Photoelectric Switch for Use With a Machine Control Circuit

I. Field of the Invention

The invention is related to electrical switches and in particular to a photoelectric switch for initiating or changing a mode of operation of a machine in a user safe manner.

II. Description of the Prior Art

It has long been known to use a pair of spatially separated manually operable switches to activate the operational modes of machines such as stamping presses, riveters, tube benders, spot welders or like machines. The purpose of the spatially separated manually operated switches is to ensure that both of the operator's hands are positioned away from the machine prior to its operation. To further ensure the safety of the operator, the pair of switches must be activated simultaneously and must be placed apart a distance sufficient to prevent simultaneous operation of both switches by a single hand. The Federal Occupational Safety and Health Administration (OSHA) has set the criteria that must be met by such machine operating equipment to ensure adequate operator safety.

The prior art initially used a mechanically actuated single pole double throw palm button switch. These palm button switches are biased to a normally open position and constant depression is required to close the normally open contact. The use of a pair of spatially separated palm buttons has been adequate for the intended safety purposes. However, because the operator must apply significant pressure to the palm button switches in order to achieve actuation, the operators are experiencing inflammation of the tendons due to the repetitive and forceful action of the fingers and wrists. This inflammation of the tendons of the fingers and wrists is known as Carpal Tunnel Syndrome. This ailment has increased worker fatigue and complaints, has decreased productivity and has increased the frequency of disability claims under workmen's compensation laws and programs.

The problems associated with the palm button switch has been substantially eliminated by the photoelectric switch taught by Herman et al in U.S. Pat. No. 4,939,358. This switch apparatus employs an infrared light source and a photodiode placed on opposite sides of a slot designed to receive one or more fingers of the operator. The fingers of the operator, when in the slot, occlude or break the infrared light path between the light source and the photodiode. The photodiode, in response to breaking or occlusion of the infrared beam, actuates a relay to switch its pole from a normally closed contact to a normally open contact. Conversely, when the infrared beam is restored, the relay is deactivated and the pole of the relay returns to the normally closed contact. Although the photoelectric switch taught by Herman et al resolved most of the problems associated with the palm button switches, the insertion of the fingers into the slot has been found to be uncomfortable for the operator and the sides of the fingers became sore when they are repeatedly banged against the bottom of the slot.

The invention is an improved photoelectric switch which is user friendly and incorporates a plurality of safety features not normally provided by the prior art switches.

The invention is a photoelectric switch for use in conjunction with a machine control circuit which controls the operation of a machine capable of performing a machining operation. The machine control circuit is responsive to inputs received from a pair of photoelectric switches to activate the machine. The machine control circuit prevents the machine from performing its machining operation unless said machine control circuit receives input signals from both of the photoelectric switches within a predetermined period of time. The machine is actuated by an AC electrical power source providing an AC voltage within a predetermined AC voltage range. Each photoelectric switch comprises a cover having a contoured finger rest surface, means for generating a light beam above the finger rest surface, which is occluded by an operator placing at least one finger on the finger rest surface of the cover, and a relay having an activated state in which it generates the input signal to the machine control circuit. Each photoelectric switch also has means for activating the relay in response to the beam being occluded by at least one finger of the operator placed on the finger rest surface of the cover and means for deactivating the relay in response to the termination of the beam from being occluded.

The preferred embodiment of the photoelectric switch includes a low voltage protection circuit which prevents the activation of the relay when the AC voltage being applied to the machine is less than a predetermined voltage, a high ambient light protection circuit for preventing the actuation of the relay when the ambient light level is excessive, end means for energizing a fault light when one or more of the relay contacts are stuck or welded in an energized position.

The primary object of the invention is a user friendly photoelectric switch which incorporates at least one or more fault detection circuits.

Another object of the invention is a contoured cover having a user friendly finger rest surface. The photoelectric switch producing an output signal when the operator places one or more fingers on the finger rest surface.

Another object of the invention is a pair of raised finger guides formed on opposite sides of the finger rest surface to guide the placement of the operator's fingers on the finger rest surface.

