Brake and turn signal adaptor for trailers

An electrical circuit for adapting the independent brake lights and turn signal lights on a towing vehicle to the combined brake and turn signal lights on an associated trailer. An isolation network and a first CMOS exclusive OR integrated circuit are connected between the right turn signal light connection and brake light connection of the towing vehicle and the combined right turn signal and brake light connection on the trailer. An isolation network and a second CMOS exclusive OR integrated circuit are similarly connected between the left turn signal light connection and brake light connection of the towing vehicle and the combined left turn signal and brake light connection on the trailer. The isolation network prevents feedback from the electrical lighting system of the trailer to the electrical lighting system of the towing vehicle.

TECHNICAL FIELD 
The present invention relates to a trailer light electrical circuit which 
adapts the independent turn signal and brake lights on a towing vehicle to 
the combined turn signal and brake lights on a trailer. Moreover, the 
invention specifically relates to using an isolation network between the 
electrical lighting system of the towing vehicle and the electrical 
lighting system of the trailer to prevent electrical feedback from the 
trailer to the towing vehicle. 
BACKGROUND 
The electrical lighting system of a towing vehicle typically includes brake 
and turn signal lights mounted on the rear of the vehicle for indicating 
vehicle braking or directional changes. Specifically, illuminated brake 
lights indicate a slowing or stopping vehicle, while illuminated and 
flashing turn signal lights indicate a turning vehicle. Typically in late 
model vehicles, the turn signal lights and brake lights are independently 
connected to the electrical lighting system of the towing vehicle and have 
separate filaments. The brake light filament is typically connected 
directly to the vehicle brake, while the turn signal light filament is 
typically connected directly to the turn signal indicator. 
When the vehicle is used to tow a trailer, the lights on the vehicle are 
obscured, so it is necessary to include a separate set of brake and turn 
signal lights on the trailer. Each brake light and turn signal light on 
the trailer, however, is typically combined into a single light having a 
common filament. The single filament functions both as a turning indicator 
as well as a slowing or stopping indicator. 
Consequently, in order for the trailer light to indicate the correct 
directional signal, it is necessary to adapt the independent turn signal 
and brake lights of the towing vehicle to the combined turn signal and 
brake lights of the trailer. Therefore, a trailer light adaptor circuit is 
typically inserted between the electrical lighting system of the towing 
vehicle and the electrical lighting system of the trailer to adapt the 
towing vehicle's lights to the trailer's lights. The adaptor circuit, for 
example, can be inserted between the trailer tap terminal on the towing 
vehicle and the wiring harness of the trailer, as shown in U.S. Pat. No. 
4,842,524 issued to the assignee of the present invention. 
Prior art devices have attempted to adapt the electrical lighting system of 
the towing vehicle to the trailer lights. For example, Bryant, U.S. Pat. 
No. 4,006,453 shows a device for adapting the independent brake and turn 
signal lights on a towing vehicle to the combined brake and turn signal 
lights on a trailer using two logic circuits, one circuit for each side. 
Bryant discloses logic circuits having npn transistors, silicon controlled 
rectifiers or relays connected between the electrical lighting system of 
the towing vehicle and the trailer lights. 
Although Bryant discloses a circuit for adapting the electrical lighting 
system of a vehicle to the trailer lights, there remains a demand for new 
and improved electrical circuits which perform the same general function 
while providing manufacturing, operational and cost efficiencies. In this 
regard, late model vehicles typically have on-board computers and 
sophisticated electronics to control the engine function and the 
electrical lighting system in the towing vehicle. The prior art electrical 
circuits may in some instances permit some feedback from the electrical 
lighting system of the trailer to the electrical lighting system of the 
towing vehicle. 
SUMMARY OF THE INVENTION 
The present invention relates to a new electrical circuit design to adapt 
the independent turn signal and brake lights on a towing vehicle to the 
combined turn signal and brake lights on a trailer. According to one 
aspect of the invention, the adaptor circuit includes complimentary metal 
oxide semiconductor (CMOS) exclusive OR integrated circuits (ICs) for 
providing logic functions in the circuit, and an isolation network 
comprising a series of LEDs and corresponding phototransistors for 
preventing feedback to the electrical lighting system of the towing 
vehicle. The adaptor circuit is inserted between the electrical lighting 
system of the towing vehicle and the electrical lighting system of the 
trailer. 
