MAS861

The Simple Pseudo-random Sound generator

Courtesy/Thanks to: Joe Paradiso

Description: This circuit runs on a single +5 Volt supply and drives a speaker directly. The sound that's heard originates from a 555 timer in one-half of the dual-555 chip (U1), the 556. This 555 circuit drives a 4024 CMOS 7-stage binary counter U2, which produces 7 subharmonics below the frequency of the 555 (quiescently set to be around 10 kHz through the resistors and capacitors at pins 1 and 2 of U1). An 8-1 multiplexer/demultiplexer, the 4051 (U3) selects one signal out of the 7 available subharmonics from U2 and the original clock signal from U1, and routes it through an NPN emitter follower bufer (T1), which is able to drive a loudspeaker, producing the sound that we hear (I've provided a line-level audio output as well for jacking into recording device inputs, decoupled with the 1 uF capacitor and buffered by the 1K-Ohm series resistor). Note that driving a loudspeaker directly off the Class A amplifier that T1 forms isn't good form without isolating the speaker from the transistor with a big (e.g., 1000 uF or more) capacitor (remember that T1 would still need an emitter resistor for DC bias), as the 0-5 volt square waves appearing at the transistor output have an average DC voltage of 2.5 volts, which does nothing but heat up the voice coil in the speaker. I get away with it here as the voltages are so low and the speaker isn't of high quality, but note that it's very poor form.