Another object of the invention is to generate a pulsed infrared light beam above the finger rest surface which is broken or occluded when the operator places at least one finger on the finger rest surface.

Another object of the photoelectric switch is that the cover is made from a transparent plastic material.

Another object of the invention is that the cover is dyed or colored red for high visibility in the work place.

Another object of the photoelectric switch is that an infrared light emitting diode is disposed in a raised finger guide disposed on one side of the finger rest surface and a phototransistor is disposed in the raised finger guide on the opposite side of finger rest surface to detect the placement of at least one finger on the finger rest surface of the cover.

Another object of the photoelectric switch is to use a relay to generate an output signal which is immune to noise and line transients.

Still another object of the photoelectric switch is to prevent the actuation of the relay when the AC voltage is less than a predetermined voltage to prevent actuation of the machine when the AC voltage is below its minimum operating voltage.

Yet another object of the invention is a photoelectric switch having a high ambient light protection circuit to prevent erratic or unreliable activation of the photoelectric switch under high ambient infrared light conditions.

Still another object of the invention is a photoelectric switch having a stuck or welded contact protection circuit which energizes a fault light when the contacts of the relay are stuck or welded in the activated state.

Still another object of the invention is an anti-tease circuit arrangement which prevents the actuation the switch when the time required to break the light beam exceeds a predetermined period of time.

These and other objects of the invention will become apparent from a reading of the specification in conjunction with the drawings.

FIG. 1 is an exploded view of the photoelectric switch.

FIG. 2 shows two photoelectric switches mounted on a switch panel and the placement of the fingers on the finger rest surfaces to activate the photoelectric switches.

FIG. 3 shows an alternate mounting arrangement of the photoelectric switch to a switch panel.

FIG. 4A and FIG. 4B are complementary portions of the circuit diagram of the circuit contained on the circuit board.

FIG. 5 is a block diagram showing the relationship of the photoelectric switches to the control and logic circuit of a machine.

FIG. 5A is a graph showing the decay curves of the two RC circuits used to actuate the relay flip-flop.

FIG. 1 is an exploded view of the improved photoelectric switch 10. The photoelectric switch 10 has an electronic circuit board 12 to which is attached a cover 14. The circuit board 12 and the cover 14 are cemented to each other and mounted on a housing 16 by means of screw type fasteners 18 at the four corners thereof. Alternatively, the circuit board 12 and the transparent cover 14 may be attached directly to a switch panel 48, as shown in FIG. 3. The switch panel 48 has a rectangular aperture 50 into which the photoelectric switch 10 is received and threaded bores 52 for receiving the threaded fasteners 18.

The cover 14 has a flat base 20 which is cemented to the periphery of the circuit board 12 and a protruding finger rest 22. The protruding finger rest has a centrally located finger rest surface 24 contoured to comfortably support at least two fingers of an operator, as shown in FIG. 2. A pair of spatially separated finger guides 26 and 28 are provided on opposite sides of the finger rest surface 24 and provide for positive location of one or more fingers on the finger rest surface 24.

The finger guides 26 and 28 are hollow and respectively enclose a light emitting diode 30 and a phototransistor 32 attached to the circuit board 12. The light emitting diode 30 and the phototransistor 32 are disposed in the finger guides 26 and 28 so that the direct light path 34 between these two elements is external to the cover 14 and across the finger rest surface 24.

Preferably, the cover 14 is made from an infrared transparent plastic material so that the light emitted from the light emitting diode 30 is transmitted through the side walls, such as side wall 36 of the finger guides 26 and 28, and is received by the phototransistor 32. Preferably, the transparent plastic material is colored or dyed red for high visibility in an industrial atmosphere.

Alternatively, as shown on FIG. 3, the cover 14 may be made from an opaque material, and infrared transparent windows, such as window 54, are located on the opposing side walls of the finger guides 26 and 28, providing a transparent infrared light path between the light emitting diode 30 and the phototransistor 32.

