Guitar Tuning Aid

The basis of this circuit is the 555 timer operating in the astable mode. The oscillating output can be used as a tuning aid for a guitar. The first string of a guitar ,E, produces a frequency of 82.4 Hz. The frequency of the oscillator is set to twice this value, 164.8 Hz. and then followed by a dived by 2 stage to provide the required frequency. The purpose of the divide by 2 stage is to guarantee that the waveform produced has a duty cycle of 50%. The frequency of oscillation of the circuit is set by the adjustment of R1,R2 and C4. The output of IC2 is fed to the emitter follower Q1 to produce current gain to drive the speaker. C3 acts as a high pass filter to attenuate harmonics and produce a natural sounding note. The circuit is powered by 5 volt dc supply and this voltage must fall within the range of 4.74 to 5.25 volts for IC2 to operate properly.

From : http://home.maine.rr.com/randylinscott/nov21.htm