Patent GB-1248620-A (GB)
Title: Electronic time period switching circuits
IPC: H03K17/955
Filed: 1969-07-07
Abstract: 1,248,620. Transistor time delayed switches. WAGNER ELECTRIC CORP. 7 July, 1969 [8 July, 1968], No. 34122/69. Heading H3T. [Also in Division G1] A timer controlling an output switch, more particularly for actuating a urinal flusher after the departure of the user and operated from A.C. supply terminals comprises a thyristor or equivalent switching device connected across the terminals and which is held off during the variable discharge period of a capacitor which was charged while an input switch was operated by an input signal, e.g. due to the presence of the user, the supply to the gate electrode of the thyristor device being reduced while the output switch is operated. As described, the input signal is produced by a capacitance responsive sensor. In Fig. 1, a complementary transistor switch circuit simulating a thyristor is normally held conductive by turn-on pulses supplied by an oscillator 30 so that a relay 54 is prevented from operation, positive half-cycles of the A.C. supply 24, 26 thereto being blocked by diodes 50 and 70 and negative half-cycles by-passed by the switch circuit. When a body is sensed relay 14 operates and a contact 16, 20 allows a timing capacitor 90 to charge to a value limited by Zener conduction of transistor 44 and sufficiently to cut-off the transistor circuit, but relay 54 is not operated as its supply is disconnected at 18. Upon release of relay 14 by the departure of the body the relay is energized and remains so until the timing and capacitor has so discharged that its voltage is insufficient to overcome the turn on pulses provided by oscillator 30. While relay 54 is operated its contact 60 disconnects a capacitor 76 from the circuit decreasing the magnitude of the turnoff pulses to the transistors so that a more complete discharge of the timing capacitor is necessary before the output relay releases. A gas tube 78 stabilizes the timing against voltage changes. In Fig. 4 transistor input switch 100, 102 normally short-circuits the timing circuit 74 and a thyristor simulating switch 44, 46 shortcircuits the output relay, the latter switch being maintained on by A.C. mains firing pulses applied through resistors 119 and 112. When a body approaches, pulses from sensor 10 decrease in magnitude allowing the input switch 100, 102 to become non-conductive so that the timing capacitor 130 charges. The thyristorlike switch is maintained non-conductive by the increase in turn-on signal level due to charge and discharge of capacitor 118. When the body departs the input switch again becomes conductive, the turn-on pulses fall in level so that the timing capacitor voltage prevents the output switch conducting, allowing the relay to operate until the capacitor discharges, the opening of contact 60 reducing the turn-on pulse level so that more complete discharge is necessary before turn-on occurs. Contact 62 is made while the relay is operated to so bias the input switch at capacitor 104 as to prevent the recharging of the timing capacitor. Switch 140 allows testing of the circuit without the timer operating a capacitor 121 prevents false operation by supply transistors. Fig. 5 again includes the transistor switches 100, 102 and 44, 46 of Fig. 4 is short circuit the timer 74 and the output relay 56. When a sensed body turns the input switch off the timer capacitor 122 charges but the output switch 44, 46 is held on by increased turn-on pulses due to current through a capacitor 118. When the body departs the turn-on pulse returns to the previous lower level and the switch is held off. The relay operates until discharge of the timing capacitor has occurred. More complete discharge is required as a result of a further reduction in turn-on pulse amplitude due to the disconnection at 60 of the turnon pulse path through resistor 168 while the output relay is operated. Switch 140 allows testing of the current without the deay and filter capacitors 158, 172 and 72 prevent false operation by power line transients. In Fig. 2 the timer 74 is normally disconnected at 18 by the input switch 12 and the output relay is by-passed by transistor output switch 44, 45. The presence of a body operates the input-switch allowing the timer capacitor 90 to charge. Resistor 98 is now connected in parallel with the turn-on pulse path through 96 to reduce the turn-on pulse amplitude so that the output switch 44, 46 quickly becomes conductive. However, although the relay operates a load 64 (such as a lamp) is not operated as a resistor 68 is connected and series therewith. When the body departs, contact 18 removes this resistor from the load path and as resistor 98 is no longer connected in parallel until the turn-on pulse path the capacitor must more completely discharge before the output switch conducts again. Fig. 3 (not shown) illustrates an alternative timing circuit for Fig. 2, including a gas-filled valve stabilizer and a diode connected across charging resistor 82 to increase the charging rate.