The circuit board 12 contains an elecronic circuit 38, shown on FIGS. 4A and 4B, and includes a multiple contact relay 40, the light emitting diode 30, the phototransistor 32, and an electrical connector 42. The electrical connector 42 permits AC electrical power to be provided to the electronic circuit 38 and permits electrical signals generated by the photoelectric switch to be transmitted to a utilization device such as a machine controller.

The housing 16 has a cavity in which the multiple contact relay 40 and the electrical connector 42 are received and has at least one electrical outlet 44. The housing 16 also has one or more mounting tabs 46 which permits the housing 16 to be mounted at an appropriate location relative to the machine being operated by the switch 10.

When operating machines, such as stamping presses, riveters, tube benders, spot welders and like machines, where the probability exists that an operator's hand may accidentally be injured if not removed from the machine prior to actuation, a pair of photoelectric switches 10 and 10', one for each hand, are normally provided to meet the criteria established by the Federal Occupational Safety and Health Administration. The two photoelectric switches 10 and 10', as shown in FIG. 2, are mounted on a switch panel 48 and are preferably separated from each other by a distance sufficient to prevent both switches from being actuated by the same hand. In general, these switches are placed 12 to 16 inches apart, which is about equal to the shoulder width of an operator. This separation has been found to be comfortable for an operator to activate both switches at the same time.

As shown in FIG. 5, the two photoelectric switches, 10 and 10', are connected to an electrical machine control and logic circuit 56 which controls the operation of a machine 58. The machine control and logic circuit 56 is of a conventional design well known in the art. In the preferred embodiment of the control and logic circuit 56, two control signals are received from each photoelectric switch 10 and 10'. The first control signal is associated with a normally closed (NC) contact of the multiple contact relay 40, and the other control signal is associated with a normally open contact of the multiple contact relay 40. The state of relay 40 in which either the normally open or normally closed contacts are closed is dependent upon whether the phototransistor 32 is receiving light from the light emitting diode 30 or if the light path between the light emitting diode 30 and the phototransistor is broken or occluded by a finger of the operator being placed on the finger rest surface 24.

When the machine control and logic circuit 56 receives a signal from both photoelectric switches 10 and 10' signifying that the normally closed contacts are closed, it will inhibit the operation of the machine. Conversely, when the control and logic circuit receives a signal signifying that the relay has been activated to close the normally open contacts, it will activate the machine. As shall be explained relative to the circuit shown on FIGS. 4A and 4B, the relay 40 is activated when the light path between the light emitting diode 30 and the phototransistor 32 is broken or blocked by the operator placing at least one finger on the finger rest surface 24.

Since the machine control and logic circuit 56 does not form part of this invention and its operation is well known in the art, it need not be further discussed for a complete understanding of the invention.

The details of the electronic circuit 38 contained on the circuit board 12 are shown in FIGS. 4A and 4B. AC electrical power is received at power input terminals 100 and 102, respectively, as shown on FIG. 4B. This AC electrical power has a nominal voltage range from 90 to 140 volts. The AC electrical power is applied to a full wave rectifier 104 through capacitor 106 and inductors 108 and 110, respectively. The inductors 108 and 110, in combination with the capacitor 112 and back-to-back Zener diodes 114 and 116, form a transient suppression filter at the electrical power input end of the circuit. This transient suppression filter slows transient rise time and lowers the amplitude of high frequency noise on the AC power lines. The back-to-back Zener diodes 114 and 116 also limit the maximum AC voltage to approximately 150 volts and further blocks power line noise and transient voltage spikes from entering the circuit causing false triggering or activation of the relay 40.

The full wave rectifier 104 rectifies the AC electrical power and produces a positive DC voltage output which is used to electrically power the light emitting diode 30, the phototransistor 32 and the remainder of the circuit. A Zener diode 118, as shown on FIG. 4A, limits the output voltage of the full wave rectifier 104 to 12 volts DC. The capacitor 120 filters the 12 volt DC output of the full wave rectifier 104.

The filtered 12 volt DC output of the full wave rectifier 104 is applied to B+ lines 122 and 124, as shown.

