GB902835A - Improvements in electronic detection and signalling systems - Google Patents

Abstract

902,835. Automatic light intensity control systems. MINNEAPOLIS-HONEYWELL REGULATOR CO. May 19, 1961 [May 19, 1960], No. 18406/61. Class 38 (4). [Also in Group XL (b)] An electrical detection and signalling apparatus for objects passing along a predetermined path comprises means for projecting a beam of radiation towards the path and means for receiving reflections from the path and from an object to be detected, when present, and generating corresponding signals which are arranged to be distinguishable in conjunction with means responsive to the signals generated as a result of reflections from the path to change the intensity of radiation from the beam projecting means in accordance with the changes in intensity of those reflections and means responsive to the signals generated as a result of reflections from an object to produce an output signal. The arrangement is primarily utilized to count the number of vehicles passing through a selected location on a highway. In the arrangement, Fig. 2, radiation from an infrared source and reflected from the path or by a passing vehicle, is received by a photo-cell, indicated at 10, the input of which is passed to an amplifying chain of transistors 22 . . . 30 including circuit elements 32, 34 tuned to a frequency of 400 c.p.s. The output of the transistor 30 is passed via a transformer 38 to oppositely connected transistors 42 and 44, constituting a full wave rectifier, and connected respectively by diodes 46, 50 to transistors 48, 52 and by oppositely poled diodes to transistors 56, 60-the transistors 48, 60 being further connected to transistors 74, 76 oppositely connected to the winding 84 of a relay 86. The bias voltages of the transistors 48, 52, 56, 60 are derived from a series arrangement of resistors 62, 64 ... 70, 72. The transistors 52, 56 are connected to transistors 78, 80 controlling relays 94, 98 having armatures electrically connected together, and engaging fixed contacts so that operation of relay 94 energizes the winding 108 of a servomotor 110 whilst a second motor 114 is driven in the opposite direction from its winding 112 by operation of the relay 98. The motors 110, 114 vary the position of the slider 116 of a potentiometer 118 to alter the bias of a transistor 126 controlling the intensity of an infra-red source 128 having its beam projected by a lens 130 on to the path or roadway. The beam is chopped at 400 c.p.s. by a shutter 132 rotated by a motor 134 driven from the source 120. Heater coils 136, 138 are connected to the source 110 by a thermostatic switch 140 in cold ambient conditions to prevent the formation of ice or snow on the optical elements. In operation, the normal output of the transistors 42, 44 is such that the transistors 52, 56 are biased over the associated series resistors 62, 72 to heavy conduction with transistors 78, 80. cut off and relays 94, 98 not energized. If the signal applied by the transistors 42, 44 increases above its normal value over a considerable period, the conductivity of the transistor 52 will be reduced and transistor 78 conducts so that relay 94 operates and the motor 110 is operated to move the slider 116 so that transistor 126 is effective to reduce the intensity of the source 128. If the signal from transistors 42, 44 is reduced, transistor 80 becomes conductive, operating relay 98 to switch in the motor 114 so that the transistor 126 is controlled to increase the beam intensity from the source 128. Capacitors 96, 100 connected between the base and collector electrodes of transistors 78, 80 prevent the response of relays 94, 98 to impulses of short duration from the transistors 52, 56.