Abstract
To avoid the artefact of electric stimulation, some devices of isolation have been used in most bioelectric work. These include a simple transformer isolation and a radio frequency isolation, and the latter is now most widely used. The isolation of the transformer method is not complete and can not transmit square waves. The radio frequency method has a complete isolation and can pass square waves, but the circuit is rather complex. We devised a photo-isolation method and applied it to an all-transistorized stimulator. A neon tube is lighted by a trigger circuit, the light is transmitted from the tube to a photo-diode, and the change in the diode resistance triggers the following monostable-multivibrator, in which the width of the pulse is set. The pulse is amplified by the final transistor, variably attenuated and led to output terminals.
The maximum output voltage is almost the same as that of the battery supplying the final transistor. All batteries of the circuit after the photo-diode are floating from the earth and separated from those of the trigger circuit. The isolation is accomplished between the neon tube and the photo-diode. The capacitance between the output and the earth is about 7 pF and is in the same order as an ordinary isolator. Only one defect is a very small leak DC voltage from the output, derived from the dark current of the final transistor. It is about 4 mV in our device. But it can be decreased if a silicon transistor is used in the final one, and it can be neglected if a condenser with enough capacitance to pass the used square pulses is inserted in series with the output. Therefore, the present device is very likely to be used instead of the ordinary radio frequency method.
