the other end of dropping resistor 64 is connected to the anode of green led 61 which has its cathode connected to a junction between respective anodes of silicon controlled rectifier scr 51 and blocking diode 65 the cathode of scr 51 is connected to ground while the cathode of diode 65 is connected to the cathode of a zener diode 66 which has its anode connected to the 34 led 34 output of integrated circuit 20 the gate 75 of the scr 51 is connected through isolation resistor 56 to the collector 53 of an npn transistor 52 which has its emitter 54 connected to ground the base 55 of transistor 52 is coupled to the cathode of a diode 60 through isolation resistor 57 and the anode of diode 60 is connected to the 34 i o 34 terminal of the integrated circuit 20 the junction of resistor 57 and diode 60 is also connected to the cathode of diode 44 which has its anode connected to the junction between the switch 26 and resistor 27 the voltages at the junction between the resistor 57 and the diode 60 and the voltage at the collector 53 of the transistor 52 are conventionally stabilized against electrical noise by capacitors 59 and 58 respectively diode 69 and resistor 68 are series connected between the 34 brass 34 output of the integrated circuit 20 and the collector 53 of the.