For a simple RPi-powered APRS station (not mentioning the radio) you would need:
- Raspberry Pi - SDcard (4GB or more) - 5V Power supply to the raspberry pi (micro-usb connector) - Network connection (for iGating and/or getting accurate Internet time)
Now, here are some of the options available:
Raspberry Pi model choice:
The Models A and A+ do not have an on-board Ethernet adapter. They can be used as stand-alone units just fine but the main issue is getting the time correct. One method is to use a WiFi adapter. These are fairly cheap but rely on a nearby WiFi network. It that network becomes unavailable the rpi will slowly drift in time. Once the network is present again it will just auto-sync with the Network Time Protocol (NTP) on the Internet. If the WiFi network is unavailable at start-up of the RPi it will default to 1 January 1970.
One other option for timekeeping is to make use of an add-on module attached to the GPIO header pins on the RPi. At this stage I can think of two possible modules that could give the RPi accurate time: 1) a battery backed-up Real-time Clock (RTC) or 2) a GPS module. The RTC would need to be set either by an initial clock synced from the Internet over NTP or set manually from the command prompt in linux (may write a web-gui page for that at some stage). The benefit of the RTC is that it keeps the time even if the RPi is switched off. So you could bring the RPi to an location where there is no GPS signal and still have near-accurate time. Most RTC's are fairly accurate but they do drift. Once a week (or month) syncing over NTP would be nice.
The second option, a GPS module would give you the most accurate time when a signal is available. It's like an Internet NTP connection. If none is available at startup, the RPi would live in 1970.
From a power point of view I would think that the RTC would be more efficient than a GPS module but they're all down to only a few mA's.
Testing and setup:
Of course you may wish to see what the RPi is showing on screen and especially for debugging network connectivity it is easiest if you have an external monitor and keyboard available:
- LCD monitor with DVI connector and cable. (or you could find one of those VGA to DVI adapters I got and use an old VGA monitor) - USB keyboard - USB mouse
Even though none of these are required for normal operation of the APRS station they could make life a little easier.
5V power suppy to raspberry pi (micro usb):
The RPi draws up to 700mA. Add to that any additional peripherals (WiFi adapter, Keyboard, Mouse, TNC-Pi, Realtime Clock) and you may run quickly over the maximum of 1A per usb port. In those cases it is recommended to use a Powered USB Hub. Several of these will allow 'backfeeding' to the RPi. Meaning, that you don't need a micro-usb connection to the RPi but the powered usb hub will just put the juice back in the RPi over the USB port it is connected to. Although this saves cables and power supplies it comes at a cost. The designers of the RPi have put quite a bit of effort in protecting the power input of the RPi and backfeeding it though USB will negate all of that. You are reliant on the powered usb hub's power supply and can only hope that it's equally well protected.
The older RPi models A and B have a full size SDCARD slot, the newer A+ and B+ models have a micro-SD slot. Don't use those SD-to-micro-SD adapter cards, just get the right size for your board.
When it comes to the operating system (Raspbian) I will be more than happy to flash the image onto it as my version will have the web-gui, the statistics and the maintenance scripts installed. For the die-hards I am happy to make the sources and scripts available, but I haven't written an installation guide for it.
One thing I haven't mentioned yet is the TNC.
At home I am running the software using a USB Soundcard as a TNC. This seems to work very well but I haven't finished testing yet. The sound card was purchased of eBay and was only $10 or so. I prefer this method as it does not involve any propietary 3rd party hardware. If you simply wish to be on-air before I have finished testing then the TNC-Pi from Coastal Chipworks is what Warren and Russell are using. These are the units that we have come to known over the last couple of months.
The TNC-Pi uses a DB9 connector for which you will have to make up an interfacing cable. It's pretty simple with RX_Audio, TX_Audio, PTT and GND. Output levels from the TNC-Pi can be adjusted with a simple trimpot and the incoming signal may need to be adjusted on the radio side. RADIO
The UV-5R is not a 12V or a 5V device. It has VOX which is probably what you want to use but you may need to come up with a clean way of continuously powering it. I believe there are 'battery eliminator' packs available which just plug in the back like a battery but will accept power from 12V. This may be the easiest method of running at 12V.
Apart from a decent antenna, that's pretty much it.