CA2038898A1 - Out-of-bounds monitor for court games - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus for monitoring a designated surface area of a playing court comprises a plurality of transmitters for producing electromagnetic pulses alongparallel paths in a plane paralleling the area and in close proximity thereto, the paths extending between opposite ends of the area and being transversely spaced apart by a distance which is less than or equal to the diameter of a playing ball used in the court, the transmitters being disposed along one of the ends of the plane, a plurality of receivers, one associated with each transmitter to form a transmitter-receiver pair, disposed at the opposite end of the plane from its associated transmitter and along the path of the pulse produced by its associated transmitter and being operable to produce an output signal upon detecting a pulse emitted by its associated transmitter, a control mechanism for energizing each transmitter-receiver pair in predetermined timed sequence while maintaining all other transmitter-receiver pairs in an inactive state whereby only the pulse produced by the associated transmitter is present when a receiver is active, anda circuit responsive to the absence of output signal from one of the receivers at a predetermined time for producing an alarm activating signal.

Description

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The present invention relates to an apparatus for monitoring an area, such as an out-of-bounds area, of a playing coure such as a squash or racquetball court.

BACKGROUNDOFT~EINVENTION
Electrically operated detection devices have been used extensively in a variety of applications, such as line monitors for tennis, intrusion detectors and the like. These devices typically use light or ultrasonic beams transmitted across an area to be monitored. Interruption of one or more beams triggers an alarrn. A
plurality of parallel beams are required to cover a large area. Typically, each beam is produced by a light emitter or transmitter positioned on one side of thearea. A receiver is positioned on the other side of the area and is aligned withthe transmitter along the path of its associated transmitter. The spacing of thebeams is determined by the size of the object to be deteceed. If the object to be detected is very small, the spacing of the beams must be comparably small. Less expensive emitters are not highly focused and, therefore, produce a divergent beam. As a result, if the path between a transmitter and its associated receiveris blocked, then a portion of the beam of an adjacent transmitter could impinge on the associated receiver and not detect the interruption. An obvious solution to this problem is to use highly focused emitters so that there is no interference 2û between adjacent transmitter-receiver pairs. However, this solution results in greater expense which may be unnecessary. Other known solutions to this problem have been at the expense of greater complexity.
A number of devices have been developed for monitoring base, side and service lines on tennis courts. The problems associated with tennis courts are different from those associated with an out-o-f-bounds area of squash, racquetball and like courts. Tennis balls are of relatively large diameter. This allows a single beam to be projected along a line to be monitored. Further, because the angle of approach of a tennis ball to the ground is often small in comparison to the angle at which a squash ball approaches the front wall of a squash court, the 3n tennis ball will interrupt at least one beam even if the beams have a spacing equivalent to the diameter of the ball.

