the oscillator 98 in the range from 500 to 10 000 hertz a preferred fixed frequency for the oscillator 94 is about 200 hertz the resonant frequency of the crystal 58 and the other parts carried thereby preferably should be within plus or minus 25 hertz of the frequency of oscillator 94 the photoconductor 42 functions primarily as an 34 on off 34 switch which responds to brightness levels above a predetermined threshold level to cause operation of the oscillator 94 at brightness below this threshold level the oscillator 94 is quiescent the photoconductor 44 is so connected in the oscillator 98 that a change in brightness on the photoconductor 44 causes a change in the output frequency the circuit is so arranged that increasing brightness on the photoconductor 44 causes a corresponding increase in the frequency of the output signal the amplifier 96 is used for the purpose of increasing the amplitude of the output signal of the oscillator 94 thus the signal applied to the transducer 46 58 from the fixed frequency channel 42 94 96 is of a greater magnitude than that from the channel of oscillator 98 since the skin is acutely sensitive to tactile vibration in the frequency range from about 200 to 300 hertz the frequency of the channel 42 94 96 is selected to be within this range also the resonant frequency of the tactile transducer 58 74 76 is selected to be within this same range whereby vigorous tactile excitation is produced at a frequency.