Single wire power/signal system for vehicle auxiliary devices

Both power to and detection signals from a remotely located condition monitoring device are provided via a single wire with return through the vehicle frame. A load switching circuit connected and responsive to the output of the condition monitoring device causes a particular current to be drawn from the line when the device and signal status are normal than when either is not. An indicator circuit connected into the line and responsive to the current drawn by the device and loading circuit provides a failsafe indication to the operator of the monitored condition and/or operational status of the system circuitry.

BACKGROUND OF THE INVENTION 
This invention generally relates to electrical signalling systems and more 
specifically to a single wire signalling scheme for vehicle condition 
monitoring devices wherein power for the device is transmitted to and 
detection signals are received from the remotely located device using the 
single wire with ground return through the vehicle frame. 
In recent years the number of auxiliary electrical devices marketed for 
vehicle use has dramatically increased. Among these are electronic 
apparatus for monitoring all phases of vehicle operation. Past practice 
has been to design apparatus that may be installed on or adpated to any 
type vehicle and thus marketed as optional equipment. Presently, however, 
vehicle manufacturers have increased the number of hazard warning and/or 
condition monitoring devices that are installed as original equipment . 
Whether original or optional equipment, the installation poses a problem 
in that wiring must be provided such that power may be made available for 
device operation and the signals indicative of a particular condition 
occurrence may be transmitted back to the operator and timely action 
taken. 
It is thus an object of this invention to simplify and reduce the costs of 
installing auxiliary devices by providing apparatus for single wire 
transmission of power to and detection signals from remotely located 
devices.