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Canadian Patent No. 1,168,667 granted on June 12, 1g88 to Carlton et al discloses an electrically operated line-monitor for tennis and provides an apparatus in which a transmitter positioned at one end of the critical line directs a master beam or ray of infra-red light to a receiver positioned at the other end 5 of the critical line. The receiver operates an alarm when the beam is broken.
Subsidiary beams controlled by a master beam are provided over an area adjoining the critical line. Interruption of the master ray prevents the receivers associated with the subsidiary rays from activating the alarm. The distance between the axis of any other rays at their receivers need not be less than the 10 diameter of a tennis ball. This patent does not disclose any way in which theapparatus could be adapted to monitor a large planar surface such as that of a squash court where the beam spacing must be small not only because the size of the ball is small but also because the beams have to be spaced less than the diameter of the ball. Furthermore, the patent does not disclose a circuit 15 configuration for the receivers or transmitters.
United States Patent No. 4,528,5~8 granted on July 9, 1985 to Oberan discloses a device for determining the validity of a kill shot in racquetball orhandball. The patentee is particularly concerned with determining the sequence of contact of a ball with the front wall and the floor. Light source and ]ight 20 receptor elements are disposed along the side walls of the court in close proximity to the juncture of the sidewalls and the floor. In order to avoid interference between iight beams traversing the front wall with those traversing the floor and determine whether the ball hit the floor or wall first, the beams along the front wall are pulsed at a different frequency from that of the beams across the floor.
25 The wall receptor elements are responsive only to beams produced by the wall transmitters whereas the floor receptor elements are responsive only to beams produced by the floor transmitters. Various methods are suggested to achieve this principle. One method uses Infrared light emitting diodes pulsed at different frequencies and phototransistors associated with the receptor elements are tuned30 to the frequency of their respective light emitting diodes, thus preventing interference between the wall and floor light curtains. Another method uses polarized lenses for the front wall and the floor. Oberan is primarily concerned " 2~3~8 with avoiding interference between the wall and the floor light curtains. In thelight curtains he discloses, the receivers receive light from a plurality of transmitters. The alarm is tripped by the received light level being reduced slightly. Furthermore, receivers and transmitters are required on both sides of the court to provide full light coverage by the fan shaped rays.
United States Patent 3,810,148 granted on May 7, 1974 to Karsten et al discloses an indicator apparatus for detecting and indicating the presence of anobject such as a ball, on a line or boundary of a tennis court. The apparatus provides six parallel, pulsed beams of light disposed on either side of a line or boundary being monitored. The beams are produced simultaneously. The patent provides complex circuitry to distinguish between good and bad shots and uses a relatively ~omplex arrangement of mirrors and collimating lens to focus light from a transmitter onto its associated receiver and a condensing lens at the receiver to collect the light. Thus, while this arrangement may overcome the interference difficulty mentioned earlier, it does so at the considerable expense.
United States Patent No. 3,746,863 granted on July 17, 1973 to Pronovost is not particularly concerned with monitoring shots in ball games but is concerned with the problem of interference between adjacent beams of a light curtain used to control a switch. The solution proposed by the patent is to transmit light from one side of the area being monitored to the other side where it is received and re-transmitted back, at a different horizontal level, to the first side. This process continues until all the transmitters have been activated and is then repeated.
Pronovost's invention does permit the use of relatively non-sharp or non-focusedlight and allows for misalignment of the two columns, but overlooks the possibility of having all the transmitters on one side of the area being monitored and all the receivers on the other side. He than has to deal with the possibility that light will be received by more than one receiver in the same column and be re-transmitted out of sequence. He presents several possibilities to correct this but misses the possibility of totally separating the transmitters from the receivers.
There is also the potential of Pronovost's system malfunctioning due to a reflection from an object. The signal coul~ be transmitted from one column, reflected from the object, back to the same column, to an enabled receiver where ~3~

it is re-transrnitted, above or below the interfering object, to the opposing column.
It is not really known how much of a problem this really is, since the object may eventually be detected by another transmitter-receiver pair. Having the transmitters and receivers separated by the area being monitored avoids this issue 5 altogether.
In summary, the prior art does not provide a simple, economical, easily-aligned system for monitoring an out-of-bounds area in an indoor racket sport.

10 SUMMARY OF T~E IN~115NTION
The present invention seeks to provide a simple and economical area monitoring apparatus which is easily aligned and avoids the problem of interference caused by closely spaced, non~focused light beams. This is achievedby providing a control circuit which energies only one transmitter-receiver pair at 15 a time but energies and de-energizes each pair in rapid succession so that, in the interval within which it is active, the active receiver can only receive the beam transmitted from its associated transmitter.
In accordance with the present invention, there is provided an apparatus for monitoring a designated surface area of a playing court comprising transmitter 20 means for producing electromagnetic pulses along parallel paths in a plane paralleling the area and in close proximity thereto, the paths extending betweenopposite ends of the area and being transversely spaced apart by a distance which is less than or equal to the diarneter of a playing ball used in the court, the transmitter means being disposed along one of the ends of the plane, receiver 25 means associated with each transmitter means to form a transmitter-receiver pair, each receiver means being disposed at the opposite end of the plane from its associated transmitter means and along the path of the pulse produced by its associated transmitter means and being operable to produce an output signal upon detecting a pulse emitted by its associated transmitter means, control means 30 for energizing each transmitter-receiver pair in predetermined timed sequencewhile maintaining all other transmitter-receiver pairs in an inactive state whereby only the pulse produced by the associated transmitter means is present when a '~ g ~ ~

receiver means is active, and means responsive to the absence of output signal from one of the receiver means at a predetermined time for producing an alarm activating signal.

