Design Goals & Background

A project log for Appliance Timer

Rugged & simple timer to control everything from room lighting to air conditioners.

Brian CornellBrian Cornell 07/15/2018 at 15:140 Comments

Expanding on simple & reliable.  I wanted a small form factor and something that could be put together without custom enclosures or special milling & routing.  That meant nothing more than a hand drill.

It also had to be weatherproof and relatively inexpensive to build.  And at the time I wasn't interested in remote control or a smartphone interface.  That meant no buttons, dials, or switches.  So it would need some form of sensory input for control.   I looked at light, sound, even vibration (like tapping the box) but opted for light since that allow light-based timing cycles.

I settled on a single pole Solid State Relay (SSR) for the switching.  No relay eliminated problems with arching, carbon on the contacts, or poor conduction due to not meeting the minimum current requirement.  I stuck with the flyback gate drive from the prototype and settled on a 6.3mm dia toroid.  With 26/28AWG wire and a handful of turns it's easy to build and mounts directly to the board.

I chose the same ABS case I had used for the Triple5:  cheap & small form factor.  Seals well and easy to work with.

Surprisingly I spent a lot of time on finding the right strain reliefs.  They needed to mount in a drilled hole, be weatherproof, and not too bulky.  With a home build it seemed harder than it should have been.  I started with the compression bushings you see on a lot of small appliances but they are not weatherproof.  I then went to PG9 spiral reliefs which are strong & weatherproof but very bulky & expensive.  I finally settled on molded strain reliefs (the kind normally molded to the cord) after I found a glue that secured them to the cord.

I stuck with a PIC MCU and went thru several variants in the 8-bit midrange family before finally settling on the 16F18324.  More on that later.

The logic power supply took some time.  It needed to be cheap, compact, and simple but provide all the requisite protections (over-current, thermal, etc.).  So far the TI UCC28880 has served me well.  It's a monolithic, non-isolated buck solution that requires just over a dozen components.  The topology is flexible, with some design trade-offs, and I found it necessary to modify the reference design to temperature stabilize it.