My Organ

Over the last couple of weeks Matt and I have been fiddling around with the Hammond Organ that I was able to pickup in Rotorua for $60. Amazingly all of the functions still work and whilst the casing / frame has seen better days it is in general in quite good condition. (let me say sorry for the image quality now, someone had been fiddling with my camera…)



Our basic goal is to work out how we can hook the thing easily up to a computer to make a “Player Organ” with the following criteria.

  1. The organ must look stock
  2. The organ must still be playable by humans
  3. The communication between the organ and the computer must be simple and adaptable
  4. Any sound coming from the organ must be generated by the organ and not just played through the organ’s speakers

The two main methods of turning a piano / harpsichord / organ depend on whether the instrument includes mechanical and electrical components or just mechanical. With a standard piano for example you’re pretty much limited to manipulating the keys or hammers with some form of motor / servo / solenoid in order to create the sound. With this organ however (being electrical) it is possible to generate the sounds by manipulating the electrical signals and components within and due to the cost of a large number of servos / motors / solenoids this method was chosen. From there it was decided that some form of MCU interfacing via MIDI to the computer would be the way to go.



Once the organ was opened up we had a look at the underside of the Manual (keyboard). You can just see the underside of the keys at the bottom of the image but basically when a key is depressed the metal lever spring things contact the wire bar that runs across the lot of them. First assumption therefore was that the metal bar was tied to ground and that a tone was generated when the circuit was completed. Under this assumption we decided that the easiest method would be to hook a series of darlington arrays up to a MCU and use them to complete the circuit in parallel to the manual switches.


We started with the lower manual, soldering a wire to each of the lever spring things and then once all 44 were complete the lot were loomed up neatly following by the reinstallation of the manual.



Once all of this was completed it was decided that one of the breakout boards holding the dalington arrays should be finished next so one Manual could be hooked up to test the operation of the whole thing before work started on the upper manual and the pedals.

One of the things that we looked at whilst we were doing this was the current through the circuit and the voltage differential between the keys and the “ground” bar. As it turned out the current flow was almost nothing but the voltage differential between the keys and the “ground” was about 24v. When we checked this against the readings on the upper manual we discovered that the differential was only 20v which tempted us to do a bit more investigation which was when we discovered that there was a 4v differential between the two “ground” bars…

After much head scratching, testing and thinking we realized that all our assumptions had been wrong. From what we can figure out from looking at the organ as a whole and tracing some of the circuitry is as follows.



This is the main circuit board that sits behind the lower manual, most of the other (and there are at least 10) circuit boards connect directly to or through this one.



  1. The two white connectors on the outside front edge connect to the lower manual.
  2. The two vertical header rows just in from these connect to the “Auto Chord” circuit board. This is basically a big mess of transistors and diodes that when enabled make it possible to play a C Major chord for example by just playing a C on the lower manualIMG_5279
  3. The outer two tabs at the back edge connect through to the upper manual with the inside one connecting to some other board and then down to the pedals.
  4. The connector on the left hooks up to the board that is connected to the switches that determine the “Voice” of the particular manual.
  5. The connector on the right connects directly to a “Tone” labeled circuit board.

After googling the ICs in the middle we discovered that they were amplifiers and after poking around with a multimeter we worked out that three of the pins (which we now assume to be inputs) connect through to the “Tone” circuit board. (Side note, 4 ¬†amplifiers with 3 inputs = the 12 notes that you need on a keyboard, 7 white and 5 black)

From there we traced back from the keys and found that all the C keys for example connected through some resistors and capacitors to the same pin on one IC (these we took to be outputs)

Basically we came to accept the fact that there was more going on here than just a simple circuit that when complete generates a tone. It looks like 12 tones are generated the whole time on the “Tone” circuit board and fed to the amplifiers. Then depending on what keys are pressed the relevant tones are passed through to the circuit that determines the sound that comes from the organ and then passes that through to the amplifier and eventually the speaker. How the octaves work is still to be discovered. (and all this could be completely wrong anyway)

After all this it looked like a simple series of darlington arrays wouldn’t work as hoped. Without a complete circuit diagram there’s just too much that is unknown and if something is connected up wrong and part of the organ is damaged then there isn’t going to be mush hope for repairing it. The only solution then was some form of switch in which the logic side (in this case most likely an Arduino Mega taking a MIDI input via a USB to MIDI converter) is electronically separate from the the switching side of the organ.

Two options came up straight away, relays and optocouplers but after looking at the pricing of optocouplers vs relays on eBay plus adding the fact that each optocoupler would require a resistor on the Arduino side it was decided that (loud switching and all(and no solid stare relays are stupidly expensive on eBay) that relays would have to be the way to go.

Its not really a waste so far, all the wiring can be used and the darlingtons will need to be used as well to switch the relays. It just looks like we’re going to need to be a bit more creative in how we lay it all out as we now need to hide 116 relays in total for the manuals, the pedals and the voice switches inside the organ as well as the darlingtons, MCUs and the USB to MIDI converter…

Thats where we are at the moment, A whole mess of relays and wiring is up next and whilst its not the ideal way of doing things it should be pretty bullet proof. We do know though that whatever the signal is we shouldn’t have any problems using this method.



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