In the isolation network, an LED and corresponding phototransistor are 
inserted between the brake light connection of the towing vehicle and one 
input to each of two exclusive OR ICs. The LED is adapted to be 
illuminated in response to current flowing to the brake lights in the 
towing vehicle. The phototransistor in turn is adapted to produce a logic 
signal to the exclusive OR ICs when the LED is activated. The 
phototransistor thereby provides a high input signal to one input of each 
exclusive OR IC when the brake light on the towing vehicle is activated. 
Similar LEDs and corresponding phototransistors are inserted between the 
tail light, backup light and right and left turn signal light connections 
on the towing vehicle, and their corresponding light connections on the 
trailer. 
The exclusive OR ICs in the circuit provide for illuminating both the right 
and left combined brake and turn signal lights on the trailer when only 
the brake light on the towing vehicle is illuminated. Further, the CMOS 
exclusive OR ICs provide for inactivating the brake signal of the light on 
the side of the trailer for which the turn signal is operating. Thus, when 
the right-hand turn signal light is flashing and the brake light is on, 
only the left-hand combined turn signal and brake light of the trailer is 
continuously illuminated and the right-hand combined trailer light is 
blinking, and vice-versa. 
The foregoing adaptor circuit provides for combining the independent turn 
signal and brake lights from the towing vehicle to the combined turn 
signal and brake lights of the trailer. Moreover, the circuit provides for 
isolating the electrical lighting system of the towing vehicle from the 
electrical lighting system of the trailer. The adaptor circuit further 
simplifies the electronic logic employed to provide manufacturing and cost 
efficiencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, and initially to FIG. 1, the adaptor circuit, 
indicated generally at 1, provides for combining the independent turn 
signal lights and brake lights of a towing vehicle to the combined turn 
signal and brake lights of a trailer. Further, the adaptor circuit i 
provides for isolating the electrical lighting system of the towing 
vehicle from the electrical lighting system of the trailer with an 
isolation circuit, indicated generally at 26. 
The towing vehicle includes right and left turn signal light connections 
10R, 10L respectively, brake light connection 12, backup light connection 
14 and tail light connection 16. The trailer includes combined brake and 
right and left turn signal light connections 20R and 20L, respectively, 
tail light connection 22, and backup light connection 24. The brake light 
connection 12 and the turn signal light connections 10R, 10L on the towing 
vehicle are electronically connected to the combined brake and turn signal 
light connections 20R, 20L on the trailer through an isolation network, 
indicated generally at 26, a logic network, indicated generally at 28, and 
an amplifier network, indicated generally at 29. The tail light connection 
16 and backup light connection 14 on the towing vehicle are electronically 
connected to the tail light connection 22 and backup light connection 24 
on the trailer through isolation network 26 and amplifier network 29. 
In the preferred embodiment of the invention, the isolation network 26 
comprises five LED's 30a-30e and five corresponding npn phototransistors 
32a-32e. The LEDs and associated phototransistors cooperatively form 
optical couplers to isolate the electrical lighting system of the towing 
vehicle from the electrical lighting system of the trailer. 
Specifically, a first optical coupler, comprising LED 30a and 
phototransistor 32a, is connected between the tail light connection 16 of 
the towing vehicle and the tail light connection 22 of the trailer. A 
second optical coupler, comprising LED 30e and phototransistor 32e, is 
connected between the backup light connection 14 on the towing vehicle and 
the backup light connection 24 on the trailer. A third optical coupler, 
comprising LED 30b and phototransistor 32b, is connected between the right 
turn signal light connection 10R, and the combined right turn signal and 
brake light connection 20R. Further, a fourth optical coupler, comprising 
LED 30c and phototransistor 32c, is connected between the brake light 
connection 12 on the towing vehicle and both the combined right and left 
turn signal and brake light connections 20R, 20L respectively on the 
trailer. Finally, a fifth optical coupler, comprising LED 30d and 
phototransistor 32d, is connected between the left turn light connection 
10L on the towing vehicle and the combined left turn and brake light 
connection 20L on the trailer. 
Each optical coupler is conventional in design, such as shown in the four 
gate package manufactured by Sharp, Model No. PC-817. The cathode of each 
LED 30a-30e of the isolation network 26 is connected through a current 
limiting resistor 34 (560 Ohms) to ground. Similarly, the emitter of each 
phototransistor 32a-32e is connected directly to ground. 
