This is a rack and tray for mounting an Ambient Weather PM2.5 Wireless Outdoor Particulate Monitor outdoors in a way that you can still use it while it's plugged in. Much of the time, these sensors are solar-powered, and they can be run most places entirely on solar power. However, if you are far enough north and have enough clouds, solar power is not effective any sort of device like this over the winter. Since this device works fine plugged in, the obvious solution is to either recharge it regularly, or just keep it plugged in. But in hot weather, that stresses the batteries via overheating. Also, the charge cord comes straight out the bottom of the battery compartment, creating a very difficult situation in terms of placing the device on any surface when in use. (Think Apple Magic Mouse. Same problem while charging.) Accordingly, I have designed this tray and rack system to hold the device in a reasonable position for extended periods while driving it with a USB charging cable. Originally, the idea was that you could slide the tray into the rack even with the cord plugged in, but in some plastics the rack became too flexible. So I abandoned that part of the idea. The rest is still good though, and keeping the tray still a separate object allows more refinements by other people and also keeps water flowing away from the electronics of the device. Please note there are TWO DIFFERENT RACKS. One is for use with the tray, and is for when the battery cover of the monitor has been removed and it's sitting on the tray running while charging or being powered solely by USB. This is the rack seen in the photos. It's right next to an outlet. The other rack is for use without a tray, as it is exactly the right width for the monitor when fully assembled and running on battery and solar. I have that rack out in full sun, and the monitor just sits on it. The tray mounts are not quite symmetrical and smaller than the charging rack, and the fact that it fits the fully-assembled monitor perfectly was an accident I was happy to use. ^_^ It's sitting out in as much sun as I can get it.