39 is fed to a third input of the nand gate 35 the three gates 35 38 and 39 act as a coincidence circuit acting to input signals from the counter 28 to the bistable 31 only when they have predetermined logic levels which will be described in greater detail below the output of the bistable 31 is fed to the input of a monostable multivibrator 42 and also to the input of an inverter 43 the input of a differentiating circuit 44 and the first input 45 of a nand gate 46 the second input 48 of the nand gate 12 is fed from the output of a nand gate 49 having two inputs one of which 50 is fed from the output of the monostable 42 and the other of which 51 is fed with the output of the inverter 43 the gate 12 has three inputs the third 52 of which is connected to the output of a nand gate 54 and to the input of an inverter 56 connected to a second input 58 of the nand gate 46 the output of the nand gate 46 is fed to a first input 59 of a monostable circuit 60 the second input 62 of which is fed from the output of the differentiating circuit 44 the output of the nand gate 12 is also fed to a first input 63.