Patent GB-1305190-A (GB)
IPC: G07F15/12
Filed: 1969-07-08
Abstract: 1305190 Coin feed time switch UNITED GAS INDUSTRIES Ltd 11 May 1970 [8 July 1969] 34365/69 Heading G4V [Also in Division H3] In a modification of the time switch described in Specification 1,057,241 in which insertion of a coin permits a differential gear to be rotated through an angle corresponding to the time paid for and indicated as such on a meter, the gear is rotated back to zero by an impulse motor 16 actuated by oscillations generated by an adjacent self-contained timing device. In one embodiment. the oscillations are produced by a complementary multivibrator, Fig. 2. The small current passed by the coil of solenoid 16 and the 88 megohm resistance R 1  is sufficient to maintain the lower plate of capacitor Cl the base of PNP transistor TR 1  slightly above common line potential, so that a small current trickles through the transistor. This current makes the base of PNP transistor TR 2  slightly negative with respect to the emitter, so that a small current flows through it and establishes a potential difference across potentiometer VR 1 , Electrons are thus withdrawn from the upper plate of capacitor C 1 , thus tending to raise the potential of C 1  and of the base of TR 1 , which therefore passes a larger current. This makes the base of TR 2  more negative, enabling that transistor to pass a larger current. This cumulative action rapidly drives both transistors into their fully conductive states, and they remain so until, capacitor C 1  is almost fully charged, which takes about 16 milliseconds. During this time the base of TR 3  becomes positive, and so the transistor conducts, as in turn do transistors TR 4  and TR 5 . The latter permits a large current to flow through the coil of solenoid 16, which is thereby actuated. As C 1  becomes fully charged there is less current through R 2  ro prevent the emitter-base current through TR 1  lowering the voltage of the base towards that of the common line. The current through TR 1  is thus reduced, as is that through TR 2 . The effect is again cumulative, and both transistors become non-conducting. Resistors R 2 , R 4  and potentiometer VR 1  and the upper plate of C 1  are thus brought to earth potential. The potential difference across C 1  remains however, so that the lower plate and the base of TR 1  become negative with respect to the common line until the capacitor can slowly discharge through R 1 . The continuing current through R 1  then makes the base of TR 1  positive, thus producing another brief pulse, the pulse interval being about 42 seconds. The duration and frequency of the pulses depends upon the reistances and reactances employed. The e.m.f. of the 1À5 volt battery is boosted by the back e.m.f. across the solenoid coil, so that capacitor C 1  is charged to 6 volts. Alternatively C 1  may be replaced by a coil. In a second series of embodiments. a mechanical oscillator is used. This may comprise a spring-biased swinging arm carrying a magnet which swing pass stationary coils whose functions are to detect the oscillation as an electrical signal which is then amplified and applied to the solenoid and also returned to a second coil so as to maintain the oscillations of the swinging arm. Alternative mechanically oscillating systems include a tuning fork or a ceramic or quartz piezoelectric crystal.