I was scrolling through AliExpress and found a number toy piano kits that used a 555 IC and a series of resistors. It was a stay-at-home-because-of-COVID weekend, and, having all the parts at home, I decided to try to build one from scratch, rather than wait 4 months for a kit to arrive.

I should have built it on a breadboard first, but I thought someone somewhere must have built and tested this thing, so I went ahead and just prototyped the thing. I was wrong; I don't think it was tested at all. It made sounds, but all of the resistors were 2K (see yellow circuit diagram above, but do NOT use the yellow diagram above) and that meant that the pitch of each note was quite random.

Here is what you will need to build a toy organ that actually sounds like a toy organ:

Supplies

speaker 8 Ohm 0.5 W (or 0.25W)

momentary tactile buttons ×8

ON-OFF switch

TLC555

0.1uF ceramic capacitors (104) ×3

4.7uF electrolytic capacitor

Lots of resistors 1K5; 1K2; 180; 330; 270; 1K2; 1K; 680; 22; 380; 5K6; 820; 1K

9V battery clip

9V battery

5cm × 7cm double-sided PCB

8-Pin DIP socket (optional)

Step 1: Hours of Research

Hours of research later (you can safely skip this step), I found a formula that allowed me to calculate (after some maths to isolate for Rb) the resistance needed to produce a given pitch. The screen shot of the spreadsheet I used indicates the final values I settled on. After having built the organ, I have found it to be slightly out of tune. This is a result of faulty calculations on my part. See picture and note that I am off by over 100 Ohms in one case. I have corrected the spreadsheet and included it here.

The original circuit diagram only used one resistor per note. I only had off-the-shelf resistors in my supplies, and I did not want to go to the store for this, so I decided to find a close approximation using two resistors. I tested all of the resistors of a given denomination to find the one that most-closely approached my desired resistance.

Start with Ra (R1 in the circuit diagram). It should be 1000 Ohms, but my closet one was 992 Ohms. Measure yours and enter the value in the orange box near the bottom of the spreadsheet.

I do not have the ability to check capacitors, so I went with the stated value of a 104 in Farads. If you can measure yours, enter its actual value in Farads in the orange box near the bottom of the spreadsheet.

Then work your way UP the rows (in the yellow "According to the Label" section), starting with C6 at the bottom and ending with C5 at the top. For each row, find two or three resistors in your supplies. In my experience, resistors are often below there labelled value; rarely have I found one to be higher. Thus, you are looking for a positive number in the "Rb Off of Theory" column. The measured totals will likely be lower.

Now measure all of your resistors to find ones with measured values that will give you the best results. Again, work your way UP the column in the green "Measured on the Multimeter" section.

Organ.xls
Download

Step 2: The Build

Here is my modified circuit diagram and a Picture of what it might look like prototyped on a 5cm × 7cm double-sided PCB. The solid lines are jumpers, and the dotted lines are connections on the underside.

The TLC555 seems to be rated from 2-15 V, so I first used two AA batteries. It worked fine, but it was rather quiet. I tried three AAs and four AAs, and I finally settled on a 9V battery. The addition of a volume pot might be a future addition.

The organ is monophonic; only one note can be played at a time, and it will be the highest note that plays. This is because the resisters are hooked up in series with the lowest pitch resulting from the highest resistance and the highest pitch from the lowest resistance. If you change one resister in the series, it will affect all of the following notes in the series.

I designed a bumper and printed it on my 3D printer.