A 700 hertz oscillator consisting of operational amplifier 126, resistances 128 through 134 and capacitor 136, has its output connected to the base of a transistor 140 through a resistance 138. Transistor 140 is connected in series with the infrared light emitting diode 30 between the B+ line 122 and ground. The output of the infrared light emitting diode 30 will, therefore, be a pulsed light beam having a 40% duly cycle and a 700 hertz frequency.

As previously indicated, the pulsed infrared light beam generated by the infrared light emitting diode 30 is received by the phototransistor 32. The phototransistor 32 is connected in series with resistor 142 between the B+ line 124 and ground. The junction between phototransistor 32 and resistor 142 is connected to the negative input of operational amplifier 144 through capacitance 146 and resistor 148. The positive input to operational amplifier 144 is connected to the junction of resistors 150 and 152 which form a voltage divider between B+ line 124 and ground. The values of resistors 150 and 152 are selected such that operational amplifier 144 produces a pulsed output signal in response to the phototransistor 32 receiving the pulsed light beam from the infrared light emitting diode 30 under normal operating conditions.

The pulsed output of operational amplifier 144 is rectified by the circuit consisting of diodes 154 and 156, capacitors 158 and 160, and resistor 162. The capacitance of capacitor 160 and the resistance of resistor 162 are selected to have a relatively short RC time constant, as shown by curve 60 on FIG. 6. The potential across capacitor 160 is applied to the input of an inverter 164 by diode 166 and directly to the input of inverter 168. An RC delay circuit consisting of capacitor 170 and resistor 172 having a time constant shown as curve 62 on FIG. 6 which is longer than the time constant of curve 60.

The output of inverter 164 is applied directly to the clock input of a fault flip-flop 174 and to the D input of relay flip-flop 178 through inverter 176. The output of inverter 168 is applied directly to the clock input of relay flip-flop 178 and to one input of NAND gate 182 through capacitor 180. The output of NAND gate 182 is connected to the RESET input of relay flip-flop 178 through diode 184. The RESET input of relay flip-flop 178 is also connected to ground by resistor 186. The output of NAND gate 182 is also connected to the RESET input of fault flip-flop 174 through diode 188 via line 118.

The Q output of relay flip-flop 178 is connected to the base of a transistor 190 connected in series with the coil 192 of relay 40 between the AC input line and ground. The Q output of relay flip-flop 178 is connected to the D input of fault flip-flop 174, to an input to NAND gate 194 and to the base of transistor 196. Transistor 196 is connected in series with a pilot light, such as light emitting diode 198 between the Q output of fault flip-flop 174 and the output of a comparator 200 via line 201. Under no-fault conditions, the Q output of fault flip-flop 174 is a ground potential and the output of comparator 200 is a positive potential.

The operation of the circuit under no-fault conditions is as follows: prior to interrupting the beam, relay flip-flop 178 is in its RESET state with the Q output being a ground potential and its Q output being at a positive potential. When the Q output of the relay flip-flop 178 is a ground potential, transistor 190 is non-conductive and the coil 192 of the relay 40 is de-energized. The position of the poles of relay 40 are as shown in FIG. 4B. The positive Q output of relay flip-flop 178 will place transistor 196 in a conductive state energizing pilot light emitting diode 198 indicating that the relay 40 is de-energized. The fault flip-flop 174 is also in its reset state in which its Q output is at a ground potential and its Q output is a positive potential.

When the infrared beam is not broken, the phototransistor 32 generates a pulsed signal at the input to operational amplifier 144 which generates an amplified pulsed signal. This amplified pulsed signal is rectified by diodes 154 and 156 to produce a positive potential across capacitor 160. The positive potential across capacitor 160 is applied to the D input of relay flip-flop 178 by means of serially connected inverters 164 and 176. A negative clock signal from inverter 168 is applied to the clock input of flip-flop 178. The negative signal applied to the clock input holds flip-flop 178 in its existing reset state with its Q output at a ground potential and its Q output at a positive potential.

