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Table of Contents
CM108 Sound Interface (SMD edition)
The CM108 is a USB sound interface chip that has become popular among radio amateurs due to it's low price (~£3.50), ease of modifications, and the presence of GPIO pins for triggering things like PTT or detecting squelch opening.
There are now many different guides out there to modify them, but I recently came up with a design optimised for Dire Wolf and simplicity which I've documented below
Note: The CoS detection isn't connected in this example. It will not work with AllStar Asterisk etc, but will work with Dire Wolf for APRS and Packet
You will need
- 1x CM108 USB Sound Interface
- 1x 10k 805 SMD resistor
- 1x 1k 805 SMD resistor
- 1x 2N3904 SMD transistor in SOT-23 package
- 0.05 mm² / 30AWG Kynar Wire
- 1.8mm thick cable ties
- 4 core cable of some flavour
- Soldering Iron, Solder, associated tools
- Drill + 2mm drill bit
- Hot Air Station (Optional)
- Multimeter (Optional)
Modifications
Remove components
First of all you will need to remove the 3.5mm jacks and the unused components highlighted in the image below. You may find it easier to use a hot air station.
The large capacitor is removed to free up a pad for the PTT line. The two smaller resistors are removed to disconnect the DC biasing designed to power a microphone.