BRIEF DESCRIPTION OF T~E DRA~iVINGS
These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:
FIGURE 1 is a broken, perspective view illustrating a portion of a ball court with an infrared beam transmitter module positioned in one side wall of the court adjacent the front wall and an infrared bearn receiver rnodule positioned in the opposite side wall in opposed relation to the transmitter module;
FIGURE 2 is an electrical schematic diagram of the infrared beam transmitter circuitry in accordance with a preferred embodiment of the present invention;
FIGURE 3 is an electrical schematic diagram of the infrared beam receiver circuitry in accordance with a preferred embodiment of the present invention; and FIGURE 4 is a timing diagram illustrating the relative timing of activation of the infrared transmitters.

DESCRIPTION OF PREFERR~:D EMBODIMENT
With reference to FIGURE 1, a ball playing court 10, such as a squash court or the like, includes a front wall 12, opposed side walls 14 and 16 and a floor 18. The present invention, generally designated by reference nurneral 20, comprises an infrared beam transmitter module 24 positioned in side wall 16 and an infrared beam receiver module 22 positioned in side wall 14. The front faces of the modules are flush with their respective walls so as not to interfere with play of the game. A synch cable 26 extends between the two modules to provide synchronization signals as described later. The cable may be imbedded in the front wall or floor or tacked to the corner between the front wall and the floor in 2~3~

any suitable manner. An electrical cord and associated ac adapter 28 extends from the receiver module for connection to a suitable a.c. source of electrical power. A further electrical connector 30 extends from the receiver and is connected to a suitable audio or visual alarm device 32.
The apparatus is adapted to monitor a generally rectangular out-of-bounds area 34 at the base of ~ront wall 12. It does so by producing a plurality of infrared light pulses from transmitters located in transmitter module 24 on the right side wall 16 to associated receivers located in receiver module 22 on the le-ft side wall 14. Each transmitter and its associated receiver is located at the same vertical distance from the top of area 34. The transmitter-receiver pairs are spaced at equal vertical spacings (not greater than the diameter of the ball) sothat the entire area 34 is sensitized to the intrusion of a ball. The transmitters are activated one at a time but in rapid succession with the timing between pulses being such that any given receiver is active only sufficiently long to receive the light pulse produced by its associated transmitter.
The light emitted from each transmitter need not be focussed or highly directional since only the rays spanning the shortest distance between the transmitter and its associated receiver are responded to. Other rays are ignored.
In this way relatively inexpensive infrared light em;tting diodes can be used as a light source. In the current embodiment, the rays run parallel to the top of area 34 since the transmitter and its associated receiver are at the same height above the floor 1~. The receivers also need not be focussed or highly directional. A
reflection from the wall on the horizontal path can also be detected by the receiver, but will be interrupted along with the ray in the direct path when a ball enters the region.
The vertical alignment of the transmitters and receivers is not critical since the transmitted light is not focused or highly directional. The rays along the shortest path between the transmitter and the receiver will be detected. The most critical vertical adjustment is the transmitter-receiver pair which monitors the top of area 34. Here it is important that the rays follow the foul line.
FIGURE 2 illustrates the transmitter module 24. The transmitter module is comprised of two basic components, namely, light emitters 40 and a counter 42.

