Noise Immune Power Switch for Automotive Accessories

This invention relates in general to power switches for connecting accessory devices, such as cellular telephones, to automobile charging circuits via the vehicle cigarette lighter socket.

U.S. Pat. No. 4,733,100 discloses an automatic on/off switch circuit with time delay for powering a radar warning receiver or the like from the cigarette lighter socket of an automobile. The switch is placed in its on condition to electrically connect the radar detector to the automobile charging circuit in response to the electrical noise produced when the automobile engine is running. The switch circuit integrates the noise signals to drive a switching transistor conductive. A predetermined time after shut down of the automobile engine (and disappearance of the noise signals), the switch reverts to its off condition and disconnects the radar detector.

The patented circuit responds to a wide range of noise signals and can be made to respond to noise signals other than those produced by a running engine, e.g. an automobile fan or a clock. Noise generated by devices requiring relatively large currents, such as CB radios or cellular telephones, can be sufficient to cause the patented circuit to falsely trigger in an oscillatory manner, which could result in the automobile battery being discharged. It should be noted that the patented circuit is intended for incorporation into the radar detector proper and therefore only needs to discriminate against a limited range of signals and noises, i.e. those produced by the radar detector (or other particular product).

The present invention contemplates a stand alone switch intended for use with a wide variety of both small and large current accessory devices. Special care is taken to assure that the electronic switch is not falsely triggered by signals and noise produced by any of the types of apparatus with which the switch will be used. The inventive electronic switch is only turned on in response to a sharp drop in B+ voltage, corresponding to energization of the vehicle starter motor, and is maintained in an on condition if a unique signal from the alternator is received within a predetermined time after the turn-on transistor has initiated switch operation. When the automobile engine is turned off, the electronic switch resets after a predetermined time and cannot be reinitiated except by the turn-on transistor. The unique alternator signal that maintains the electronic switch in the on state is the negative half of the alternator ripple voltage. This signal is unique and differs from any other signal present in the automobile environment or in the accessories that would be operated from the switch.

A principal object of the invention is to provide a novel electronic power switch for operating an accessory from an automobile cigarette lighter socket.

Another object of the invention is to provide an improved accessory switch for use with an automobile battery circuit.

These and other objects and advantages of the invention will be apparent upon reading the following description in conjunction with the drawings, in which:

FIG. 1 is a simplified block diagram of an automobile power supply system and the switch of the invention; and

FIG. 2 is a schematic diagram of the switch of the invention.

Referring to FIG. 1, an automobile battery 10 is connected to an ignition switch 12 which in turn supplies a starter, an ignition circuit and an accessory circuit (all not shown) of conventional construction in an automobile. The positive (B+) terminal of battery 10 is also connected in a conventional charging circuit with an alternator 14 and a voltage regulator means 15. A direct connection exists from the positive terminal of battery 10 to a cigarette lighter socket 16 mounted in the dashboard of a vehicle (not shown). A mating plug 18, engageable with lighter socket 16, provides a B+ input connection to an electronic switch 20 which has an output connector in the form of another cigarette lighter socket 21. A mating plug 22 on an accessory device 23 connects to socket 21. Accessory device 23 may be a cellular telephone, a CB radio or any other suitable utilization device.

Under normal operating conditions and with the automobile engine not running, the B+ DC voltage of the battery appears at socket 16 because of its direct connection to battery 10. When the engine is running, alternator 14 produces a higher multiphase alternating current voltage that is rectified and supplied as charging current for the battery 10 and to operate the ignition and accessory circuits. The B+ line carries the rectified alternator voltage and a small ripple voltage. Any electrical noise, emitted by operation of the engine (ignition noise) or received from other sources will also be on the B+ line and hence appear at socket 16. Consequently, the input to electronic switch 20 may include a DC voltage, an alternator ripple voltage, ignition noise voltage and other types of noise voltages.

Referring to FIG. 2, electronic switch 20 includes an input plug 18, which is connected to socket 16. A protective fuse 19 is connected in series with the B+ line. Output socket 21 of switch 20 is connected to a terminal B which is connectible to the B+ line via the emitter-collector path of a transistor switch 46. Socket 21 and plug 19 both have one side or terminal connected to ground. A similar plug 22 is connectible to socket 21 for connecting an accessory load 23, such as a cellular telephone, to battery 10 through electronic switch 20.

The B+ lead is connected through a diode 27 to a voltage divider consisting of resistors 28, 29 and 30 connected in series to ground. The cathode of diode 27 is connected to a large filter or storage capacitor 25 and the junctions of resistors 28 and 29 and of resistors 29 and 30 are connected to ground through bypass capacitors 26 and 31, respectively. The junction of resistors 28 and 29 develops a reference voltage (indicated as REF 1) and the junction of resistors 29 and 30 develops another reference voltage (indicated as REF 2). REF 1 is connected to the positive or non-inverting terminal of an operational amplifier 36 and REF 2 is connected to positive terminal of an operational amplifier 43. The negative or inverting terminal of operational amplifier 36 is connected to B+ through a filter consisting of a capacitor 32 and a resistor 33 connected in series. The output of operational amplifier 36 is returned to its negative input terminal by means of a feedback resistor 35. The output of operational amplifier 36 is coupled through a capacitor 37 to a junction A of three diodes 38, 39 and 55. Diode 38 has its anode connected to ground and its cathode connected to junction A and diode 39 has its anode connected to junction A and its cathode connected to a voltage divider consisting of resistors 41 and 42 connected to ground. The junction of resistors 41 and 42 is connected to ground through an integrating capacitor 40 and is also connected to the negative input of operational amplifier 43. The anode of diode 55, which is referred to as a defeat diode, is connected to junction A and its cathode is connected to output terminal B. As mentioned, transistorized switch arrangement 46 bridges the B+ line and output terminal B. A resistor 45 is connected from B+ to the input of transistor switch 46 and to the output of operational amplifier 43 through an LED 48 and a resistor 44. When transistor switch 46 is supplied with base current (turned ON), LED 48 will be lighted.

