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The team begins working on an Atari Junk Keyboard, a version of the Atari Punk Console that combines 555 timers to make a simple circuit that makes Atari-like music and sound effects. Instead of using sequences to repeat Atari-like sounds they decide to make a whole keyboard of the sounds.
Episode 262: Ben Heck’s Atari Junk Keyboard Part 2: Stepped Tone Generator Episode
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The plan is to take the matrix from a set of keys to figure out how they work, add discrete logic by using a circuit to read keys instead of opting for a microcontroller, and then feed that input into a number of Atari Junk Console integrated circuit pairs resulting in multiple sound effects playing at once. They begin their project by first getting the keyboard matrix working so they can later attach it to the Atari Junk Console to make music.
Ben begins by wiring up an oscillator. An oscillator is different than a crystal in that the oscillator can output its own waveform whereas a crystal you have to attach something else to it in order to get a square wave. The 1mhz oscillator has power and ground going into it and pins for enable and output. When you hook up power and an oscilloscope to the output pin you should see a 1mhz square wave coming off of it. As that may be a little too fast for the switch matrix a CD74HC4017 Johnson Decade Counter is used to divide the frequencies. A second Johnson counter is also added. On the first Johnson counter one of the outputs is going into the clock input of the second Johnson counter. This prevents 2 useless cycles and keeps timing more consistent.
Next he hooks up the outputs of the Johnson Counter to an inverting buffer that will drive the columns as well the flip flop driver that will load the data off of the switches. He uses an Inverting buffer 74HCT540 and wires it so that it so that it goes onto his breadboard. He does more analysis on the oscilloscope after he wires outputs from the Johnson Counter to the Inverting buffer and decides to use another Johnson counter to drive the flip flops.
Ben attaches 8 octal flip flops. A flip flop holds one bit of memory, which can be set or cleared. An octal flip flop has 8 bits per flip flop but they only need the first 6 bits to line up with the rows of the switch matrix to give them 48 keys. He attaches the output enable line on each one of the flip flops and then attaches the clock line from the secondary Johnson counter to tell the flip flops when they should be loading data. The switch matrix is attached to all 8 of the flip flops but only one flip flop at a time will be loading data off of the switch matrix.
The goal is to have 48 on-off signals from the matrixed keyboard. When a switch is closed, and its column goes low (active) this pulls the signal low. Once Ben gets his keyboard matrixes working Felix takes a circuit from the breadboard and ports it over to a protoboard. He also adds six buttons and seven potentiometers. They’re planning one potentiometer per key for note, and one potentiometer per octave for effect. The tact switches will be replaced by flip-flop drivers on the final unit. Felix connects variable resistors (potentiometers) with the circuit and gives it a test run.