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For reasons of economy onl~, the emitters are split into two banks 44 and 46 of infrared light emitting diodes. Each bank includes ten diodes numbered lN to 10N. As indicated more fully below, in response to a system clock signal, the counter is operable to energize each diode in succession beginning with diode lNS of bank 44 and ending with diode 10N in bank 46.
The anodes of each of the ten LED's in bank 44 are connected together and to the collector of a bank-select transistor Tl while the anodes of each of the ten LED's in bank 46 are connected together and to the collector of a bank-select transistor T2. The emitters of bank-select transistors T1 and T2 are connected to 10 the positive terminal of a twelve volt DC supply (see FIGURE 3), via a current limiting resistor 48. The base of each transistor T1 and T2 is connected to one of the two outputs of a bank select circuit 50. Circuit 50 is arranged to receive a reset signal along a line 52, a clock signal along a line 54 and a counter output signal along a line 56. Each of the bank-select transistors is turned ON by taking 15 its base to a lower voltage potential than its emitter. By turning ON either the bank select transistor Tl or T2, the corresponding one of banks 44 and 46, respectively, is selected. Transistor T1 is turned on when a reset signal is applied to the bank select circuit. Each diode in banlc 44 is energized in succession and in synchronism with the system clock signal and in accordance with the count of 20 decade counter 4~ which also receives the c]ock signal along line ~8. At the end of its first cycle, the counter outputs a signal along line 56 which causes circuit 50 to switch transistor Tl OFF and transistor T2 ON to thereby de-energize bank 44 and energize bank 46. Each diode in bank 46is then energized in succession and in synchronism with the system clock signal and in accordance with the count of 25 decade counter 42.
The cathode of each LED bank 44 is connected to the cathode of its correspondingly numbered LED in bank 46 and each thus connected pair of LEDs is connected to the collector of one of ten drive transistors labelled T3 to T12, which operate as normally open switch means. The base of each drive 30 transistor is connected to one of the ten outputs labelled Q1 to Q10 of decade counter 42. The decade counter sequentially switches each of its outputs high and then low to thereby sequentially turn ON and then OFF each drive transistor T3 ~ B ~ g to T12 as the counter cycles through a complete count. The output derived from the tenth count of the decade counter 42 triggers either the selection of bank 44 or 46, as already explained. A single-pole, eight-way rotary switch 60 provides the option of reducing ~he number of active LEDs to a number between thirteen and twenty in the event that a smaller number of LEDs is required. The switch has each of its normally-open contacts permanently connected to the outputs numbered Q3 to QlQ at the decade counter. The common contact of the switch is logically ANDED by an AND gate 62 and the bank 44 select output to provide a RST signal that resets the decade counter when both the lower bank is selectedand the count prescribed by the switch has been reached. A reset signal RST is generated once after every total count ranging from thirteen to twenty. The output from the AND gate 62 also provides a system reset signal and is deliveredto the receiver circuit.
The receiver circuit will now be described by reference to FIGURE 3. By way of overview, the receiver circuit is generally comprised of twenty detectors, each of which is operable to sense light emitted by an associated LED 40 in the transmitter module 24 and, thus, forms with that LED a transmitter-receiver pair.
A counter selects each detector in sequence and in synchronism with the selection of that detector's associated transmitter in the manner described earlier. All other detectors are thus rendered inactive during this period. The output of the detector is compared against a predeterrnined threshold level. A pulse is produced if the detected pulse is high; no pulse is produced if the detector output is low. If no pulse is produced in synchronism with the clock signa], an alarm is energized.
The receiver unit is powered by a power regulator 70, having an input supply of twelve volts DC from an unregulated ac adaptor 28, and a first output 72 of five volts DC, a second output 74 of seven volts DC and a third output 76 of twelve volts DC which is delivered to the transmitter module over cable 26.
The detectors are in the form of an array of twenty PIN photo-diodes labelled I)1 to D20, each having their anodes connected to an associated one of twenty FET
op-amp preamplifiers labelled Al to A20, respectively. The outputs of each of the twenty preamplifiers, are multiplexed onto a common line or junction 78 by three ~o3~(7in~

