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The Ben Heck team completes work on the Atari Junk Keyboard. Previously, they took apart a keyboard and made a manually activated switch matrix to read keys. Now it’s time to take those outputs and hook them up to a 555 array to create the Atari sound effects!
For inspiration Ben and Felix view a diagram of an Atari Junk Console Circuit by the influential Maker, Forest Mims. To get polyphony, the ability to play multiple notes at one time, they are considering a 555 on every key and making it work through a combination of op-amps, resistors for outputs, and transistors to allow the switch matrix to activate the 555 circuit.
Felix works on the PNP transistor bank that will act as a trigger for the 555 timers. He laser cuts a panel for the potentiometers. In order to have as much control over sound as possible, they are going to have one of panels per octave and attach potentiometers to the 555 timers to adjust the frequency of each key individually.
There is a lot of wiring to do so Ben and Felix split up the tasks. Ben wires up the bank of 556s and 555s onto a board that will sit on top of the transistor array that Felix is wiring with a header interfacing them. The 555 drives each octave and having six 556s, 2555s in one package, gives you (6 multiplied by two) 12 different notes. There are 12 notes per octave in a musical scale.
Ben walks you through wiring up the power bus first and gives tips on soldering and order of operation. He sets up convenient power and ground rails that are close to pins so that when he wires up the 555 circuit anything that has to go to ground has a very short path. This allows him to use fewer wires which is why it’s good to wire your power rails first. Ben makes the power rails using bits of wire cut off of resistors, capacitors, and other things to make the power rails. He attaches them at a right angle on the integrated circuit side, solders them in place, and then bends them at right angles using tweezers, going up and down the length of the chip.
He has all the positive rails connected to each other on one side of the board and all the negative rails connected on the other. Now that the power wires are connected he can put all the signal traces over them. He labels them so that the input keys go 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. He adds the trigger wires from the Felix’s circuit board. Normally the triggers run off potentiometers – in this case they’re connected to transistors as well. Audio is mixed with 470 ohm resistors so you go from the timer through the resistor and then into the bus. Additional resistors are used to tie each of the four buses to each other so there will be two levels of mixing which will require additional amplification to be audible.
Ben and Felix hook up the first 12 notes of the system to test whether it is working. To get things working Felix needs to switch circuits around to allow the potentiometers in the switches to drive the frequency with the help of the master effects control. Every key has an adjustment potentiometer. The key simply turns them on or off with the transistor bank. Instead of one potentiometer controlling the 555, the keys are connected to individual pots to change the notes. Ben and Felix go to work on making additional adjustments to the board and create a totally 80s Atari Junk Keyboard!