an output lead 28 vco output lead 28 is connected to an input lead 29 of output circuit 10 leads 25 and 27 are connected together junctions 12 39 and 13 are connected to triangle wave generator input lead 24 a diode 30 is connected from transistor emitter 17 to junction 14 a single pole multiple throw switch is provided at 31 having contacts 32 33 34 35 36 and 37 all the structure described from the first mention of output circuit 10 herein to this point may be entirely conventional the same is true of vco 26 shown in fig1 and 2 triangle wave generator 23 shown in fig1 and 3 leads 38 39 and 40 and junctions 13 and 14 in accordance with the present invention a small circuit is incorporated at 41 including a transistor 42 and a diode 43 transistor 42 has a collector 44 an emitter 45 and a base 46 lead 38 is connected from junction 13 to base 46 lead 39 is connected from vco 26 to collector 44 diode 43 is connected from emitter 45 to lead 40 lead 40 being in turn connected to junction 14 triangle wave generator 23 oscillates when input lead 24 is connected to ground from junction 12 39 either through diode 22 and switch 12 or through the collector emitter circuit of transistor 15 through diode 30 contact 34 and switch 31 transistor 15 is at saturation when switch 11 is in the position shown triangle wave genrator 23 then oscillates it.