In this paper, the fundamental characteristics of tactile recognition by electrical stimulus in order to develop a vision substitution system were described. The electrical stimulus pulse or DC voltage was applied at a touch board, and a conducting band which was connected to the ground level was fastened around a root of finger. First of all, the resistance of finger by the DC voltage was measured and the equivalent circuit of a finger was estimated. It was found that the most of resistance of this mechanism was concentrated at the contact of tip of finger and its value reached to MΩ order. And this resistance widely varied by the contact condition. The resistance of finger itself was relatively low and the contact resistance of band connectoin was about 30 kΩ. Total stray capacitance was about 26-62 nF, which was calculated by our experiments. Secondly, the minimum recognition voltage to applied stimulus pulse was measured by changing frequency, duty-ratio and voltage of pulse. It was found that the most sensitive pulse was in situation of that the frequency range was within from 60 Hz to 300 Hz, the duty-ratio of 20%, and the minimum sensitive voltage was about 13V. Lastly, this electrical stimulus pulse was applied to the touch Braille board. A touch Braille board was controlled by a computer (PC8801). In this system, an input letter from keyboard is translated to Braille code data by a computer automatically, which express the letter by the 6 points for the brind. And a Braille data is output at a touch board. By touching on the contact point of the touch board, a person can recognize Braille points by electrical stimulus. It was found that the Braille recognition by electrical stimulus pulse was available as same as it could be done by raised points.

In this letter, we study on the sensitivity to the electrical stimulus pulse for biomedical electronics for the purpose to make a tactile vision substitution system for binds. We derive the equivalent circuit of finger by measuring sensitive voltages with various touch condition and various DC voltage. And we consider to the sensitivity of finger against electrical stimulus pulse. In order to convert the sense of sight to tactile sense, we consider four types of touch condition and various types of pulse. It is shown that the sensitivity of finger to electrical stimulus pulse is determined by duty-ratio, frequency, hight of pulse and the type of touch condition. In the case that duty-ratio is about 20%, frequency is within about 60-300Hz and touch condition is A-4 type, the sensitive voltage becomes the lowest. With this result, a tactile vision substitution system can be developed and the system will be used to transfer various infomations to blinds without paper.