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Old 09-15-2013, 02:52 AM   #491
SVXdc
Scooby Specialist
 
Member#: 25837
Join Date: Oct 2002
Chapter/Region: South East
Location: Atlanta, GA
Vehicle:
1996 SVX L AWD
Laguna Blue Pearl

Default

cs1986's circuit uses the relays and fixed resistors to simulate pressing all of the SWC buttons on the wheel.

When I thought about this awhile back, it occurred to me to search for "digital potentiometers", and I found that they do indeed exist. Check out the Microchip Technology Inc. MCP41X2 (single) and MCP41X2 (dual). Those are both rheostats (the variable resistors only have 2 terminals), which is all you need for this application. They also make digital pots (3-terminal resistors). I'm sure there are also many others from other manufacturers.

Using one of those would simplify the circuit considerably, yet give the flexibility to be programmed to talk to any Subaru HU (or for that matter, other cars that use resistive SWCs).

A 5 k ohm part with 8-bit divide should work. I think the OEM HUs have enough tolerance (e.g., if you send it 19 ohms, that's close enough to 22 ohms for it to read it as "mute").

Subaru SWCs have a parallel resistor that the HU sees when there are no buttons pressed. You would need to include a similar resistor in your circuit (4.7 k for cars without the phone/BT buttons, or 100 k for cars with phone/BT buttons). To design a circuit that can work in any Subaru, the digital resistor on the first channel (pin 4 on the 20-pin radio harness) would need to allow the fixed resistor to be switchable with either 4.7 k or 100 k. The fixed resistor on the second channel (Subaru pin 13 for Impreza/Forester/Crosstrek or pin 11 for Legacy/Outback) can always be 100 k. See cars' resistor values here.
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