Toy Piano

555 astable Toy Piano

The Purpose: To construct a very inexpensive 'piano' that could be assembled by enthusiastic 11 to 14 year olds that could 'play' an octave using a few fixed resistors, capacitors, a 555 timer clip, and a battery.

The Method:

An 555 astable circuit gives a pulsed digital output. Astable means that the 555 can operate repeatedly, it will switch on, then off, then on, then off, continually. The 555 astable may be called an oscillator. The positive output, mark, produces a beep on a loudspeaker followed by a period of silence, the space. When the resistance between pins seven and six are altered or the capacitance between pin 2 and the negative rail is changed, the ratio of time between each mark and space is altered and the period for them to exist in time may be extended or shortened. We hear musical notes because the energy is carried in 'waves' of compression that correspond to the vibration of the loudspeaker's cone that in turn vibrate the ear drum and stimulate the perception of sound. Each musical note is differentiated by its frequency which is the number of times per second that the mark and space are generated per second.

The frequency of the 555 astable oscillator can be worked out by the formula f = 1.44 / (R1 + 2R2) x C . In the example above R1 is 1,000Ω  R2 is 500Ω and C is 0.000001 Farads. F = 1.44/ 1000 + 1000X0.000001 which is 720Hz. The frequencies for equal-tempered scale, A = 440 Hz and this is the note an orchestra normally tunes to.

The 555 shortcomings:  The 555 is a very reasonably priced timer clip, it is robust and in its CMOS form, the 7555, it can be run at very low currents. It produces for this project a high volume output compared to other oscillator solutions such as transistors, op amp, or logic chip. However it is not the most accurate timer chip and can produce slight variances throughout the frequency range and struggles in high capacitance situations. Capacitors have tolerance values of up to 10% so in choosing one test it to ascertain its true initial capacitance. Resistors have inherent tolerance values and the resistance will change with temperature. So the theory is all very good but test the outcome frequency with some kind of chromatic tuner such as the Korg CA-30.

The other problem is that the square wave output sounds very beepy but you can smooth out the square wave with a series is three capacitors and drop down resistors, However this reduces the volume so you may need to build an amplifier.

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