Patent GB-1438040-A (GB)
Title: Circuit for the starting of a predetermined stage of an electronic sequence circuit with holding circuit
IPC: H04N5/44
Filed: 1973-08-01
Abstract: 1438040 Switching circuits; radio and TV channel selection LICENTIA PATENTVERWALTUNGS GmbH 1 Aug 1973 [1 Aug 1972] 36650/73 Headings H3T and H3Q In a starting circuit for the setting of a predetermined stage I of an electronic switching circuit having stages I, II, only one of which operates at a time, the voltage necessary for setting the stage is dropped across a component D B , D C  which is connected between the control electrode of a switching transistor T A  setting the stage I in operation and the potential connection common to the circuit. This component D B , D C  is connected in series with resistances R A , R B  forming a voltage divider and at least one constant voltage element Z DA - Z DD  to a higher value voltage source 7 and the tapping of the voltage divider R A , R B  is connected by means of a component D A  which is permeable to current in only one direction to a voltage source of lower potential 8 such that the resultant load on the higher voltage source is eliminated or at least reduced if the lower value voltage source is effective.  A negative control input at 10 turns on transistors T1-T9 in stage I. The voltage across a common resistance R18 maintains the stage I in a switched on state. The transistor T6 turns on for frequency selection and T8, T9 for the region of the desired channel or range selection. If a negative control signal is applied to 12 the transistors T1-T9 in stage II turn on and the increased voltage drop across R18 causes the transistors in the stage I to turn off. Switchover can be effected by applying a positive pulse signal to the common resistor R18 at 2. In this way the switched on stage is shut off. Simultaneously a negative switch off pulse arises at the collector of T6. This pulse is fed to the input of the subsequent stage II, e.g. by a capacitor coupling 6 and 12 so as to switch stage II on. The sequence switch is switched further by one stage with each pulse. When the supplies 7 and 8 are applied the voltage across diodes D B , D C  which may be Zener diodes turns on a transistor TA which selects the stage I transistors T4-T9. The load on the higher supply 7 such as 30 volts must not exceed a certain value. The diode D D  prevents any current flowing through the Zener diodes ZD A -ZD D  if only a low value voltage source is applied. If at first the 30 v. source 7 is present a turn on voltage drop of 1À2 v. occurs at the double diode D B , D C  and due to the resistances R A , R B  a low current of 0À30 mA then flows and this loading is not critical as long as the stage is still not operating. When the 12 volts lower voltage supply 8 is applied the voltage at the connection between R A , R B  increases in steps from 3 v. to 11À4 v. Then no more current flows through the Zener diodes and the 30 v. supply is free of loading by the starting circuit III. The circuit may be integrated in a semiconductor body and the higher voltage supply 7 may be stabilized. The sequence circuit may be used for example in radio and TV apparatus for electronic switchover of channels and ranges.