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for driving the two LEDs. Unfortunately, the multiplexing
caused timing problems when detecting the RS-232
sequence, and I had used all my ports on the PIC12F675. I
thought I would have to redesign the board using a 16-pin
micro. I solved the problem by a sequence of software
codes. If both LEDs are red or green, I used a common
ground. When one LED is green and the other is red, I turn
off the common and use the LEDs to drive each other.
Linx’s layout is extremely critical and you must use a
ground plane. Don’t try to breadboard. It won’t work. If
you modify the board layout for another project, make sure
that you don’t have any components within . 15” of the
chip, and don’t ever run any traces under the chip.
When the receiver detects a start byte, it takes the
first byte and compares it to a known code. If this passes,
the first two bits of the second byte are checked to see
what alarm caused the interruption and it’s deciphered.
This activates the LEDs and sounds the alarm. I normally
use the micro to drive the transducer. However, this again
interfered with the RS-232 sequencing. I wanted both the
LEDs and sound to be on when the receiver is checking for
Building the Units
Make a cord for the thermistor by twisting two each
of six foot lengths of wire-wrap wire. Strip about 1/2” of
the wire-wrap wires and wrap them around the leads of
the thermistor. Solder and cut the excess leads off of the
thermistor. Take the #12 wire nut and take out the metal
spring. Place the thermistor into the wire nut and fill it with
hot glue. This will make a waterproof sensor. Since the
thermistor is high resistance, the wire can be any length
and will not affect the readings.
The transmitter box mounts on the wall using a
Serpac box (Figure 1). The box comes with all the
mounting hardware. There is a template for drilling the
holes at the article link. Cut it out and paste it onto the
box using a glue stick. You will be able to remove it with
hot water. The drill sizes are located on the template.
You will need to drill two additional holes for the power
cord and the thermistor.
I will leave the location up to you, as it will vary
with the mounting of the unit. Consider drilling the hole
close to the back, and file it out to the edge. The back
cover has a lip, so give some distance from the edge.
The reason for doing this is so you can remove the
circuit board and the wires without having to de-solder
Mount the Linx transmitter first (Figure 2). Make
sure the chip is centered on pads with pin 1 next to
the 1, heat the pad, and add solder. Solder in IC2 and IC3,
noting pin 1 is the square pad. Solder the three diodes
noting their polarity, the three resistors, and the two
capacitors noting polarity on C1.
Solder the three N battery holders in series (black to
red). Use double-sided tape and place the holders into the
box (Figure 3).
Cut the plug off the end of the battery eliminator,
thread it through the strain relief hole, and solder the white
striped wire to the negative and the other to the positive;
then solder. Perform the same operation with the battery
holders with the red to the positive and black to the
negative. Repeat once more with the two thermistor wires.
They have no polarity.
Place the LED into the board with its long lead to the
square pad. DO NOT SOLDER.
Take two each 1/2” 6-32 screws and push them
through the top side of the box. Add the two spacers
and screw them into the top of the board. The board is
designed to act as the nut. Place the antenna into the box
with its flat toward the back and secure with the screw
provided. Push the LED though its hole and now solder.
Secure the N battery holders with double-sided tape. Add
the three N batteries and secure the back of the box with
the hardware provided.
Using the template, drill the box holes. Again, you will
have to make a hole for the power cord (Figure 4).
Solder the Linx receiver chip to the board. Solder the
sound transducer noting its polarity, and the resistor. Solder
in IC1 and the switch. Cut the plug off of the battery
eliminator and thread it through the hole in the box and
the strain relief hole. Solder the white wire to negative and
the other to positive (Figure 5). Place the two bi-colored
LEDs into their holes on the board. The long lead goes to
n FIGURE 4. Receiver box. n FIGURE 5. Receiver board.