A turn-on transistor 52 has its emitter-collector path connected between the positive terminal of filter capacitor 25 and the positive terminal of integrating capacitor 40. A small resistor 54 is included in the circuit to limit current flow. The base of turn-on transistor 52 is connected to B+ through a resistor 53. A manual switch 49 and a series resistor 47 are connected across the emitter-collector path of turn-on transistor 52. Under normal operating conditions, the battery voltage is approximately 12.6 volts DC (without the alternator operating) and the REF 2 voltage applied to the positive input of operational amplifier 43 is about 0.25 volts. In the absence of charge on integrating capacitor 40, the output of operational amplifier 43 is positive, transistor switch 46 is cut off and there is no connection from B+ to output terminal B. Integrating capacitor 40 is approximately one tenth the size of storage capacitor 25 (about 3.3 microfarads vs. 33 microfarads). Storage capacitor 25 is charged to a potential equal to B+ minus the voltage drop across diode 27.

Turn-on transistor 52 is normally off. Responsive to a negative-going voltage edge on the B+ line, such as that caused by energization of the starter motor circuit, diode 27 is reverse biased and turns on transistor 52, which is driven into heavy current saturation, since its base is driven negative with respect to its emitter which is held at the potential existing across filter capacitor 25. Conduction of turn-on transistor 52 results in a rapid transfer of energy from filter capacitor 25 to integrating capacitor 40. This instantaneously raises the potential at the negative input of operational amplifier causing the output of operational amplifier 43 to go negative and turn-on transistor switch 46 through LED 48, which is illuminated to indicate that transistor switch 46 is ON. Transistor switch 46, in turning on, connects the B+ line to the output terminal B and energizes the connected load 23.

Turn on transistor 52 turns off when its base-emitter junction is no longer forward biased and integrating capacitor 40 begins to discharge through resistor 42. Unless capacitor 40 has its charge replenished, its terminal voltage will drop below the REF 2 potential at the positive terminal of operational amplifier 43 and operational amplifier 43 will revert to its positive output voltage and cut off transistor switch 46 and LED 48.

Assuming that the engine is running and that an alternator ripple voltage appears on the B+ line, the frequency of that ripple voltage is such that the signal is passed by the filter amplifier (capacitor 32 and resistor 33) to the negative input of operational amplifier 36. The ripple voltage is amplified by operational amplifier 36 which produces an output signal. This output signal consists of the amplified and inverted alternator ripple voltage riding on a DC voltage level. When the output voltage of amplifier 36 is positive going, capacitor 37 is charged and diode 39 is forward biased to charge integrating capacitor 40. When the output voltage of amplifier 36 is negative going, capacitor 37 discharges through. diode 38 which becomes forward biased. (Capacitor 40 discharges through resistor 42 during this period.) The net effect is to maintain a sufficient charge on capacitor 40 to keep the negative input of amplifier 43 at a greater potential than the REF 2 voltage at its positive input. When this condition occurs, the output of operational amplifier 43 remains negative and maintains transistor switch 46 conductive and LED 48 illuminated.

As mentioned, should the alternator ripple voltage not appear on the B+ line to supply sufficient charge to integrating capacitor 40, capacitor 40 will discharge through resistor 42. The output of operational amplifier 43 will switch to positive, LED 48 will be extinguished and transistor switch 46 will open to disconnect B+ from output terminal B. This, of course, occurs when the automobile engine is turned off.

Protection against inadvertent operation of electronic switch 20 is provided by defeat diode 55 which connects junction A to output terminal B. With a connected load, such as load 23, supplying a resistance connection to ground across output terminal B, and with transistor switch 46 open, positive potential buildup at terminal A will be inhibited. This is because defeat diode 55 will conduct current through the connected load resistance as soon as the positive potential exceeds the breakdown voltage of defeat diode 55. With terminal A being effectively coupled to ground, there is no possibility of integrating capacitor 40 achieving a sufficient charge to enable conduction of transistor switch 46. When turn-on transistor 52 operates however, integrating capacitor 40 is immediately charged through the alternate path from filter capacitor 25 to turn transistor switch 46 on. When this occurs, terminal B goes to B+ potential, which back biases defeat diode 55 and permits the potential at terminal A to follow the output signals from operational amplifier 36.

Manual switch 49 is not involved with the invention and is normally in the open position. When switch 49 is closed, it connects resistor 47 between the emitter and collector of turn-on transistor 52, which simulates a continuous motor starting condition that turns on LED 48 and transistor switch 46.

Thus the circuit of the invention not only is capable of switching on relatively large current accessory devices when the ignition key of the automobile is turned to the start position, and of switching them off when the engine is shut down, it is protected from inadvertent turn on by extraneous noise signals and by signals that may be generated by the accessory devices themselves. This is achieved with a turn-on circuit which responds only to a negative-going edge on the B+ line to initiate the transistor switch operation. A unique signal must be received shortly thereafter to maintain the transistor switch in its on state. This unique signal (to the automotive environment) consists of the negative portions of the alternator ripple voltage.

What has been described is a novel electronic switch for use in connecting accessories to the cigarette lighter socket of an automobile. It is recognized that numerous changes and modifications in the described embodiment of the invention will be apparent to those skilled in the art without departing from its true spirit and scope. The invention is to be limited only as defined in the claims.