When the operator places one or more fingers on the finger rest surface 24 of the cover 14, the light beam is broken, terminating the pulsed signal at the input to operational amplifier 144. The potential across capacitor 160 will start to decay at a rate faster than the potential across capacitor 170. The flip-flop 178 will respond to the leading edge of the positive signal output by inverter 168 and clock flip-flop 178 while the signal across capacitor 170 is still positive. This will cause relay flip-flop 178 to change state and its Q output will assume a positive potential and its Q output will assume a ground potential. The positive potential at the Q output of relay flip-flop 178 will place transistor 190 in a conductive state. The coil 192 of relay 40 will now be energized by the current from the AC input terminal 100 through inductance 108, diode 109 and transistor 192. The poles in the relay 40 will change state. Also the Q output of relay flip-flop 178 will place a ground potential at the base of transistor 196 de-energizing pilot light emitting diode 198 indicating the relay 40 has been actuated.

When the operator removes his finger from between the infrared light emitting diode 30 and the phototransistor 32, a pulsating signal will again be applied to the negative input of operational amplifier 144 which will produce an amplified pulsating output. This pulsating output will be rectified by diode 154 and 156 placing a positive potential at the inputs of inverters 164 and 168. Inverter 168 will produce a negative output signal which is applied to an input of NAND gate 182 through a capacitor 180. The output of NAND gate 182 will become a positive potential in response to the negative potential applied to its input which is applied to the RESET input of relay flip-flop 178. The positive potential applied to the RESET input will reset relay flip-flop 178, returning its Q output to a ground potential and its Q output to a positive potential.

The ground potential at the Q output of relay flip-flop 178 is applied to the base of transistor 190 rendering it non-conductive, and thereby, deactivating relay coil 192 of relay 40. This returns the switch to its nonactivated state which returns the contacts of relay 40 to their unactivated positions, i.e. the normally closed contacts will close and the normally open contacts will open. The positive potential at the Q output of relay flip-flop 178 places transistor 196 in the conductive state energizing pilot light emitting diode 198 which produces a visual signal indicating that the relay 40 is deactivated.

Diode 111 and capacitor 113 connected in parallel with relay coil 192 are provided to surpress the inductive flyback potential generated when the relay coil 192 is de-energized.

The photoelectric switch 10 has several fault protection circuits which enhance the safety and reliability of the photoelectric switch. The faults protection is for the following conditions:

1) when the AC voltage is below the minimum operating voltage of the device being controlled;

2) when there is excessive ambient infrared light present which would adversely effect the operation of the switch;

3) when one or more contacts in the relay 40 are welded or become stuck in the activated position; and

4) when the infrared light beam is blocked or otherwise occluded by dirt in the side walls of the finger guides;

5) upon initial power up, if the beam is not broken the relay will not energize.

A comparator 202 is used to generate a fault signal when the AC voltage is below the minimum operating potential of the device being controlled. This fault signal will disable relay 40 and energize a fault light emitting diode 204, as shall be explained.

Comparator 202 has its positive input connected to the 12 volt B+ line 122 by a resistor 206 and to ground by Zener diode 208. The Zener diode 208 maintains the potential at the positive input of comparator 202 at a predetermined reference voltage of approximately 5.1 volts. The negative input to comparator 202 is a connected junction between fixed resistor 210 and variable resistor 212 which form a voltage divider between the B+ line 122 and ground. Variable resistor 212 is adjusted so that the output of comparator 202 on line 203 is a positive potential when the AC voltage across input terminals 100 and 102 is below the minimum operating voltage of the device being controlled.

The positive potential output of comparator 202 resets relay flip-flop 178 through diode 213 so that its Q output becomes a ground potential and its Q output becomes a positive potential. The positive potential at the output of comparator 202 on line 203a also places transistor 214, FIG. 4B, in the conductive state which places the base of transistor 190 at a ground potential rendering transistor 190 non-conductive, thereby de-energizing relay 40. Therefore, during the initial application of electrical power to the electronic circuit, transistor 190 remains non-conductive preventing accidental activation of the relay 40. Further, the positive potential at the output of comparator 202 on line 203b sets fault flip-flop 174 causing the Q output to become a positive potential and its Q output to become a ground potential. When the the Q output of fault flip-flop 178 becomes a positive potential, transistor 196 becomes nonconductive de-energizing pilot light emitting diode 198. The ground potential at the Q output of fault flip-flop 174 applied to an input of NAND gate 216 causes its output to become a positive potential, placing transistor 218 in a conductive state. This energizes a fault light, such as light emitting diode 204, signifying a fault has been detected.

