GB690503A - Improvements in or relating to electrical timing circuits employing thermionic valves - Google Patents

Abstract

690,503. Road signals for controlling traffic. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd. July 3, 1951 [July 31, 1950], No. 19094/50. Class 118 (ii) [Also in Group XL(c)] Fig. 1 shows a timer circuit which may be used to control traffic signals, contacts S1 being controlled by the passage of vehicles over a detector in a traffic lane possessing right of way whereby the timer is re-set after the passage of a vehicle, so that the timing period is extended. Contacts S2 are controlled so that the timer is ineffective until right of way to a traffic lane has been granted. With contact Sl open and contact S2 closed the timing period is started, capacitor C1 is charged from the H.T. supply until the grid potential of valve V2 rises to such a level that valve V2 conduits, relay RA operates and its contact RA1 changes over. The grid potential of valve VI is thus driven negative but commences to rise as capacitor Cl discharges through resistor R1 until valve V1 conducts, its anode potential falls and valve V2 is thus rendered non-conductive due to the negative voltage applied to its control grid via rectifier MRA and capacitor C4, relay RA is released and the timing period is terminated. The duration of the timing period may be varied by adjustment of resistor R4. Due to the cross coupling between valves V2 and V1 the' latter valve is rendered conductive and a trigger action thus occurs between valves V1 and V2 with a period dependent upon the time constant of network R1, C1. If however contacts S1 are closed by the passage of a vehicle over a detector in a traffic lane possessing right of way positive battery is applied to the grid of valve V2 and the relay RA remains operative. The timer is thus re-set and a new timing period may be initiated by opening contacts S1. If contacts S2 remain open the timer remains operative. In a modification shown in Fig. 2 at the commencement of a timing period capacitor C10 is charged by grid current flow in valve V3 and valve V4 is rendered conductive by reason of the positive battery applied to its grid via rectifier MRB. Relay RB thus operates, contacts RB1 are opened and the grid of valve V3 is driven negative to render the valve non-conductive until capacitor C10 has discharged sufficiently via resistor R10 when valve V3 conducts and oscillates by reason of the feedback via timed transformer TR. The amplitude of oscillations are sufficient to render valve V4 non- conducting so that relay RB is released and the timing period is terminated. The timing period may be varied by adjustment of potentiometer R13. If contact S1 is closed capacitor C10 is recharged from positive battery, oscillations are not produced or are terminated and relay RB remains released. The operation of the timer may also be prevented by the shorting of the grid circuit winding of transformer TR by closing contacts S2.