Referring to the drawings, FIG. 1 generally illustrates the principle of 
the invention as it may be applied to a vehicle condition monitoring 
system comprising a source of D.C. power 10, a sensor 12 for monitoring a 
specific condition occurrence, a detector 14 responsive to the state of 
the sensor 12, an electrical loading circuit 16 responsive to the state of 
the detector 14, and an indicator 18 for providing a visual indication of 
the monitored condition and/or state of the circuitry. 
More specifically, the invention is directed to providing power by reason 
of a battery 10 for the remote detector 14 via a single connecting wire 20 
with return through the vehicle frame 22 while also providing a signal to 
the indicator 18 indicative of the sensed change in condition using the 
same connector 20. The invention is accomplished in a circuit arrangement 
which will hereinafter be described in conjunction with condition 
monitoring detector circuits 14 as exemplified in U.S. Pat. Nos. 3,665,387 
issued May 23, 1972 and/or 3,831,161 issued Aug. 20, 1974, to Robert S. 
Enabnit and assigned to The Goodyear Tire & Rubber Company, Akron, Ohio, 
wherein the presence or absence of pulses from the detector circuit as 
indicative of a sensed condition. 
Referring to FIG. 2, a detector 14 in accordance with either of the 
above-cited patents and showing only a portion of its output circuitry, 
provides an output signal that maintains a capacitor C-1 in a charged 
state as long as the condition being monitored by the sensor 12 is normal. 
Power for the detector is provided via line 20 by the battery 10 with 
ground return through the vehicle frame indicated by reference numeral 22. 
Absent circuitry comprising the instant invention, the system draws a 
current i.sub.1 (conventional current direction shown) to power the 
detector 14. In accordance with the teaching of this invention, however, a 
switched loading circuit generally indicated by reference numeral 24 is 
connected to the detector output at node 26 and is responsive to the 
charged state of capacitor C-1. Circuit 24, in a negative ground system, 
comprises a common-emitter connected NPN type transistor Q-1 that has its 
base lead connected through a resistor R-1 to the detector output at 26, 
its emitter lead connected to ground 22, and its collector lead connected 
to line 20 through a resistor R-2. Further, circuit 24 includes a second 
transistor Q-2 having its base lead connected and responsive to the 
collector output of transistor Q-1, its emitter lead connected to ground 
22, and its collector lead connected to line 20 through a resistor R-3. 
Also, on line 20, is an indicator circuit 18 that is remotely located from 
the detector and series connected between the detector and battery 10, 
which indicator circuit comprises a light emitting diode LED-1 in parallel 
with a resistor R-4. 
In normal operation, i.e. the absence of an abnormal condition occurrence, 
the charge on capacitor C-1 is maintained such that transistor Q-1 is 
conductive and a current i.sub.2a exists. This current biases transistor 
Q-2 to cut-off by virtue of the voltage drop across resistor R-2 and a 
current i.sub.3 = i.sub.1 + i.sub.2a is drawn on line 20 through the 
indicator circuit 18. Line current i.sub.3 provides a voltage across 
resistor R-4 insufficient to exceed the reverse breakdown level of LED-1 
which is a red luminous energy emitter. The breakdown voltage of the LED 
is relatively independent of current and in the normal condition of the 
sensor 12, no luminous energy is emitted. Alternately, when the charge on 
capacitor C-1 is not maintained by the detector output, i.e., an abnormal 
condition occurrence is sensed by sensor 12, the bias on transistor Q-1 is 
lowered to cut off current i.sub.2a and the base voltage on transistor Q-2 
increases to its "switch-on" level. Transistor Q-2, therefore, conducts 
and a much larger current i.sub.2b is drawn on line 20. A current i.sub.3' 
= i.sub.1 + i.sub.2b is drawn through the indicator circuit 18 and the 
voltage across resistor R-4 is increased above the reverse breakdown level 
of the LED. LED-1, therefore, emits red luminous energy and the occurrence 
of an abnormal condition is recognized. 
Further, with respect to the load switching circuit of FIG. 2, an optional 
red light emitting diode LED-2 (shown as ghost lines) may be connected in 
the emitter circuit of transistor Q-2. In this circumstance, LED-2 acts as 
a status indicator at the detector site having the same indicating sense 
as the remote indicator comprising LED-1. This arrangement provides easy 
troubleshooting when multiple detectors are involved, such as, for 
example, in an individual vehicle tire monitoring application, as 
exemplified in the before-mentioned patents. 
Continuing now with reference to FIG. 3, a second embodiment of the 
indicator circuit 18 is illustrated and generally indicated by reference 
numeral 18'. This embodiment, again, with reference to a negative ground 
system, comprises a PNP type transistor Q-3 having its base connected to 
line 20 through a red energy emitter LED-4, its collector lead connected 
to ground 22 through a resistor R-6, and its emitter lead connected to the 
positive side of the D.C. supply 10. Across the emitter-collector junction 
of Q-3 is a green energy emitter LED-3 while across the emitter of Q-3 and 
LED-4 is a resistor R-5. Again, the monitored condition status and 
detector output are normal when the current on line 20 is low compared to 
an abnormal condition status when the current is relatively high. Now, 
therefore, a lower current is drawn through resistor R-5 than when the 
condition is abnormal and the resultant voltage drop across R-5 and the 
emitter-base junction of Q-3 is insufficient for ignition of red LED-4 and 
it is "off". By the same token, the voltage across the emitter-collector 
junction of Q-3 exceeds the ignition level for green LED-3 and it is "on" 
indicating the normal condition. Alternately, when an abnormal condition 
is sensed and an output indicative of such abnormal condition is provided 
by the detector 14, a higher current is drawn on line 20 through resistor 
R-5 which provides a turn-on voltage for red LED-4 while the current 
through Q-3 is also increased lowering the voltage across green LED-3 
below its ignition level and it is turned "off". Thus, it should be 
appreciated that both the normal and abnormal conditions are recognized by 
indicator circuit 18' by virtue of the magnitude of the current existing 
on line 20. 
Further, with respect to FIG. 3, an NPN type transistor Q-4 is shown base 
connected to line 20 through a diode D-1 and having its emitter lead 
connected to the source of D.C. power 10 and its collector lead connected 
to ground 22 through resistor R-6. If, for example, line 20 should become 
open-circuited so that no current is drawn, the base of transistor Q-4 is 
biased positively through diode D-1 and the reduced voltage drop across 
resistor R-5 and the resutling voltage drop across the emitter-collector 
of Q-4 is insufficient to maintain the green emitting LED-3 in a "lit" 
state. Thus, the open circuit malfunction is recognized independent of 
either the normal or abnormal monitored conditions. 
FIG. 4 illustrates a load circuit 24' arranged to draw more i.sub.3 current 
when a normal monitored condition exists while drawing relatively less 
current when an abnormal condition is sensed. In this circumstance, a PNP 
type transistor Q-5 is provided that is base connected and responsive to 
the charge on detector capacitor C-1 such that a second transistor Q-6 is 
conductive, drawing a high current on line 20. To indicate the normal 
status condition, a green LED-5 is connected in line 20 in parallel with a 
resistor R-9 that provides a voltage drop across the LED exceeding the 
ignition level and the LED is "on" indicating the normal condition. 
Alternately, an abnormal condition is indicated by a drop in line current 
resulting in a corresponding drop in voltage across R-9 which is below the 
ignition level of LED-5 and it is "off". Thus, a no-light indication 
provides the operator with information relative to the presence of an 
abnormal condition occurrence sensed by sensor 12. 
Now, therefore, while the preceding circuits may be applied to detectors 14 
that communicate information by reason of a substantially constant signal 
level maintained on line 20, FIG. 5 illustrates a circuit embodiment 30 
for the situation wherein a pulse signal 32 is drawn on the line 20. In 
this circumstance, capacitor C-1 in the detector circuit 14 is of such low 
value as to permit the i.sub.3 current to vary with the pulses produced in 
the detector. This pulsed current produces a voltage drop across a 
resistor R-10 sufficient to cause a PNP type transistor Q-7 to conduct 
during the same pulse period and a corresponding voltage drop appears 
across resistor R-11. These pulses are isolated from any steady state 
level by a capacitor C-2 and the resulting pulses 34 are used to operate a 
hold circuit 28 similar to that shown in FIG. 2 comprising C-1 a switched 
loading circuit 16; and an indicator circuit 18" similar to either of 
those herebefore discussed. 
While the preceding description was primarily directed to vehicle 
electrical systems of the negative ground type, it is anticipated that the 
invention is equally applicable to positive ground type systems. The 
changes and modifications necessary to accomplish this will be apparent to 
those skilled in the electronic arts and, therefore, the invention is 
considered limited only by the scope of the appended claims.