Each optical coupler effectively isolates the electrical lighting system of 
the towing vehicle from the electrical lighting system of the trailer to 
prevent feedback from the trailer. Each LED of an optical coupler is 
responsive to current in its respective light connection. The current 
causes the LED to be illuminated, which in turn causes a corresponding 
phototransistor to become activated to produce a current flow from its 
collector. For example, if the right turn light is activated for flashing, 
current will intermittently flow through right turn light connection 10R 
and illuminate LED 30b. This illumination of LED 30b will optically 
activate photoresistor 32b across the gap therebetween to produce a 
current from the collector of phototransistor 32b. The LED-phototransistor 
pair precludes any feedback of electrical signals from the trailer to the 
towing vehicle. 
The collectors of phototransistors 32b, 32c, 32d are connected to logic 
network 28 through resistors 38, 40 and 42, respectively. Resistors 38, 40 
and 42, cooperate respectively with resistors 44, 46 and 48 to form three 
voltage dividers. Preferably resistors 38, 40 and 42 have a value of 22 K 
Ohm, while resistors 44, 46 and 48 have a value of 470 K Ohm. Resistors 
44, 46 and 48 are connected to supply voltage 60, which is preferably a 
fuse protected, +12 VDC source. 
The first voltage divider, comprising resistors 38 and 44, is connected 
between the collector of phototransistor 32b and a first input 39a to the 
logic network 28. Similarly, the second voltage divider, comprising 
resistors 42 and 48, is connected between the collector of phototransistor 
32d and a second input 41a to the logic network 28. Finally, the third 
voltage divider, comprising resistors 40 and 46, is connected between the 
collector of phototransistor 32c and third and fourth inputs 39b, 41b 
respectively, to the logic network 28. The three voltage dividers are 
selected to provide the logic network 28 and the amplifier network 29 with 
the appropriate level of voltage to operate effectively. 
The four inputs are connected to CMOS exclusive OR ICs 39, 41 in a 
prepackaged logic circuit. In particular, inputs 39a, 39b are connected to 
exclusive OR IC 39, while inputs 41a, 41b are connected to exclusive OR IC 
41. The exclusive OR ICs 39, 41 are formed in CMOS transistors 
manufactured by a variety of companies under Model No. JIN 74C86. The 
exclusive OR ICs, however, can be formed or manufactured in a number of 
different ways, such as, for example in a programmable logic circuit. This 
invention is not limited in the technique for forming or manufacturing the 
exclusive OR ICs and all such different techniques are within the scope of 
this invention. 
The exclusive OR ICs perform the logic function for the right and left 
combined turn signal and brake lights on the trailer. For example, when 
either one of the two inputs to an exclusive OR IC is high, the output 
from the exclusive or IC is high. Further, when both of the two inputs are 
high (or both are low), the output is low. 
The output from each exclusive OR IC 39, 41 in the logic network 28 is 
connected through resistors 50, 52 respectively (1 K Ohm each) to the 
amplifier network 29. Similarly, the collector of phototransistor 32a from 
tail light connection 16 is connected through a fourth voltage divider, 
comprising resistor 62, (1 K Ohm) and resistor 63 (4.7 K Ohm), to the 
amplifier network 29. Resistor 63 of the fourth voltage divider is also 
connected to supply voltage 60. Further, the collector of phototransistor 
32e from backup light connection 14 is connected through a fifth voltage 
divider, comprising resistor 66 (1 K Ohm) and resistor 67 (4.7 K Ohm), to 
the amplifier network 29. Resistor 67 of the fifth voltage divider is also 
connected to supply voltage 60. The fourth and fifth voltage dividers are 
selected to provide the amplifier network 29 with the appropriate level of 
voltage to operate effectively. 
The amplifier network 29 preferably comprises four Darlington amplifiers 
54, 55, 64, 68 which are selected to provide the proper output current 
levels to the filaments in the trailer lights. Each Darlington amplifier 
is conventional in design, and typically consists of two transistors. 
Darlington amplifiers 54 and 68 are manufactured by a variety of companies 
under Model No. TIP 120, while Darlington amplifiers 55, 64 are 
manufactured by a variety of companies under Model No. TIP 125. The four 
Darlington amplifiers are connected by output leads 56 to supply voltage 
60. 
The operation of the logic network 28 is as follows. When the right turn 
signal light on the towing vehicle is illuminated and the brake lights are 
inactive, current flows through right turn signal light connection 10R, 
and LED 30b is illuminated. Current thereby flows from the corresponding 
phototransistor 32b to the first voltage divider and produces a high input 
signal to input 39a of exclusive OR IC 39. The other input 39b to the 
exclusive OR IC 39, through the third voltage divider, is held low because 
the brake light is inactive. The exclusive OR IC 39 accordingly provides a 
high output pulse through resistor 50 to Darlington amplifier 55. The 
Darlington amplifier 55 in turn activates the right combined brake and 
turn signal light on the trailer through connection 20R. When the right 
turn signal light on the towing vehicle alternately flashes on and off, 
the right combined turn signal and brake light on the trailer also flashes 
on and off in synchronization. 
