4 TO 3 WIRE TAILLIGHT CONVERSION
Connect brake and turn signal lights of trailers to foreign cars
BY MICHAEL S. CRANMER
VIRTUALLY all foreign cars and trucks have taillights that use separate bulbs for the turn signals and brakes. This type of system uses a 4-wire configuration, which makes it possible to have the turn-signal light blinking and the brake light on at the same time on the same side. A problem occurs, however, if you want to hook up a U.S. - made trailer to the foreign car or truck. Trailer lights are usually wired on a standard 3-wire system. That is, the turn signal and Circuit Operation. The circuit is powered by the signal display system on the foreign car or truck. The three diodes, in parallel with the inputs, conduct current to the power Darlingtons and the EX-OR power supply pin. The diodes also prevent current from conducting back to another gate input. (These diodes are not needed if a separate 12-volt line is run from the battery.) The three l-kilohm resistors, tied to the inputs of the gates, are used to pull to ground the input gate volt age. (This is needed to keep the gate inputs from floating high.) The other brake light share the same bulb. With this configuration, if the brake is on and the right signal is on, the right bulb blinks while the left bulb indicates brake operation.
The problem of connecting a 4-wire signal system to a 3-wire system can be solved by using an EXCLUSIVEOR gate between the two systems. The truth table for this logic gate is given in Table 1 while the basic connections to the gate are shown in Fig. 1. Figure 2 shows all possible input conditions of left or right signal and brake light with the resultant output of the EXCLUSIVE-OR gate. Note that the ground wire is not shown in Fig. 2. The complete schematic of the circuit is shown in Fig. 3.
1-kilohm resistors are used as current limiters. Unused input pins on each gate must be tied to or ground. Also it would be advisable to place an in-line fuse on the 12-volt supply.
Building this circuit is simple and inexpensive. Most of the parts can be mounted on a piece of perf board with the power Darlingtons mounted on heat sinks. Usual care should be taken with the CMOS IC to insure that it is not damaged by static electricity.
Copyright by Bill Bytheway, K7TTY February 2012