When my son Michael was young, I made a little black box with binary counter, shift register and LED's. There were two astable multivibrators, one used as the shift register clock with 0.1 to 10 second period. It had a potentiometer to set the speed of the clock and a load switch that set the input to the shift register. The second astable multivibrator was set for a base frequency of 200Hz to 2kHz and had it's own potentiometer, this one drive a small loudspeaker. The output of the shift register summed to offset the base frequency of the second multivibrator oscillator. So basically the thing was a light and sound making gadget, and the lights and sound was affeced by the switches and knobs. Had a NiCad battery and a 12 volt wall pack transformer to recharge it.Adults were endlessly fascinated, toddlers, maybe two minutes. But now that I think about it, the two minutes seemed like an eternity. 
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