Now you don't think I just cooked this up in my head one day and built it, do you? Where's the fun in that? I definitely faced more than a couple challenges in getting this thing to work.
The Basic Stamp
Originally, I had planned to use a Parallax Basic Stamp I in my project. It had many features I desired: a simple programming language, me already knowing that language, and the fact that I already had three. But as I built the first test circuit (consisting of the Stamp, a timer, a shift register, and a light), I realized that the Stamp could not reliably set the brightness of the light at any high frequency of the clock. The only solution was to get a faster microcontroller. So after much browsing for microcontrollers, I decided that the low-level assembly-programmed PIC16F84 was for me. This was no small decision - this meant I'd have to learn assembly language (something I was only partially familiar with; even then with the x86 assembly, not PIC) and buy a programmer (Those aren't too cheap!). So I hunkered down one night and read the entire PIC16F84A datasheet - just about the driest reading I'd ever read, but quite interesting once I understood it. I had also ordered a PIC programming kit off of QKits - that same place in Canada where I had ordered the TX and RX433 modules. Once built, it was really a no-brainer to put these programs together. (Note: programming software is not included, and link provided to download software is dead... I found an alternate link once but forgive me... I can't find it again; so try the USB version instead) I had those lights blinking in no time.
Imperfect radio transmission
Well nothing's perfect... the TX/RX pair have a few caveats. The TX433's constant current draw (whether transmitting or not) can be ignored, although it was an issue in my wireless sound system remote control project. The RX433, when not actively receiving a transmission, can sometimes fire random pulses that can be mistaken as RS232 start bits. This is an issue when a transmission starts shortly after one of these pulses - everything will be shifted off. Also, when the receiver is very close to the transmitter, high pulses last a little longer than they should, and when it's just at the edge of reception, the high pulses last a very short time. My compensation for this is less than perfect, and is explained in the code section.
The RX433's power requirements
Apparently the RX433 can't tolerate pulses in its power supply - these get translated to signal pulses. Imagine my surprise when I found out the LEDs turning on would cause a square wave to come out of the RX433's signal pin! I fixed this by placing a 220 ohm resistor and a 100 microfarad capacitor in the fashion shown on the schematics page. This attenuates the high-frequency interference quite a bit, while only dropping the voltage about a half a volt from the supply. The remaining voltage is sufficient to power the receiver for the entire range of operating voltage for the PIC.
So now that I'm finished telling you what went wrong, you can reap the benefits of what went right.
On to Page 3 - Schematics
Copyright © 2003 by Nathan True - wlcolor at natetrue.com