Patent GB-1253804-A (GB)
Title: Control of traffic lights
Inventor: STUBBS RAYMOND FREDERICK
IPC: G08G1/08
Filed: 1967-09-21
Abstract: 1,253,804. Road traffic signals. D. SIGNS & MAINTENANCE Ltd. 19 Dec., 1968 [21 Dec., 1967], no. 42945/67. Heading G4Q. In a control system for controlling first and second traffic signals, having associated vehicle sensing means, by means of first and second sets of solid state multivibrators, each set of multivibrator circuits includes a multivibrator for each light in the associated traffic light signal which multivibrator circuit produces an output signal to energize that light when in one of its two states and a multivibrator circuit corresponding to the associated vehicle sensing means which multivibrator circuit is set in one of its two stable states by an input signal from the vehicle sensing means when triggered by a vehicle and the multivibrator circuits are interconnected so that a predetermined cycle of operations is triggered by the multivibrator corresponding to the vehicle sensing means and so that the cycle of operations can only proceed in one set of multivibrator circuits when it is not proceeding in the other set of multivibrator circuits. The control system may be portable and may be used at road works for single line working or at cross-roads and road junctions for temporary replacement of faulty permanent traffic lights. Traffic control.-The red, amber and green lights are turned ON by their associated bistable, monostable and bi-stable circuits respectively being in an ON state. The lights are energized by relays, the coils of which are energized by output transistors fed from driver transistors so as not to load the multivibrator circuits. When a vehicle is detected by the road pad or interruption of an infra-red beam at say the left-hand end of the road and assuming both the red lights are ON the bi-stable memory associated with the left-hand end switches ON. This causes the amber monostable circuit associated with the left-hand end to be triggered ON and turn ON the amber light so that both left amber and red lights are ON. After a delay the monostable circuit goes OFF and causes the bi-stable circuits associated with the left red and green lights to turn OFF and ON respectively so as to turn OFF the red and amber lights and turn ON the green light. If no other vehicle arrives at the left end a unijunction transistor or double base diode which is charged from a constant current source when the green signal is ON is fired after the required delay time and the amber light is then turned ON for a period followed by the red light turning ON. Also the left-hand demand memory is reset OFF and at the end of the cycle a monostable left-hand red hold circuit is turned ON for a delay time so as to prevent the amber signal at the other end being lit until the predetermined delay time has ended. AND gates 2 are associated with the demand memories and amber circuits at each end to prevent the go cycle being started at the associated end if the go cycle is in operation at the other end. Delay circuits ensure that the amber circuit is always switched ON before the demand memory is reset to ensure correct operation. Extension of green time.-If further vehicles are detected at the left-end pad while the left green signal is still ON a monostable green extend circuit is triggered ON and disconnects the charging circuit from the injunction transistor for a time period determined by the green extend circuit so that the green period is extended. A three-position switch adjusts the extension. Maximum green time-Outputs occurring from each demand memory are fed via AND gates 3 to turn ON a bi-stable hold memory which allows a unijunetion interrupt circuit at the end where the green signal is ON to be charged until the unijunction fires to terminate the green period. Also the output from the hold memory inhibits AND gate 1 so that on the left green signal going OFF the demand memory at that end is not reset. This enables the lights and interrupt circuit at the other end to start operating so that the lights at that end are only held for a time determined by that interrupt circuit before the signals at the left end are again reset. As a safety precaution the hold memory may also interrupt the power to the pads and also the hold memory may interrupt the supply of extend pulses to the green extend circuit a predetermined time after the hold memory is activated so as to reduce the maximum green time below that set by the interrupt circuits. Light traffic flow.-When the gap between vehicles is large on changing of the signals at one end from green, through amber to red the control would normally pass to the other end. However on detection of another vehicle at the one end the outputs from the demand memory and red signal at that end are passed via AND gate 4 to cause the amber lamp at that end to re-light and the cycle to repeat. If however a vehicle is detected at the other end that AND gate 4 is inhibited and control passes to the other end. Time controlled.-By operating switch S the unijunction transistors in the green circuits are disconnected, the pad power is disconnected and the demand memories are activated to hold them ON so that the green light is turned on for a fixed time at each end which is determined by the interrupt circuits.