Milestag sensor PCBs

As I've mentioned, I was fortunate to be in the USA last year and was able to get an American friend to order Milestag RevH PCBs for me, which saved me about 35UKP on postage. However, Jim was out of stock of sensor PCBs at the time. So I still needed to source these.

I decided to make my own. First I used Kicad to implement the simple schematic published on the Milestag site:



Then I designed a PCB around it. As I intend to make this myself, I changed the diameter to 31mm as this is the internal diameter of the 35mm holesaw that I have, and I replaced the two through hole resistors with surface mount ones to give me some more real estate to work with. Here is the design:



I repeated this 8 times onto an area of 160x100mm which is a standard dual layer size supplied by Maplin.

I then printed the top and bottom layers onto tracing paper with my laser printer:



The top layer is cut and overlayed onto the bottom layer. I align them using a lightbox to make sure the alignment is as accurate as possible:




When aligned, I fastened them together to form a pocket using cellotape and inserted the pre-sensitised PCB, fastening it with tape to prevent it from moving:



I then exposed the PCB to UV light from a single sided UV source, turning the board to expose the other side.

I develop the PCB with commercial developer, in the past I have used 99% sodium hydroxide crystals but this is nasty stuff. I got this from Mega Electronics. They do commercial grade PCB stuff but are willing to deal with individuals:



I etch my PCBs in a home made tank:

It's tuppaware with tropical fish tank heater and air pump with a wooden surround. Basic, but it works. God help me if it ever leaks though:



Here is the etched PCB:



I then cut each unit out with the holecutter:



After cleaning with wire wool, I used this stuff from Mega Electronics:



To tin plate the copper tracks to prevent oxidisation.

Here is the finished sensor PCB: