. AVR microcontrollers are most commonly
February 2009 39
PARTS LIST
ITEM QTY DESCRIPTION
❑ IC1 1 ATtiny2313 (SparkFun)
❑ KP1 1 Keypad, 4x4, matrix format (All Elect: Cat# KP-23)
❑ VR1 1 LM7805 three terminal linear voltage regulator
❑ D4-D6 3 LEDs
❑ PZ1 1 Piezo beeper, 5V (All Elect: Cat# PE-52)
❑ Q1,Q2 2 2N2222 NPN transistor
❑ RLY1, RLY2 2 Relay, 5V coil, DPDT, 16 pin DIP profile
(All Elect: Cat# RLY-625)
❑ D1-D3 3 1N4004 diode
❑ SW1 1 On/off switch, PCB mounted
❑ SW2 1 Reset switch, normally open, PCB mounted
❑ H1 1 10 pin programming header, PCB mounted
❑ H2 1 2 pin header, PCB mounted
❑ R1, R4-R7 5 10K
❑ R2, R3, R10 3 470Ω
❑ R8, R9 2 1K
❑ R11 1 47Ω
❑ C4, C5 2 220 µF 16V
❑ C6 1 100 µF 16V
❑ C8 1 10 µF 10V
❑ C1-C3, C7 4 0.1 µF 16V
Miscellaneous:
9V 300 mA wall wart power supply; printed circuit board; waterproof
enclosure (PacTec OD56 Kit; gray); header shorting connector; grommet for
wires to exit enclosure; board, keypad and enclosure mounting hardware;
silicon sealant; twin lead wire to wall wart and garage door opener to
switch contacts
Websites for further information
Atmel www.atmel.com
AVR Freaks Forum www.avrfreaks.net
Bascom-AVR www.mcselec.com
ButtLoad www.fourwalledcubicle.com
ExpressPCB www.expresspcb.com
Nuts & Volts www.nutsvolts.com
PacTec Enclosures www.pactecenclosures.com
PonyProg www.lancos.com
Spark Fun Electronics www.sparkfun.com
Jay Carter www.docjc.us
Footnotes
The PonyProg Serial Device Programmer is available from Claudio
Lanconelli. The program and further information is available at
www.lancos.com
ButtLoad (Butterfly software Loader) is available from Dean Camera.
The program and further information are available at
www.fourwalledcubicle.com
Parts Suppliers
All Electronics www.allelectronics.com
Digi-Key Corp. www.digikey.com
SparkFun Electronics www.sparkfun.com
programmed in assembly language, C, or Basic. This
project was programmed using Bascom-AVR Basic.
Roughly 2/3's of the available memory is used by the
program, allowing room for future revisions or
enhancements.
The current software includes several convenience
features. By first entering a "Programming Code," the
microcontroller beeps three times and the user then
enters the access code of their choice. This code is saved
in the microcontroller's EEPROM memory for future use.
The access code can be changed at any time by simply
re-programming it.
Many programs contain a "back door" or secret access
code, and this project is no exception. A secret access
code also exists which will always work and can not be
changed by user re-programming described above.
If the user presses the second key on the keypad prior
to entering their access code, the second door (Door #2) is
activated instead of Door #1. A single keypad and access
code can therefore be used to open or close either door.
Safety First
Once a valid access code is entered and a door is
activated to either open or close, one can press any key
on the keypad to instantly stop the door. This feature is
active for 30 seconds after entering a valid access code.
If the incorrect door was activated or if one needed to
stop the door quickly, it can be easily done without
remembering and re-entering the entire access code.
Programming the Microcontroller
The operating program — written in Basic — must
be compiled to generate a hex file which can be loaded
into the ATtiny2313 microcontroller. Both versions of
the program are available on the Nuts & Volts website
(www.nutsvolts.com). If you are familiar with
programming AVRs, you are all set. A programmer is used
to load the hex file into the microcontroller and many
different programmers exist. A low cost approach is the
AVR STK Serial Port Dongle Programmer available from
SparkFun Electronics (~$12.95). This device is used with
the free PonyProg software. An ATMEL Butterfly
demonstration board (~$21.00) can be easily converted
into an AVR ISP programmer using the free ButtLoad
program. The ATMEL STK500 (~$85.00) development
board/programmer provides extensive testbed and
programming capabilities.
Additional options and further information on
programming can be found on the ATMEL and AVR
Freaks websites given in the parts list. Instructions on
downloading the program (hex file) into the
microcontroller chip are provided with each of the above
programmers, and in tutorials on the ATMEL, AVR Freaks,
and SparkFun Electronics websites.
A circuit board was developed for this project using
ExpressPCB and their software. The board layout is
available and is posted with the software. The board
includes a 10 pin programming header for use with the
above programmers. Additional pads are provided on the
board for adding an external crystal, its capacitors, an LCD
contrast potentiometer, and for connecting to the USART
pins. These are unused in this project, but are useful for
experimentation.
Let Me In!
This project demonstrates the incredible power
and capability of small, inexpensive microcontrollers
to perform everyday tasks and make our lives easier.
With just a handful of components, a truly useful and
convenient project can be assembled in a weekend. It
demonstrates interfacing to a low cost matrix keypad and
the software techniques required to read and debounce
the keypad input. Additionally, the project demonstrates
interfacing a microcontroller to LEDs, relays, and
piezoelectric beepers.
Keypad access is both a great convenience and
security measure. This project shows the ease of its
implementation. But stay tuned for Garage Access Version
2, using ATMEL's biometric fingerprint Fingerchip sensor
to make keypads a relic from the past! NV
40 February 2009
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