Before finishing the dry wall in my garage, I pulled some speaker wire to connect a pair of ceiling speakers in the garage to the receiver in the family room. Having some degree of foresight, I also pulled some thermostat wire to a wall plate in the garage. Running back and forth between the garage and living room to adjust the volume would get old real quick. Initially I was planning on adding an IR repeater to control the receiver from the garage. While looking into the commercial IR repeater interface, I stumbled upon a DB9 RS232 connector on the back of the receiver. This connector allowed for control of and feedback from the receiver, plus a great opportunity for a microcontroller project. A set of built in controls using this serial connector had two big advantages of the IR repeater: no remote to lose or break in the garage and bi-directional communication from the receiver for feedback.

I could not locate a manual for my Denon AVR-2805 serial connection but the manual for the serial protocol for model 3805 was available. First step was to hook up the laptop to the receiver and test it with a USB serial adapter. The commands and feedback worked mostly as described in the manual. The terminal text was saved and annotated a bit to help with the software development later.

I had run two 5-wire thermostat cables between the wall plate in the living room and the one in the garage. On the garage side of things, I think I could have just used buttons and leds with the electronic portion located in a project box behind the TV. I was looking for a cleaner solution than an extra box behind the TV and thermostat wire running out from the living room wall plate. I had some S-Video keystone inserts that would support 4 wires: Power, Common, RX, & TX. This would allow the whole project to be contained behind the wall switch.

Next decision was how to power the in wall unit. Putting batteries in the wall easiest option and likely the most efficient. The need to change the batteries, having to remove the wall plate to change the batteries, and the potential to have a leaking battery inside the wall killed this option. Second option was putting a power supply in the living room behind the TV. This violated the clean solution goal. Don't need any more clutter or cables crammed behind the TV and available electrical outlets are scarce. Started looking into using one of the 12V relay outputs from the receiver. One cable to the back of the receiver would be optimal solution even with a pig tail at the end. After some investigation it seems the 12V power supply for the relay outputs was only energized when the receiver was powered on. So if the receiver was in stand-by mode, the wall switch would not be powered. The 2 relay trigger outputs are only rated for 25mA a piece as well. The 6V stand-by power is always energized, perfect. (Well it's always energized as long as the physical button switch on the unit is on.)

So, all the wires from the receiver were tagged and disconnected. The receiver was pulled out and opened up. A miniature transformer, rectifier, and regulator supplied a 6V stand-by power. The 6V stand-by power was connected to pin 9 on the RS232 connector. The RS232 connector was on its own little PCB allowing for a fuse and reverse polarity protection diode to be added. Only the RX, TX, and GND pins on this board had traces running from the connector. I didn't get any photos of the modification; wasn't the cleanest modification as it was pretty tight access. Removing the mains power board looked like way too much work and too much risk of breaking something else. The modification worked and is hidden out of sight. So now one DB9 connector can provide the 4 connections needed to the wall switch in the garage.

The thermostat wire was a bit too thick for the S-Video keystone punch down connections. Ended up ripping the punch down connectors apart and soldering the connections. One end was cut off an S Video cable and the DB9 connector added....