multiplexers 80, 8~ and 84, which operate as normally open switches. Only the selected switch is closed. Standard off-the-shelf analog multiplexers are available in an eight input single output configuration. Hence, to accommodate the twenty inputs in this output configuration, three multiplexers are required. Two of the5 three multiplexers have all eight inputs used and the third multiplexer has only four of its inputs used. Junction 78 is connected to the input to a buffer amplifier 86 whose output is applied to one input of a comparator 90.
A two-decade counter 92 and associated circuitry (not shown) are operable to select the appropriate multiplexer and route the appropriate one of its inputs 10 to its single output terminal 78. Two-decade counter g2 is reset by a RST signal received over cable 26 from AND gate 62. A system clock oscillator 94 provides a clock signal at a frequency, fs, of 28 kHz and delivers this signal to counter 92, a missing pulse detector circuit 100 and to the transmitter module 24 over cable26.
The other input 102 to the comparator is a preset sensitivity threshold level signal. If the input signal to the comparator has an amplitude greater than the threshold, the comparator outputs a high level signal. It will be noted that theinfrared beams are transmitted as pulses; hence, the photodiodes output pulses which, in turn, translate into pulses at the comparator output. Hence, if a pulse 20 is at a lower amplitude than the threshold leve] set at the comparator, the comparator will not output a high pulse. For the purpose of this discussion, this output will be termed a "missing pulse". The output of comparator 90 is fed intomissing pulse detector circuit 100. The missing pulse detector 35 is synchronized with the system clock oscillator to look for a "missing pulse" only on the 25 occurrence of a clock signal and to activate alarm 32 if a "missing pulse" occurs.
A timer (not shown) associated the missing pulse detector circuit, activates thealarm for a fixed period of time such as one second, from the detection of the "missing pulse".
FIGUl~E 4 is a diagram illustrating the timing waveforms and control 30 signals used to control the transmitters and receiver. The clock signal waveform is shown generally by numeral 110. Each infrared LED in the transmitter and its associated detector in the receiver is energized for about thirty-six microseconds o ~ ~ ~

in sequence from the first LED/photodiode pair to the last LED/photodiode pair.
These pulsed signals are shown generally by numeral 112. At the end of the last pulse, RST reset signal is generated to restart the counting sequences in both the transmitter and receiver.
The clock frequency is selected so as to allow the entire area 34 to be scanned at least once in the time it would take a ball to break the plane containing the beam paths, impact on the wall and return through the plane.
Those skilled in the art will appreciate that this time is a function of the elasticity of the ball and of the distance of the plane, containing the beam paths, from the 1() wall. For the particular ball used and the number of transmitter-receiver pairs, a clock frequency of 28 kHz proved to be suitable. The transmitter-receiver pairs are positioned about 1/4 to 1/2 inch from the front wall and spaced apart by a distance not greater than the diameter of the ball.
It will be seen from the foregoing that the present invention provides a structurally and operationally simple area monitoring apparatus which is assembled from commercially available products and which overcomes the interference problems addressed earlier.
It will be understood that various modifications may be made to the invention without departing from the spirit of the present invention. For example, the three multiplexers in the receiver circuit could be replaced by a single multiplexer having twenty or more inputs thereby further simplifying the circuitry associated with the two-decade counter.

Claims (21)

1. An apparatus for monitoring a designated surface area of a playing court comprising:
transmitter means for producing electromagnetic pulses along parallel paths in aplane paralleling said area and in close proximity thereto, said paths extending between opposite ends of said area and being transversely spaced apart by a distance which is less than or equal to the diameter of a playing ball used in said court, said transmitter means being disposed along one of said ends of said plane;
receiver means associated with each said transmitter means to form a transmitter-receiver pair, each said receiver means being disposed at the opposite end of said plane from its associated transmitter means and along the path of the pulse produced by its associated transmitter means and being operable to produce an output signal upon detecting a pulse emitted by its associated transmitter means;
control means for energizing each said transmitter-receiver pair in predetermined timed sequence while maintaining all other transmitter-receiver pairs in an inactive state whereby only the pulse produced by said associated transmitter means is present when a receiver means is active; and means responsive to the absence of output signal from one of said receiver means at a predetermined time for producing an alarm activating signal.

2. An apparatus as defined in claim 1, said control means including means for producing timing signals of predetermined duration and at a predetermined frequency.

3. An apparatus as defined in claim 2, said control means including means for simultaneously energizing said transmitter means and said receiver means of eachof said transmitter-receiver pair in synchronism with said timing signal.

4. An apparatus as defined in claim 3, said energizing means including:
counter means having a plurality of output terminals and being responsive to said timing signals to produce an output signal at each said plurality of output terminals in sequence and in synchronism with said timing signals; and a plurality of normally open switch means, each said switch means being associated with one of said counter output terminals and one of said transmitter means and being responsive to an output signal at its associated counter terminal to energize its associated transmitter means.

