Abstract: Each of two channels has a sine wave oscillator, an electronic key, an amplifier and a pair of electrodes, electrically connected in series. A pair of threshold circuits are provided. Each of the threshold circuits has an input electrically connected to the amplifier of a corresponding one of the channels and an output. A first coincidence circuit has a pair of inputs each electrically connected to the output of a corresponding one of the threshold circuits, and an output. The output of the first coincidence circuit is electrically connected to a first input of a second coincidence circuit. The second coincidence circuit has a second input to which signals are fed from the output of a first time delay element. The output of the first time delay element is also connected to the input of a second time delay element. The output of the second time delay element is electrically connected to the control inputs of the electronic keys of the channels.