Touch-Key Organ

64 EE February 1989 TOUCH-KEY ORGAN A simple-to-build, inexpensive, two-octave organ with touch- sensitive keys etched on the printed circuit board. In spite of the fact that the present organ is monophone, and does not provide semitones, it is, none the less, a perfect musical instruction aid. In the circuit diagram shown in Fig. 1, the central oscillator of the mini organ is formed by IC5, the well-known timer by T. Wig mo re Type 555. The output frequency of the 555 in astable multivibrator configura- tion depends on the values used in the R- C network connected between pins 6 and 7. In the present application, the output frequency of the organ is, therefore, de- termined by C3 and the binary code ap- plied to inputs A to D of 16-channel analogue multiplexer IC2. This CMOS IC acts as a large, single-pole, rotary switch that connects one of resistors R12 to R26 incl. between RI! and R9. If, for example, the one but highest tone is to be generated, the one but lowest resistor val...