5. An apparatus as defined in claim 4, said timing means being an oscillator.

6. An apparatus as defined in claim 4, each said switch means being a drive transistor, said transmitters means being infrared light emitting diodes and said receiver means being PIN photodiodes.

7. An apparatus as defined in claim 4, each said transmitter means including a plurality of infrared light emitting diodes equally spaced apart by said distance along said one end of said plane, said diodes being divided into first and second banks of diodes, said control means including bank select circuit means responsive to said timing signals and a predetermined count of said counter means for alternately activating one of said banks and de-activating the other of said banks to permit all diodes in said one bank to be energized and de-energized in sequence.

8. An apparatus as defined in claim 7, further including a first bank select switch means for connecting all of the diodes of one of said banks to a source of power and a second bank select switch means for connecting all of the diodes of the other of said banks to said source of power, said first and second switch means being responsive to first and second control signals produced by said bankselect circuit means, respectively.

9. An apparatus as defined in claim 8, said first and second banks having equal numbers of diodes, each diode in one of said banks being associated with a diode in the other of said banks and being connected to a common one of said plurality of normally open switch means.

10. An apparatus as defined in claim 3, said energizing means including:
normally open switch means associated with each said receiver means, said switchmeans being operable to connect the outputs of their associated receiver means to a common junction; and counter means for selectively closing and opening each said switch means in sequence and in synchronism with said timing signals so that the output of only one of said receiver means is connected to said common junction at a time.

11. An apparatus as defined in claim 10, said means responsive to the absence of receiver output signal further including:
a comparator having a first input terminal connected to said common junction, a second input terminal having a threshold signal applied thereto, and an output terminal, said comparator being operable to produce a first output signal when the signal at said first input terminal is greater than said threshold signal and a second output signal when the signal at said first input terminal is less than said threshold signal; and detector means for producing an alarm activating signal upon simultaneously receiving a timing signal and said second comparator output signal.

12. An apparatus as defined in claim 10, said normally open switch means being at least one multiplexer.

13. An apparatus for monitoring a designated surface area of a playing court comprising:
a plurality of transmitter means for producing electromagnetic pulses along parallel paths in a plane paralleling said area and in close proximity thereto, said paths extending between opposite ends of said area and being transversely spacedapart by a distance which is less than or equal to the diameter of a playing ball used in said court, said transmitter means being disposed along one of said endsof said plane;
a plurality of detector means disposed along the other of said ends of said plane, each said receiver means being associated with one of said transmitter means to form a transmitter-receiver pair and being operable to produce an output signal upon detecting an electromagnetic pulse emitted by its associated transmitter means along one of said paths; and control means for energizing each said transmitter-receiver pair in predetermined timed sequence while maintaining all other transmitter-receiver pairs in an inactive state so that the only the pulse produced by said associated transmitter means is present when an energized receiver is active, said control means including means for producing timing signals of predetermined duration and at a predetermined frequency, means for sequentially energizing said transmitter-receiver pairs in synchronism with said timing signal including first counter means having a plurality of output terminals and being responsive to said timing signals to produce an output signal at said plurality of output terminals in sequence and in synchronism with said timing signals, a first plurality of normally open switch means, each said switch means being associated with one of said counter output terminals and one of said transmitter means and being responsive to an output at its associated terminal to energize its associated transmitter means, a second plurality of normally open switch means associated with said receiver means, each said switch means of said second plurality of normally open switch means being operable to connect the outputs of their associated receiver means to a common junction, and second counter means responsive to said timing signalsfor closing and then opening each said switch means in sequence and in synchronism with said timing signals so that the output of only one of said receiver means is connected to said common junction at a time; and means responsive to the absence of receiver output signal at a predetermined time for producing an alarm activating signal.

14. An apparatus as defined in claim 13, said timing means being an oscillator.

15. An apparatus as defined in claim 13, each said switch means of said first plurality of normally open switch means being a drive transistor, said transmitters means being infrared light emitting diodes and said receiver means being PIN
photodiodes.

16. An apparatus as defined in claim 13, each said transmitter means being an infrared light emitting diode equally spaced apart by said distance along said one end of said plane, said diodes being divided into first and second banks of diodes, said control means including bank select circuit means responsive to said timingsignals and a predetermined count of said counter means for alternately activating one of said banks and de-activating the other of said banks to permit all diodesin said one bank to be energized and de-energized in sequence.

