Monday, October 8, 2012

Christmas Lights

I finally got fed up. We bought a Christmas light "timer". This thing would do essentially 3 functions, the first was simply turning the outlet on, the second to use a photo sensor and turn off when it's light outside, and the final was a set number of hours (there being anywhere from 1 to 8 hours).  
The Problem
The problem with the hours one is that you have to manually turn it to off, then back to the number of hours you would like. This is frustrating. The photo sensor would seem like a neat idea, however when you don't get good lighting where the photo sensor is, you end up with Christmas lights on all the time. I (and in part due to my wifes request) decided this had to be rectified. 
Attack Number 1
My first run at it was to simply solder onto the point where the timer is on, and try to set and clear the 4 hour timer (but since the system needed to go back to off, then back to on, this failed miserably). 
Attack Number 2
Then I realized that the photo sensor should handle the turning off and on automatically, and I wouldn't need to turn it all the way off. I decided to remove the photo sensor, and put a pair of wires in there and a relay instead. Now the fun part. I have a PIC16F684 that I decided to control this with. With a multimeter I tested putting the photo sensor under the light in my room, when it was under direct light, the resistance went up really high, when I covered it, it seemed to reduce to a low amount of resistance. My guess is that I can put a resistor in line with the relay and whenever there is a high amount of resistance it turns the system off. When the resistance is low it turns the system on. Essentially what I needed was my microcontroller to activate my relay for 4 (or so) hours then turn it off. Then 24 - 4 hours later, it would attempt to turn on the relay again. If I can get this working on a short run, I.E. 10 minutes on, then 2 hours off, then 10 minutes on. I may consider putting in a pseudo random number into there, so the lights are on somewhere between 3 and 7 hours.
Unfortunately I plugged it in, tried shorting the pins for the photosensor (hoping it would activate the lights), well it didn't seem to matter whether the wires were shorted or not, the lights just stayed on.
Final Solution
I finally actually did what any smart engineer* should do. I took a look at what was already there. I saw 2 chips the first was a LM393 a dual voltage comparator. And the second was a programmable timer. JACKPOT. (Here it is for those of you interested: cd4541be). Looking at the datasheet I see there is a master reset line. I tested it out by turning the system on for 1 hour, then when the lights turned off, I simply tapped 3.5 volts to the MR line and bam lights were on again. I now have my "in".

On to the Microcontroller
I decided to use my Pic 16F684, I have a smaller one (can't recall the number at the moment), but I didn't have time to make sure my code ported smoothly as well as verify the timing. I did a couple tests using a couple nested for loops for a wait (I know, I know, I could have used a timer interrupt, and in fact i would like to go fix this, but I didn't have a lot of time to mess around with this and well, hey it was a hack)
So after some timing, I simply put an initial wait in there to get it to hold off to about 7pm from when I plugged it in, and then set the timer to 4 hours. The lights turned on for 4 hours, then a few hours later 7pm hit, and bam we have light.

However over the long term the timing was off for sure, when I got back from vacation it was definately wrong, I'll have to play with it this christmas again!

*Smart engineers keep things simple, and just using a reset line is MUCH simpler then the previous methods I tried.