A 555 Synthesizer

When I was young, my parents gave me a simple electronic piano, with some metal pads for the keys and a metal pen. It was a kit and it felt like magic with all theses electronic components, which produced sound when soldered correctly. Unfortunately I can't find it anymore, but building my own simple analog synthesizer sounds cool for a project for the 555 contest. This is the result:

My old synthesizer had a switchable vibrato, but I didn't manage to create a good one with a second 555 by changing the CV input of the first 555 with a LFO, it doesn't sound right. But instead my synth has some other nice features, like a filter and a volume envelope. This is the schematic, with a 555 timer chip:

For a stable oscillating frequency, I've used a 7805 voltage regulator, which allows to supply it from 7 V to 20 V. When some sound is generated with the speaker, it needs about 60 mA. For higher input voltages you may need a heat sink for the 7805, but not e.g. for a 9 V battery.

The oscillator part is a 555 with a standard oscillator circuit. T2 and C1 is for the lowpass filter. T3 is for the volume and T4 and T5, with Q1 and the rest around Q1, is for the volume envelope with attack and release. Q2 is a standard class A amplifier for the speaker. S2, S3 and S4 are the "Note On" buttons in the video and the picture. Two of it are pushbuttons on the left and the right side and one is a toggle button, for constant signal generation.

The volume envelope works like this: when all "Note on" switches are off, C3 is charged by T5 and Q1 is conducting, which inhibits Q2. If you turn on one of the three switches, C3 is discharged and Q1 is not conducting any more, so Q2 is conducting and amplifying the sound. With T4 and T5 you can change the time constants how long it takes to charge and discharge C3.

The keyboard:

And the PCB:

The Eagle files: eagle.zip. Compared to the version in the video below, this version 0.2 has the additional resistor R1 to avoid burning T4 and T5 and all values for the fixed resistors are entered. I've used resistors of the E96 series and 1 k below the calculated value, which was nearly the same as the measured value, so it is very linear, once you know the exact value of the factor for the capacitor. With the 2 k trimmers it can be adjusted very accurate.

Parts list:

name value description
C1 100n ceramic capacitor
C2 100n ceramic capacitor
C3 22 elco
C4 22 elco
C5 1 ceramic capacitor
C6 100n ceramic capacitor
C7 47 elco
IC1 7805T voltage regulator
IC2 NE555N timer chip
LED1 blue 5 mm LED
LED2 blue 5 mm LED
LED3 blue 5 mm LED
LED4 blue 5 mm LED
Q1 BC557A PNP transistor
Q2 BC557A PNP transistor
R1 1k resistor
R2 390 resistor
R3 1k resistor
R4 390 resistor
R5 10k resistor
R6 1k resistor
R7 2k2 resistor
R8 150 resistor
R9 390 resistor
R10 390 resistor
R100 4k12 resistor
R101 4k99 resistor
R102 5k9 resistor
R103 6k98 resistor
R104 7k5 resistor
R105 8k87 resistor
R106 10k2 resistor
R107 11k resistor
R108 12k7 resistor
R109 14k7 resistor
R110 16k9 resistor
R111 18k2 resistor
R112 20k5 resistor
R113 23k7 resistor
R114 24k9 resistor
R115 28k7 resistor
R116 32k4 resistor
R117 36k5 resistor
R118 39k2 resistor
R119 44k2 resistor
R120 49k9 resistor
R121-R156 2k trimmer
R157 4k53 resistor
R158 5k49 resistor
R159 6k49 resistor
R160 8k25 resistor
R161 9k53 resistor
R162 11k8 resistor
R163 13k7 resistor
R164 15k8 resistor
R165 19k1 resistor
R166 22k1 resistor
R167 26k7 resistor
R168 30k1 resistor
R169 34k8 resistor
R170 41k2 resistor
R171 46k4 resistor
S1 SWITCH switch
S2 SWITCH switch
S3 SWITCH button
S4 SWITCH button
SP1 8 ohm speaker
T1 100k potentiometer
T2 25k potentiometer
T3 25k potentiometer
T4 100k potentiometer
T5 500k potentiometer

The small 2 k SMD trimmers are from Murata, PVZ2A202A01B00. You can buy it from Farnell. You'll need it, if you use the same layout, but otherwise you can use any type. Same for the other parts, they are not critical.

For my setup I've measured that the required resistance R per key is R=1/f/(1.45*C2)-R7 (with f=the frequency of the key, R7=1860 and C2=1e-7). Maybe your setup may vary, so some resistor kits are useful, like this one from Digikey with 0805 SMD resistors, or you could use two pots per note, one for coarse and one for fine tuning. You'll need at least a pot per note for fine tuning, otherwise it can sound very out of tune with 1% E96 resistor values.

I've tuned it for equal temperament. This means every pair of adjacent notes has a frequency ratio of 1.059463 (12th-root of 2, because of 12 semitones per octave). See this Excel table for the resistors and the frequency values of the notes: notes.xls

The synth in action and a time lapsed full documentation how I built it:

The volume envelope, low pass filter and amplifier are very simple and as a programmer I don't know much about analog electronics, so the values are most trial and error. I think there are many things to improve, e.g. I don't like that the volume pot changes the filter, too. But it was a fun project and you can use it for whatever you want. Please send me links of your construction or improvements, I'll add it to this page for reference, if someone wants to build it again.

11. März 2011, Frank Buß