Buy the PCB set:
First and foremost, thanks to Roman Sowa who designed the circuit. It is a truly brilliant and simple design. He posted the schematics to the Synth DIY Facebook group. I highly recommend his website, MIDI-Hardware.com. He has some excellent MIDI solutions to about any MIDI need you have. THANK YOU ROMAN!
The project is discussed on Muff Wiggler here (page 15).
Do you have an old synthesizer that has a Pratt-Read or similar “buss” type keybed that has seen much better days? You can replace the old one with a new FATAR matrix type keybed using this circuit board and an additional PCB with some reed type relays. This PCB uses basic logic ICs to scan and decode the output of a FATAR 4 or 5 octave keybed and produce a TTL level output for each individual key that is held down. With a 5 octave, 61 key keybed, you will have 61 independent TTL outputs that can be used to drive a bank of reed relays (these act as the buss in the Pratt-Read system) or any other device/circuit that can be driven by the outputs.
I don’t have the schematic and component layout documents bundled yet. If you buy one, I’ll send you the schematics and component layout separately.
The purpose of this circuit is to scan a FATAR keybed and then produce individual TTL level outputs when ever one or more keys is pressed. Ultimately, the drive for this board was to enable the use of vintage keyboard electronics, e.g.: Moog 952, ARP 3640, Steiner Duophonic, Aries AR313, or any vintage keyboard that used a buss type “organ” keybed with a buss (busses) with a flexible “throw” (or throws) that would make contact with the buss while the key was pressed.
This circuit is an intermediary circuit that provides a way to get an individual “on” or “pressed” TTL level signal when any key or keys are pressed.
The input is a standard FATAR 4 or 5 octave cable set. The output is wither 49 or 61 individual TTL level signals that will be active only when the corresponding key is pressed and will stay active so long as the key is pressed regardless how many keys are pressed.
There are several ways to connect this PCB. I’ve added some photos of how I connected this PCB to a reed relay PCB to emulate a 61 note, single buss, Pratt-Read keyboard. Basically, there are two ribbon connectors, which are polarized, that run from the keybed itself to connectors on the decoder PCB. There is a power connector that will be fine taking anything between the typical 12 to 15 volt supplies in most analog synthesizers and the typical Euro Rack and MOTM connectors are provided. Only a positive supply is used. This is regulated to +5 Volts. There are also two power outputs in case you need to run 5 volts to your reed relay PCB.
I highly recommend using angled connectors as there is typically little space between this PCB and the bottom of the keybed. The orientation of the connectors is such that the wires point toward the middle of the PCB except for the MOTM power connector on the end of the PCB which points off the PCB. This orientation allows the wires to be collected over the center of the PCB and then run to either/both ends of the PCB toward what ever other circuit is being driven by the decoder circuit. You can see what I did in the attached photos. In my build, I used a Synthesizers.COM keyboard, removed the existing decoder circuitry and mounted this decoder PCB under the keybed. I then ran the control wires out of the case to a reed relay PCB mounted under the brackets behind the keybed, again, see photos.
NOTE: If you use a 4 octave, 49 note keybed, the lowest key on the keybed corresponds to output #13 not #1.
See the Component Notes page for more information.
Nothing special. Standard resistors here.
There is one resistor network, 10k resistors. This needs to have 8 resistors with a common connection. I used and laid out a 9 resistor network. make sure the common is installed toward the “output” side of the PCB
Nothing special here either. Standard ceramic capacitors are fine. Note the 0.2 inch spacing
Ferrite beads on the input line. Not strictly needed. See Component Notes page for more information.
Make sure you get the right voltage and power ratings, the “HC” variety.
One diode for input protection. 1N4001 type.
There are 8 1N4148 diodes which can be used to feed an external gate/trigger circuit if one is desired. I opted to just do the control voltage and trigger/gate with a two or three buss reed relay PCB. But, they are there if you want them.
I assume you know the basics of soldering. I like to insert the low lying parts first, like resistors, diodes, etc. After these, I install the IC sockets. Next capacitors, transistors, connectors. Use a good solder, either an organic flux, which you should wash regularly, or a no-wash flux.
Take a break every so often, wash off the flux if you are using a flux which required cleaning. Double and triple check orientations, pins, and solder joints.
Power Supply Regulation/Filtering:
The regulator has room to lay flat and have a small heat sink. The heat sink is only needed if you are driving a lot of outputs at one time or driving loads at the upper end of the specification of the SN74HC563N chips. If you are using this regulator to drive external circuitry, be sure you don’t overload the capacity of the 7805 and heat sink.
The PCB runs on +5 volts from the 7805 regulator. You can use +12 or +15 volts as input voltage. There are two additional connectors, a 2 pin MTA connector on 0.1 inch spacing and a 4 pin “MOTM” on 0.156 inch spacing that provide a ground and +5 volt source if you want for a relay PCB or other external components.
The 4 pin MOTM and 10 pin “DOEPFER” connectors should be right angle. The DOEPFER should point towards the middle of the PCB. The MOTM should point off the PCB. make sure you pay attention to which end is the +12 or +15 volts and pick/mount the appropriate connector.
COMPONENT PLACEMENT and CONNECTION:
I left room so that the regulator can be bent over to give additional vertical clearance under a keybed. I also laid out the connectors so that right angle connectors can be used and so that they would mount facing the center of the PCB except the power connector on the end of the PCB. It was made to point off the PCB, but a right angle connector all the same.
There are two connectors for the upper and lower halves of the keybed. Note that this decoder only works on 4 and 5 octave keybeds.
As mentioned above, the polarization of the connectors is such that the connectors should be mounted with 90 degree angled connectors such that the wires point toward the center of the PCB, not off the PCB. The PCB should be narrow enough to fit under any FATAR keybed. Depending on your specific implementation, you may need (or may just want) to cut one of the mounting legs off the keybed to give better clearance for wires.