IS YOUR GARAGE DOOR
By Ron Newton
We live in a two-story house with two separate garages, each with two
doors. One garage houses an electronics laboratory and the other a
machine shop. We are not able to see them from the house without going
outside. During the summer, I often leave the doors open for ventilation.
Our neighbors (bless their hearts) call us and tell us our garage doors are
open. Sometimes in the middle of the night I wake up and have to get up
to see if I have left them open.
Hard-wiring is always a problem, especially with an old house. With the new Linx transmitters and
transceivers, this can be done wirelessly. The cost of the
project is less than $65 + boards. Pre-programmed chips
and boards are available in the Nuts & Volts Webstore.
The primary design shown here was made for a single
garage door. Installation is simple. Using Velcro tape, the
transmitter is placed on the back of one of the panels of
the door, and the receiver is in the bedroom. When the
door goes up or down, the receiver indicates its position.
There’s no wiring of switches as it uses an internal tilt
If you have more than one garage door, the
transmitter can be hard-wired for up to three doors using
magnetic switches; it will indicate each door’s position. A
single receiver receives and decodes three garage doors,
and they don’t have to be in the same location. The
transmitter and the receiver use two AAA batteries which
should last six months to a year. Both the transmitter and
receiver have an audio indicator to let you know when
the batteries need replacing.
The receiver also has a switch that can be activated
to be an alarm if any of the doors are opened by a
burglar. See Figure 1.
38 November 2010
In both the transmitter and receiver are two ICs, a
Linx transceiver, and Microchip PIC16F505 which
directs the transceiver. A voltage detector is used to
determine if the batteries are getting low.
The transmitter is put to sleep to save power. It draws
12 micro-amps when sleeping. The transmitter activates
every 2. 3 seconds by use of a watch dog timer (WDT).
When the WDT times out, the unit checks the battery
voltage. If the voltage drops below 2. 4 volts, it beeps. The
transducer resonates at 2.730 kHz. The micro proves a
square wave at this frequency; the 10 µF capacitor blocks
DC but allows the AC to pass.
If the tilt switch or any of the magnetic switches
activate, it transmits a code to the receiver. The code is 16
bits wide and the first three lower bits determine which
door is open. This leaves 13 bits for scrambling 132 =
8,192 combinations. The transmitter turns on the Linx
transceiver and sends the codes. It transmits for about five
seconds and goes back to sleep. The code combination
flow sheet is located on the NV website.
The receiver basically has the same parts but uses
three bi-colored LEDs to indicate door positions. See