The low voltage fault detection circuit will also momentarily turn on the fault light emitting diode 204 each time power is first applied to the photoelectric switch 10. However, after the AC voltage exceeds the minimum operating voltage of the device being controlled, the output of comparator 202 will become a ground potential which terminates the positive potential applied to the input of inverter 220 and its input will now, via resistor 248, become a ground potential. Inverter 220, in response to its input being at a ground potential, will generate a positive potential at its output which is appled to the RESET input of fault flip-flop 174 via line 221. The Q output of fault flip-flop 174 will now become a ground potential and its Q output becomes a positive potential.

As previously discussed, when the output of comparator 202 is a positive potential, relay flip-flop 178 is reset so that its Q output is a positive potential which is applied to the base of transistor 196 causing it to become conductive. The emitter of transistor 196 is connected to the Q output of fault relay which is at a ground potential. This causes transistor 196 to become conductive, energizing pilot light emitting diode 198. Simultaneously, the output of NAND gate 194 is a positive potential in response to the ground potential generated at the output of inverter 250. The input to inverter 250 is connected to B+ line through a normally closed contact 252 of relay 40. The input to inverter 250 is also connected to ground through resistor 254.

The other input to NAND gate 216 is connected to the Q output of fault flip-flop 174 which also is a positive potential. With both inputs to NAND gate 216 being positive, its output will be a ground potential, rendering transistor 218 nonconductive de-energizing fault light emitting diode 204. This places the circuit in its normal mode of operation prior to breaking the infrared light beam between the infrared light emitting diode 30 and phototransistor 32.

In the event of a high ambient infrared illumination being present which would prevent normal operation or unreliable operation of the circuit, the circuit 38 includes a high ambient infrared fault detection circuit responsive to the high ambient infrared illumination to disable relay 40 and to actuate the fault light emitting diode 204. This fault detection circuit consists of comparator 200 which has its positive input connected to a voltage divider consisting of resistors 226 and 228 connected between B+ line 124 and ground. The negative input to comparator 200 is connected to the junction between phototransistor 32 and resistor 142. Under normal operating conditions, the peak or maximum potential at the junction between phototransistor 32 and resistor 142 is less than the potential at the positive input to comparator 200 so that the output of comparator 200 is a continuous positive potential. This positive potential at the output of comparator 200 is applied to one input to NAND gate 230 and to the anode of pilot light emitting diode 198, as shown in FIG. 4B, via line 232, as previously described. The other input to NAND gate 230 is connected to B+ line 124 through resistor 234. When the output of comparator 200 is a positive potential, NAND gate 230 will produce a ground potential at its output which is inverted to a positive potential by inverter 236. This positive potential at the output of inverter 236 is applied to one input of NAND gate 182. The other input to NAND gate 182 is connected to the B+ line 124 through resistor 238 and to the output of inverter 168 through capacitor 180.

When the ambient infrared illumination exceeds a predetermined level, the potential at the junction beween the phototransistor 32 and resistor 142 becomes greater than the potential at the positive input to comparator 200 causing its output to become a ground potential. This ground potential de-energizes pilot light emitting diode 198 and causes the output of NAND gate 230 to become a positive potential which is inverted to a ground potential by inverter 236. The output of NAND gate 182 will now become a positive potential which resets relay flip-flop 178 deactivating the relay 40, as previously described. Because the output of comparator 200 on line 232 is a ground potential, the fault light emitting diode 204 will be activated by diode 240 having its anode connected to fault light emitting diode 198 through resistor 242 and its cathode connected to the ground potential at the output of comparator 200 by means of line 232.

