Good Things Come in Small Packages

A project log for LoFi

LoFi is a very low cost ($5), small, auto-transmitting module. Preassembled and preprogrammed. Simply attach to your appliances or projects!

David CookDavid Cook 09/02/2014 at 05:010 Comments

I’m experimenting with some novel enclosures for various installations of LoFi around the house and yard. I need to make an impact for the stage 3 judging.

I ordered some more m&m’s candy tins, because they are an excellent size and have a clear top window. When you buy them, you can customize the candy pieces with logos and writing. So, I used the Hackaday Prize logo.


I appreciate the industrial design talent at Apple. Here is LoFi inside of a standard Apple power adapter, which is approximately 1 inch cubed (25 mm). In the USB slot, a small panel holds a light sensor, an ice cube LED, and the antenna. Yes, it really works.

The main board holds the CR2032 coin cell and has a standard 0.1 inch header for the transmitter and accessories. There is enough room leftover in the center for additional sensors.

LoFi runs for over a year on a single coin cell. Although I usually use a plastic holder, metal holders are cheaper and use less space. I designed a LoFi board with pads on the back for the ground terminal of the cell, and then bent the metal holder tabs around to the front of the board to attach the positive terminal.

Unfortunately, I made a mistake. Upon inserting the coin cell, nothing happened! I checked my connections, looked for shorts, and even wrongly suspected the new ultra low dropout diode.

The problem turned out to be my failure to account for the height of the solder mask. You see, the solder mask goes on top of the copper layer, and thus is slightly higher. The back of the coin cell was resting on the solder mask and not making contact with the pads on the back of the board. Applying a thin layer of solder to the pads raised it up enough to make contact.


I have a really interesting application where LoFi needs to fit inside of a 1-inch circular tube, rather than 1-inch square. The tube was machined from a green medical grade Delrin/acetal block.

As you can see, LoFi managed to squeeze inside. A blue lens is machined to fit on top.

To make LoFi fit in the smaller space, I needed to insert a longer header in the front of the transmitter and bend it back 180 degrees (see red arrow below). However, this would not be necessary in a production device, as you'd use a smaller connector. Anyway, bending the connector allows the socket to overlap the bottom of the board and close the gap (blue arrow). There is enough room in the tube to fit a helical antenna – with the accompanying reduction in range, of course.

LoFi can now fit into a standard prescription pill bottle. A vibration sensor (added after the photos were taken) detects when someone is opening the bottle. The light sensor with the blue filter detects a significant change in brightness compared to when it was within the orange bottle.

From this prototype, I determined that a pushbutton switch recessed in the side of the tube would be superior, as it would detect that the cap had been removed (rather than the bottle just shaken) and it would work in darkness.