2. Brown wire is general headlights (taillights).
3. Yellow wire is left turn and stop.
4. Green wire is right turn and stop.
The transmitter sends codes to the receivers which
decode the signals and turn on the LEDS. The range of the
transmitter/receivers is about 300 ft.
The transmitter consists of a microcontroller,
transmitter, three diodes, three resistors, a voltage
regulator, and two capacitors. Each trailer connector wire
provides 12 volts when switched. Diodes are connected
to each wire and their cathodes are connected together.
The diodes prevent the return of current to the other
wires. A three volt linear voltage regulator is used to
reduce the voltage to the ICs. The large 470 capacitor
stores just enough energy to transmit an off signal when
the lights are turned off. The 10 µF capacitor filters the
output of the voltage regulator.
With three lines, there is a possible combination of six
states ( 23 = 6). The 12F508 decodes which wires are hot
and converts this into the first three bits of a byte. The
micro is programmed to produce a serial data output of
16 bits. This allows 213 = 8,192 transmitter combinations.
The first key of the combination is the first byte. The
second key uses the last five bits of the second byte (the
first three bits being the wire codes). The bytes are
transmitted using a 16-bit asynchronous serial stream with
a stop and start byte. This data is fed into IC2 and
transmitted. The data is transmitted at 1200 baud.
To keep in compliance with FCC regulations, the
“lights are on” signal only transmits once every 10
seconds when the lights are turned on. The brake and turn
signals transmit when needed. This keeps the transmission
rate to a minimum.
The Linx Technologies
TXM-433-LR transmitter
module that I’m using here
(see Parts List) only needs an
antenna and does not require
any other external components
to transmit. However, the
antenna and chip do need a
ground plane (which both the
board and chassis provide).
if the key is identical. If this data matches, it decodes the
last three bits which determine what the taillights do.
The Linx Technologies RXM-418-LR receiver was
constantly being bombarded with noise and gave me fits
as I kept getting random turn on and off action. I took
care of this by adding a piece of code in the software
which insures that the signal is high for a period of time
before it starts decoding the data.
The taillights have two rings of LEDS. The outer ring is
for the taillights; the inter ring is for the brake or turn
signals. Only two signals need to be deciphered per unit.
The signal for the opposite unit is ignored. The receiver
chips are coded for left and right turn signals.
The LED rings are controlled by two NPN transistors.
Two D batteries power both IC1 and IC2. Each transistor
has a voltage drop of . 8 volts. This allows the LEDS to be
driven at 2. 2 volts 20 milliamps each. The ground plane
acts as a reflector.
I have included an RSSI (received signal strength
indicator) pad for those who want to troubleshoot or
check the power of the transmitted signal. (See the RXM-
418-LR specification sheet for further information.)
The software is programmed so that if no signals are
received within a one minute period (lights on), it will turn
off the LEDS.
Construction Time
On the Nuts & Volts website, there are two assembly
files. You will need MPLAB and a programmer for the
chips. For the transmitter, solder IC1 and IC2 to the top
side of the board. Solder the three resistors, three diodes
■ FIGURE 2. Completed Transmitter.
■ FIGURE 3.
Receiver Board.
Meet the Receiver
The receiver acquires the
signals and passes the data to
the 12F508 which decodes the
serial data. All three micros
must be coded with the same
keys as the transmitter for the
system to work. Two bytes are
extracted from the serial data.
The first byte and part of the
second byte is checked to see
July 2010 35
