through a transistor 96 and the scr 76 when the transistor 96 is turned on a third multivibrator turns transistor 96 on and off by applying a signal to the base of transistor through resistor 100 the signal applied to the base of the transistor 96 is the signal that appears at the output of an amplifier connected between terminals 13 and 12 of chip 68 that amplifier and the amplifier in chip 80 which appears between terminals 1 and 2 are connected in a multivibrator circuit which is formed by connecting the output of the chip 80 amplifier to the input of the chip 68 amplifier through a connection 102 which incorporates no time delay the output of the chip 68 amplifier is connected to the input of the chip 80 amplifier through the series combination of a capacitor 104 and a resistor which comprises the combination of a resistor 106 in parallel with the series combination of resistors 108 and 110 having in mind that there are ground return circuits associated with the amplifiers within the chips 68 and 80 there is a current reversal in the resistance capacitance circuit the diode 112 short circuits resistor 108 in one direction of flow but not the other and that circumstance is used to provide different on and off times in that multivibrator arrangement summarizing the operation to this point when the scr 76 is rendered conductive power is applied to all three multivibrators to the end that sounder 52 operates continuously at one frequency and sounder 50 operates at another frequency 100 cycles different the operation of sounder 50 is interrupted periodically at a frequency controlled by the frequency of.