Friday, October 21, 2016

A simple board for a project

I've decided to shelve the energy monitor for now, as I need to do some consideration on how to actually get data into the system. I've decided to use the wall box with the battery charger for something else - a power supply for the existing wireless sensor transmitters that I have running on the network.

In the past, I'd probably have used a Radio Shack protoboard to lay circuits out on, but the maker economy has brought a number of low volume PCB vendors to the market. It's easy enough to lay out a board, and get 3 boards for $5/sq. inch.

One such house is OSH Park, which manufactures boards on community panels - that is, your order gets added to a bunch of other people's orders, and the fab house makes everything on one big panel. You get your boards, and it's cheaper than what conventional prototype houses offer. It just takes a while (few weeks) to get your boards.

My first board design with them is a simple unit. This design features a low ripple 5V DC-DC converter to power existing wireless nodes. The regulator (U1) is 1/2A, which is fine as the nodes draw about 60mA, and there are currently only 3. It's fully bypassed and has some extra filtering onboard just in case.

The other section of the board is a simple rectifier and regulator to power a relay. This side takes the AC directly from the battery charger's transformer, rectifies and regulates it, and turns a relay on to trip an alarm. AC goes away, relay contacts fall, and alarm is triggered, the alarm itself running off the battery that was just being charged.

The circuit itself is nothing fancy, a simple LM78xx is used here. The only thing I decided to do was lay pads for either a TO-220 or a TO-92 regulator. The relay itself is a Hamlin 3700 reed relay, a batch of which were purchased on eBay (but the footprint is standard SIP.)

In all, not a whole lot is going on here, but doing it this way simply provides a nice, clean layout.