Patent GB-1170753-A (GB)
Title: Improvements in or relating to permutation switching units
Inventor: BENJAMIN BRIAN RALPH
IPC: H03K17/62
Filed: 1966-05-11
Abstract: 1,170,753. Selective switching circuits. B. R. BENJAMIN. 11 Aug., 1967 [11 May, 1966], No. 20886/66. Heading G4H. A permutation switching unit having a plurality of input terminals to which input signals are applied in a predetermined sequence to produce an output signal comprises at least three bi-state devices, each connected to an input terminal, and so interconnected that when any of them is in one state, the others are in the other state, and inhibit means responsive to one of the devices being in one state to prevent switching of another of the devices to the one state. In the form of Fig. 1 transistors Q 1  and Q 2  form a bi-stable device and three more transistors Q 3 , Q 4 , Q 5  connected to input terminals 1, 2, 4 are interconnected by NOR logic circuits so that when one is switched " off " the others are " on ". The unit has an initial condition in which transistor Q 3  is off and Q 4  and Q 5  are on, the bi-stable Q 1 , Q 2  being in a first state applying a biasing signal via resistor R to diode D 3  so that a signal on terminal 2 may pass via capacitor C and diode D 3  to switch diode Q 4  off. In this state the bi-stable Q 1 , Q 2  also biases diode D 5  to prevent transistor Q 5  being switched off by a signal on terminal 4. The bi-stable Q 1 , Q 2  is held in this first state by transistor Q 3  which being off, causes diode D 5  to prevent signals on terminal 3 reaching the bi-stable. In this initial condition a signal is applied through diode D 2  to the input of the output circuit OC, thereby preventing an output. Only an input on terminal 2 can now change the condition of the unit, causing transistor Q 4  to switch off and transistor Q 3  to switch on. The next change can only be made by a signal on terminal 3. A signal on terminal 1 would reset the unit and signals on terminals 2 and 4 would have no effect. The signal on terminal 3 switches the bi-stable Q 1 , Q 2  to the second state, biasing the diode D 5  so that Q 5  can now be switched off by a signal on terminal 4 and biasing D 3  to prevent an input at terminal 2 switching Q 4  off. In this condition a signal on 1 returns the unit to its initial condition, signals on 2 and 3 have no effect and a signal on terminal 4 switches Q 5  off so that transistor Q 4  is switched on. Since both Q 2  and Q 4  are now on, no signals are applied to the output circuit OC via diodes D 2 , D 4  and an output signal is therefore given to indicate that the unit has been correctly switched. The inputs to terminals 1, 2, 3, 4 may be from a rotary switch and a push-button. The switch may have ten positions only four of them being used. A timing device may be provided actuated by the first signal which gives an alarm signal if the correct sequence is not given within a certain time. A further input terminal may be provided to the bi-stable device to switch it back to its first condition irrespective of the state of transistor Q 3 . The output circuit may be made to respond not only to Q 3  and Q 4  being on but the bi-stable Q 1 , Q 2  being in its first state. The sequence will then have five steps instead of four. Such a form is described in Fig. 2 (not shown). The unit may be used for example to control the ignition circuit of a motor vehicle.