Brrrt… Bleep Bloop: Forrest Mims’s 3909-based toy organ

  • October 10, 2016
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If you give the MP3 file embedded above a play, you’ll hear me noodling around with my slightly modified version of the “toy organ” DC circuit featured on page 105 of Forrest Mims Engineer’s Notebook [ISBN 9781878707031].

I had three types of small 8Ω speaker on hand.

Using alligator clips, I attached each of the three different types of small 8Ω speaker (left to right: 0.25W, 0.5W, and 1W) in turn and got slightly different results. The recording included here was made with the rectangular, 1W speaker.

Mims's 3909-based toy organ assembled on a solderless breadboard, powered by a AAA battery and outputting to a 1W speaker.

The resistors labeled R4 (spanning terminals F16 and E16 on the solderless breadboard in the image above) and R5 (connecting F20 to E20) in Mims’s book are specified as 150Ω and 47Ω, respectively. I couldn’t hear anything when I pressed the button feeding juice to R5 and the tone emitted when I pressed R4’s button was so high-pitched that it was barely audible, so I swapped them out for a 4.7kΩ resistor (R4) and a 2.2kΩ resistor (R5).

I also trimmed the leads on all of the resistors to make the breadboarded circuit easier to see and used the J-column-side negative rail to connect the switches, capacitor, and pin #8 on the LM3909N. Electricity, from a 1.5V AAA battery, enters the + and – rails on the a-column side of the breadboard.