Work's got interesting again
Posted by stephen on Sunday, 18th March, 2007 @ 22:05
The Problem
Our client is building houses on three plots of land, and demolishing two existing buildings. As part of the project they want time-lapse photography of the process to be used in an exhibition later in the year. Ground works are already underway on the new builds and demolition is scheduled for a week after our initial contact with them. Multiple points of view will be needed to cover the whole construction area.
The Solution
Three cameras placed to cover approx. 270° of the site. There is no power or network reaching the three locations, all of which are on masts above existing buildings, two of which are very high up. To simplify the process we agree to provide equipment that can run from a DC power supply and that two of the cameras will connect wirelessly via the third to their network, and on to our server which will control the cameras and store the captured images.
We'd heard of a similar project using a digital still camera controllable via it's USB port, a USB device server that connects a remote USB device to a machine over IP, all stored in a standard CCTV camera housing. With a little bit of research we discovered that the company that makes the USB device server also makes a wireless variant with a standard antenna connector. So presumably it'd be possible to put these in the CCTV housing with two of the cameras along with an access point to receive the signal in the third.
With a tight deadline looming, orders for all the various parts were placed as soon as the purchase order was received, including a nice chunky PC with 1Tb of storage for lots of pictures. Annoyingly the cheapest digital camera that could be remote controlled and was currently available for sale was £200 as opposed to our intended £120 budget for it. As gphoto supports controlling the camera we were hopeful we could use Linux on the server and save on the cost of licenses for Windows and the camera control software, offsetting some of the increased costs of the cameras. The only unknown was the USB device servers.
Everything except the antennas and wireless access point was ordered on the Friday and the first stuff to arrive was the digital cameras on the Saturday. A quick play with gphoto and sure enough I could get the camera to take a picture and store it to the memory card, but gphoto dropped out with an error before downloading it. Some prior research had indicated that it might be necessary to use current development source code, so after a long and fiddly compile process I tried again, and sure enough everything worked. The camera took the picture and downloaded it immediately without writing to it's memory card. And even better gphoto has built in options to take a series of photos with a given time delay. Perfect!
The next problem to think about was power. In two of the cameras we need to power the CCTV housing's built-in heater (to avoid condensation of the lens), the camera itself and the USB device server. In the third we add a wireless access point. We've agreed to take a DC power source and that'll we'll specify the PSU and cabling they'll need to install for us. The heater is 12V @ 1.2A, the camera is 4.3V @ 1A, the USB device server is 5V @ 1.5A and a wireless access point (WAP) is 5V @ 2A. So we're looking at up to just under 40 watts in one of the units. A 60W 12V power supply is commonly available and seems like a sensible option.
So the next question becomes how do we get from 12V down to 5V for the USB server and WAP and 4.3V for the digital camera? Well, it's been years since I've done anything with electronics, I was probably in my mid-teens and was never any good at it anyway. I didn't get much further than lighting LEDs and opening up things, looking at the circuits and realising I didn't have a clue what was going on. Fortunately Chris's recent experience with the Modtronix stuff has introduced me to the concept of voltage regulators. These are wonderful little ICs that promise to give you a constant voltage output if you can provide anything from a little over the voltage you want up to something several times bigger. In this case a 5V regulator can take anything from 6V to 20V. Ideal!
So that gets me my 12V supply for the heater and a 5V supply for the USB device server and WAP. A little research with Maplin shows that 1A and 2A regulators are available, so the current demands won't be a problem. But how am I going to get down to 4.3V for the digital cameras? Maplin don't sell regulators to get to that voltage. Perhaps a resistor after a 5V regulator will do the trick?
