We’ll start with the heart of the project — the DSP-G1 synth chip and the Feather M0 Express to send it MIDI commands. On its own, the synth chip can receive MIDI messages from any device that can send them, such as a MIDI keyboard or sequencer. But, the addition of the Feather M0 Express allows us to use a wide array of sensors, buttons, knobs, and more as input devices. The Feather can be programmed to read input devices and then output MIDI messages to the synth chip.
You could even code and arpeggiator, sequencer, or MIDI song player with the Feather and DSP-G1 combo!
Download the application manual for the DSP-G1 for more details.
This is the synth chip's schematic, including a resistor-capacitor (RC) circuit to smooth the signal to the audio output. Instead of a typical MIDI input circuit connected to the DSP-G1's MIDI in leg, we can see the Feather M0 Express will send MIDI messages from it's TX pin to the chip.
It's a good idea to test the circuit on a breadboard before committing to soldering it onto a Perma-Proto board.
First, we'll need to solder the short male header pins to the Feather M0 Express so that it can be attached to a breadboard for testing and later to the Perma-Proto board.
Follow this guide for detailed instructions on soldering in the male header pins.
If you would like to breadboard the circuit for testing before moving it to the Perma-Proto board, follow this diagram, then jump to the coding test on the next page to try it out. Once you're satisfied everything is in working order you can return here to assemble the circuit onto the Perma-Proto for greater permanence!
Instead of soldering the Feather directly to the board, we'll used female headers. This allows the Feather to be easily removed during assembly, or even swapped out at a later date for a different one.
- Use the Feather M0 Express to properly align the female headers on the Perma-Proto board as shown here
- Use tape or poster putty to prevent the headers from falling out when you turn the board over
- Solder all of the pins in carefully
- Solder in the IC socket as shown, so that the DSP-G1 chip can be inserted later. Follow the column numbers and row letters as seen in the picture
- Be mindful of the half circle mark at the top of the socket -- it must be oriented with the mark on the left side as shown here. This will help us orient the chip properly -- the dot in the upper left corner of the chip indicates the first leg, and will be placed nearest the half-circle mark of the socket
- Follow these pictures to wire the chip's power, ground, and RX legs to the proper output pins of the Feather
- Be sure to clip off any excess wire after soldering each point
- Note the tiny wire used to jumper legs 5 and 6
- Add the resistor and both capacitors as shown here, soldering each in place, and clipping their excess leads
- Note the polarity of the 10uF electrolytic capacitor -- make sure the negative side with the white stripe is on the right side, with its leg in point F 26
- Run short lengths of hook-up wire to the positions as shown in order to connect the chip's audio output and ground to the 3.5mm stereo out jack
The breadboard-friendly headphone jack functions the same as any 1/8' / 3.5mm TRS (tip-ring-sleeve) stereo audio jack. We can use it to listen to the synth on headphones, or connect to any amplified speaker.
Insert the jack as shown, then solder it in place.
Carefully insert the DSP-G1 chip into the IC socket with the dot and half-circle marks oriented as shown-- you may need to bend the legs inward slightly first for a good, clean fit. Once you're sure the legs are all aligned well in the socket, gently push the chip down until it is snuggly nestled in place.
You can now re-insert the Feather M0 Express.
You've assembled the core circuit, next we'll run a simple software test.