Helpful In-the-Field Engineering Tips
Here's some interesting information sent from our deployment coordinator, Paul Smith, Technical Director for the Center for Neighborhood Technology:
The small switches, routers, and wireless accesspoints are typically powered from low voltage, external power supplies of the wall-wart kind. The power consumption ranges usually between one and two amps, the voltage is usually 5V, 12V, or more rarely 6 or 9V.
To operate such stuff without the mains, you need a suitable battery; I usually use 12V DC from a car, more rarely 24V DC from a truck, or 12V DC from a sealed lead cell battery cannibalized from a dead UPS.
To power any given piece of equipment of the above, a grab bag of things is needed.
* Power connectors. Depends on the boxes you want to keep in operation. They are fairly standardized. In case of lacking them, you can solder wires directly on the boards, but it voids warranty and is ugly. Also clamps for connecting to the car battery terminals.
* Integrated stabilizers. Get some 78S05 (2-amp) or 78T05 (3-amp) and some 78S12, which will make 5 or 12V from the battery. (12V battery has about 14V typically, and a car generator gives a little over that, so even the drop on the 7812 won't matter much). Some 78S09 just for case won't hurt. Some diodes, as with two diodes on the middle pin of the 7805 chip you can shift the voltage up to about 6.4V if needed. Fairly every electronics hacker should know how to connect these.
* Capacitors for the stabilizers. 100 nF and 100 uF in pairs will do for vast majority of cases. You can't go wrong with these.
* HEATSINKS! The stabilizers will make a lot of heat when under higher load. Use bigger heatsinks, at least of the size cannibalized from dead PC power supplies. The ones from old Celeron CPUs are even better, but you have to drill a hole in them so you can mount the TO220 cases on it. Do not forget these! You will need them. A bit of thermal grease can be a big improvement here.
* Gas-powered soldering iron, flux and tin, for putting it all together on-site.
* Voltmeters, one for each installation, to keep them attached to the batteries used. The cheapest $5-7 a piece will do, the purpose is to have the power supply unit checkable by a single glance without having to debug around. Important for high-stress situations.
* Some transils. In case of using generator power, transils for protecting the sensitive equipment from voltage spikes from the generator coils can save you money. Good to use by default when you don't trust the power supply. Use the unidirectional ones that behave as diodes in the other direction (similar to Zener diodes as a whole), in combination with the Polyswitch fuses below they will also protect you from the operator error, eg. connecting a battery with a wrong polarity.
* Polyswitch reversible fuses. Get some for 0.5, 1, 1.5, and 2.5 amp. Put them before the stabilizers, as close as reasonable to the battery. In the field conditions where ad-hoc installations are prone to shorts, they will save you many smoky sparkly surprises. In addition, they become conductive again when the power is interrupted, so you can't run out of fuses even when you get tired and less than attentive.
* Laptop, for configuring the boxes. Get a wifi card for it, and install Kismet or Netstumbler or other wifi analyzer software, for quickly seeing of what goes from the equipment into the air and what channels are available to claim. Also software like tcpdump or Ethereal, for network analysis.
* Cables. Lots of spare cables. Power, Ethernet patch cables, bare CAT5 and connectors, for quick deployment of ad-hoc LANs as needed. It's way too easy to forget about cables. Don't forget a crossover cable, for direct connection of the laptop with the accesspoint/router.
* Glue sticks for hot glue gun. The hot glue, melted with eg. a propane torch, is good for insulation of naked wires and solder joints and their waterproofing.
* MANUALS for the boxes you are going to install. They contain the default passwords.
* Some means for the team to communicate. Depends on what the crew likes, may be handheld transceivers or a whistle for Morse code or binoculars for communication via ASL over direct visibility.
* Some LEDs with suitable resistors, as power indicators. If they won't help, they won't hurt.
* Power invertor 12V to 110V. Will allow running fairly any mains-powered equipment from a car battery.
* Perhaps some UPS units. When running on a generator, the power is not exactly stable. Line-conditioning UPS units will improve state.
That's about all. A Fry's store should have it all. Maybe even Radio Shack.
When connecting cables, tie them both into a knot and then solder together the wires. The mechanical load on the joint will then be absorbed by the knot instead of the individual wires, greatly enhancing reliability. Same can apply to connecting things via connectors - depends on if you prefer pulling at the cables or disconnecting the connection when somebody stumbles over the cable.
Engineer defensively, and have a lot of luck there
