GB1018083A - Improvements in or relating to electrical timing devices - Google Patents

Abstract

1,018,083. Road-traffic signals. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. April 3, 1962 [May 4, 1961], No. 16146/61. Heading G4Q. [Also in Division H3] In a Miller integrator timing circuit for use in a road-traffic signalling system, the capacitive feedback path between the collector and base of a first transistor comprises the base-emitter path of a second transistor connected to function as an emitter follower. In the circuit of Fig. 2, Miller feedback from the collector of transistor TX2 to its base through emitterfollower TX1 is initiated by changeover of contacts X1, X2, capacitor C then charging negatively until the potential at the base of transistor TX3 goes positive with respect to earth, transistor TX3 then conducting and releasing differentially-energized relay Y. Capacitor C is normally discharged through resistor R4; a timing cycle is started by the change-over of contacts X1, X2 whereupon charging current for the capacitor C through resistor R1 causes transistors TX1, TX2 to be switched on, the fall of potential at the collector of transistor TX2 switching transistor TX3 off. C charges negatively until transistor TX3 conducts whereupon further charging ceases since the collector of transistor TX2 is then clamped at earth potential by transistor TX3. Transistors TX1, TX2 are consequently switched off and C remains charged to a fixed value until contacts X1, X2 are released whereupon it is discharged through resistor R4. By virtue of the further stage TX1, the charging rate for the capacitor C, even for long delays of the order of 10 seconds, can be made substantially independent of leakage current fluctuations in the transistor TX2. In a modified circuit, Fig. 3, the delay introduced may be made dependent upon the rate at which vehicles pass. In this circuit, the capacitor C is not discharged at the beginning of the timing interval but is charged to a voltage having a value between those established by the tapping points on potentiometers RC, RD. At the end of a timing interval when contacts X1, X2 are released the capacitor C is discharged through diode DA to the potential determined by potentiometer RD. Thereafter each arrival of a vehicle at a traffic signal showing " Red " causes momentarily operation of a contact X, thus partially discharging capacitor CA, the charge on which capacitor subsequently discharges through resistor RB. The state of charge of capacitor C thus depends upon the number of actuations of contact Z; however, it is prevented by conduction through diode DB from discharging below a level determined by the setting of potentiometer RC. Hence upon a subsequent actuation of contacts X1, X2 the time taken for capacitor C to charge to a voltage causing transistor TX3 to be switched on depends upon the number of previous actuations of contact Z.