17. An apparatus as defined in claim 16, further including a first bank select switch means for connecting all of the diodes of one of said banks to a source of power and a second bank select switch means for connecting all of the diodes of the other of said banks to said source of power, said first and second switch means being responsive to first and second control signals produced by said bankselect circuit means, respectively.

18. An apparatus as defined in claim 17, said first and second banks having equal numbers of diodes, each diode in one of said banks being associated with a diode in the other of said banks and being connected to a common one of said first plurality of normally open switch means.

19. An apparatus as defined in claim 13, said means responsive to the absence of receiver output signal further including:
a comparator having a first input terminal connected to said common junction, a second input terminal having a threshold signal applied thereto, and an output terminal, said comparator being operable to produce a first output signal when the signal at said first input terminal is greater than said threshold signal and a second output signal when the signal at said first input terminal is less than said threshold signal; and detector means for producing an alarm activating signal upon simultaneously receiving a timing signal and said second comparator output signal.

20. An apparatus as defined in claim 13, said normally open switch means being at least one multiplexer.

21. An apparatus for monitoring a designated surface area of a playing court comprising:
a plurality of infrared light emitting diodes for producing infrared light pulses along parallel paths in a plane paralleling said area and in close proximity thereto, said paths extending between opposite ends of said area and being transversely spaced apart by a distance which is less than or equal to the diameter of a playing ball used in said court, and said transmitter means being equally spaced apart along one of said ends of said plane and being divided into first and second banks of diodes;
a plurality of PIN photodiodes equally spaced apart along the other of said ends of said plane, each said photodiodes being associated with one of said light emitting diodes to form a transmitter-receiver pair and being operable to produce an output signal upon detecting a light pulse emitted by its associated light emitting diode along one of said paths; and control means for energizing each said transmitter-receiver pair in predetermined timed sequence while maintaining all other transmitter-receiver pairs in an inactive state so that the only the pulse produced by said associated light emitting diode is present when an energized photodiode is active, said control means including an oscillator for producing timing signals of predetermined duration and at a predetermined frequency such that all of transmitter-receiver pairs are energized at least once in the time in which a ball crosses said plane, rebounds from said area and returns through said plane, means for sequentially energizing said transmitter-receiver pairs in synchronism with said timing signals including:
first counter means having a plurality of output terminals and being responsive to said timing signals to produce an output signal at said plurality of output terminals in sequence and in synchronism with said timing signals;
a plurality of drive transistors operable as normally open switch means, each said drive transistor being associated with one of said counter output terminals and one of said light emitting diode and being responsive to an output at its associated terminal to energize its associated light emitting diode;
bank select circuit means responsive to said timing signals and a predetermined count of said first counter means for producing first and second control signals for alternately activating one of said banks and de-activating the other of said banks to permit all diodes in said one bank to be energized and de-energized in sequence;
a first bank select transistor responsive to said first control signals for connecting all of the diodes of one of said banks to a source of power and to said second control signals for disconnecting all of the diodes of one of said banks from said source of power;
a second bank select transistor responsive to said first control signals for disconnecting all of the diodes of the other of said banks from said source of power and to said second control signals for connecting all of the diodes of the other of said banks to said source of power;
multiplexer means operable as a second plurality of normally open switch means associated with said photodiodes, said multiplexer means having a plurality of input terminals each connected to the output of one of said photodiodes and a single output terminal connected to a common junction; and second counter means responsive to said timing signals for causing said multiplexer means to transmit in sequence and in synchronism with said timing signals the output of each said photodiodes to said common junction whereby the output of only one of said photodiode is connected to said common junction at a time; and means responsive to the absence of receiver output signal at a predetermined time for producing an alarm activating signal including:
a comparator having a first input terminal connected to said common junction, a second input terminal having a threshold signal applied thereto, and an output terminal, said comparator being operable to produce a first output signal when the signal at said first input terminal is greater than said threshold signal and a second output signal when the signal at said first input terminal is less than said threshold signal; and detector means for producing an alarm activating signal upon simultaneously receiving a timing signal and said second comparator output signal.