When one or more of the contacts in relay 40 becomes welded or stuck in the activated position, normally closed contact 252 of relay 40 connected to the input of inverter 250 will be stuck in a normally open state, terminating the positive potential at the input to inverter 250. The input to inverter 250 will now become a ground potential and its output will become a positive potential. This positive potential causes the output of NAND gate 194 to become a negative potential which in turn causes the output of NAND gate 216 to become positive. The positive output of NAND gate 216 causes transistor 218 to become conductive, energizing fault light emitting diode 204.

When the windows between the light emitting diode 30 and the phototransistor 32 are dirty or the light beam is otherwise continously occluded, the potential across capacitor 160, FIG. 4A, will be a ground or near ground potential. This places a ground potential at the input of inverter 164 and a positive potential at the input of inverter 176 which produces a ground potential at the D input of relay flip-flop 178. The ground potential across capacitor 160 will cause inverter 168 to produce a positive clock pulse which will clock relay flip-flop 178 to have a ground potential at its Q output, deactivating relay 40, as previously described, and to have a positive potential at its Q output which is applied to the D input of fault flip-flop 174. The clock input from the output of inverter 164, via line 165, will clock the fault flip-flop 174 to its set state with its Q output assuming a positive potential and its Q output assuming a ground potential. As previously described, when the Q output of fault flip-flop 174 is a positive potential, pilot light emitting diode 198 is de-energized, and when the Q output of flip-flop 174 is a ground signal, fault light emitting diode 204 is energized. Also, the positive output of inverter 164 is applied to the input of inverter 220 via diode 244 causing its output to become a ground signal. This ground signal is applied to the RESET input of fault flip-flop 174 via capacitor 246 and line 221 to assure fault flip-flop 174 will be clocked by the leading edge of the clock pulse received from the output of inverter 164.

The different time constants of the RC circuits connected to the D input of relay flip-flop 178, in particular the first RC circuit comprising capacitor 160 and resistance 162 and the second RC circuit comprising capacitor 170 and resistance 172, also constitutes an anti-teasing circuit which prohibits the operator from trying to produce an abnormal mode of operation of the photoelectric switch 10. The photoelectric switch 10 is intended for applications in which the infrared light beam is completely broken in less than 20 milliseconds by placing one or more fingers on the finger rest surface 24 between the finger guides 26 and 28. The operator may try to tease the photoelectric switch 10 by slowly edging his finger along the finger rest surface to slowly reduce the intensity of the infrared light beam from its maximum value to zero. To prevent abnormal operation of the light switch, the decay rate of the first RC circuit is selected to be significantly shorter than the decay rate of the second RC circuit, as shown in FIG. 6, so that the clock pulse generated by the first RC circuit will occur at least 20 milliseconds before the potential generated by the second RC circuit decays to a value less than that required to toggle the relay flip-flop 178 to change state in response to the clock signal. If the intensity of the light signal received by the photo diode is slowly decreased at rate equal to or less than the decay rate of the second RC circuit, then both RC circuits will decay at approximately the same rate and the clock signal will be generated when the potential at the D input of relay flip-flop 178 is below the potential required to cause relay flip-flop 178 to change state in response to the clock signal. Therefore, when the infrared light beam is slowly broken, i.e. teased, the relay flip-flop will not change state and the relay 40 will not be energized.

In summary, the electronic circuit for the photoelectric switch 10 is a DC relay drive circuit which is easily presettable for a predetermined minimum voltage and easily set to eliminate relay chattering. This circuit is relatively immune from noise and input line transients. The photoelectric switch has at least four fault detection circuits which inhibit the activation of the relay 40 and will energize or light a fault indicator when the input AC voltage is below the minimum operating voltage of the device being controlled, when the ambient light level is above predetermined levels which would interfere with the normal operation of the photoelectric switch, when one or more contacts of the relay become welded or stuck in the energized position, or when the windows in the line of sight between the infrared light emitting diode become dirty or are continuously occluded.

It is not intended that the invention be limited to the exact embodiment shown in the drawings or discussed in the specification. It is acknowledged that those skilled in the art can make certain changes and improvements thereto without departing from the concept of the invention as described above and set forth in the appended claims.