When both the right turn signal light and the brake light on the towing 
vehicle are illuminated, a high input signal is received in both inputs 
39a and 39b to exclusive OR IC 39 from the right turn signal light 
connection 10R and the brake light connection 12. Exclusive OR IC 39 
accordingly provides a low output pulse through resistor 50 to Darlington 
amplifier 55. Consequently, the right combined turn signal and brake light 
on the trailer will be inactive. At the same time, a high input signal 
from the brake light on the towing vehicle is applied to the second input 
41b of exclusive OR IC 41, while a low input signal is applied to the 
first input 41a from the inactive left turn signal light. The exclusive OR 
IC 41 accordingly provides a high output pulse to Darlington amplifier 64 
to continuously illuminate the left combined turn signal and brake light 
of the trailer. 
Further, when the right turn signal light on the towing vehicle turns off, 
the input to exclusive OR IC 39 from the right turn signal light 
connection 10R goes low, and the exclusive OR IC 39 accordingly provides a 
high output signal through resistor 50 to Darlington amplifier 55, and to 
the combined right turn signal and brake light. Accordingly, as the right 
turn signal light on the towing vehicle alternatingly flashes on and off, 
the combined right turn signal and brake light on the trailer 
alternatingly flashes off and on, in reverse synchronization with the turn 
signal light on the towing vehicle. 
The left turn signal light and brake light on the towing vehicle operate in 
the same fashion as the right turn signal light and brake light through 
exclusive OR IC 41, and therefore their operation will not be described 
further. Finally, the tail lights and backup lights on the trailer operate 
independently of logic network 28, and hence are adapted to be illuminated 
when their corresponding lights on the towing vehicle are illuminated. 
In an additional embodiment of the invention, as shown in FIG. 2, LED's 
30a-30e and phototransistors 32a-30e in the isolation network can be 
replaced by a series of five high-impedance resistors 70a-70e. The high 
impedance resistors effectively shunt any feedback from the electrical 
system of the trailer to prevent the feedback from interfering with the 
electrical circuitry of the towing vehicle. Each of the high impedance 
resistors preferably has a value of at least 1 M Ohm, and is connected 
between each electrical connection 16, 10R, 12, 10L, 14 in the towing 
vehicle and their corresponding electrical connections in the trailer. 
Additionally, resistors 44, 46, 48, 67 and 63 are each replaced with 
resistors having values of at least 1 M Ohm, and are connected to ground. 
In particular, high impedance resistor 70a is connected between tail light 
connection 16 on the towing vehicle and tail light connection 22 on the 
trailer. In the event the current through resistor 70a to tail light 
connection 22 is insufficient to drive the Darlington amplifier 54, lead 
71a can be connected to a buffer 71, for example an unused exclusive OR IC 
in the logic circuit 28, to provide additional current to drive the 
Darlington amplifier 54. The other input 71b to the exclusive OR IC 71 can 
be connected to ground. Similarly, resistor 70e, connected between backup 
light connection 14 on the towing vehicle and backup light connection 24 
on the trailer, can be connected through lead 72a to buffer 72, for 
example an additional unused exclusive OR IC in the logic circuit 28, to 
provide additional current to drive Darlington amplifier 68. Similarly, 
the other input 72b to the exclusive OR IC 72 can be connected to ground. 
Consequently, the logic network 28, comprising exclusive OR ICs 39, 41, 
operates to combine the independent turn signal and brake lights from the 
towing vehicle to the combined turn signal and brake lights of the 
trailer. Further, the isolation network isolates the electrical circuitry 
of the towing vehicle from the electrical circuitry of the trailer to 
prevent feedback. 
The principles, preferred embodiment and modes of operation of the present 
invention have been described in foregoing specification. The invention 
which is intended to protected herein should not, however, be construed as 
limited to the particular form described as it is to be regarded as 
illustrative rather than restrictive. Variations and changes may be made 
by those skilled in art without departing from the spirit of the present 
invention. Accordingly, the foregoing detailed description should be 
considered exemplary in nature and not as limiting to the scope and spirit 
of the invention set forth in the appended claims.