|All News -> Designer News|
|Written by Nicholas McClanahan|
|Wednesday, 15 December 2010 10:10|
Experiments with RF
XBee's are nice, but they're too expensive for the stuff many folks want to do, they're also pretty complicated. Fortunately, frequency synthesis for broadcast is actually pretty straightforward. When firing up the TV, you just specify the base frequency & change the pin mode, and bam, you've got a video signal on your TV. Here's how to set it up;
You'll need a Propeller, 12 feet of hookup wire & 3 resistors; 1.1k, 560, and 270. The circuit is below. Don't worry, it's the standard Propeller video DAC, if you're using a ProtoPlus, you've already got the circuit, you just jam about 6 feet of hookup wire into the center of the RCA TV jack.
Here's the broadcast antenna;
It's just 6 feet of braided wire jammed into the RCA video jack
And here's the receive antenna
This is a pretty pathetic antenna, just a standard coax cable with the braid stripped off.
Total range is about 30 feet, but that could improve significantly with a better receive antenna.
First, writing a TV driver from scratch is quite an exercise & requires a solid understanding of NTSC. Fortunately, the Propeller Tool includes tv.spin, which we'll use for our task. In fact, I'm going to use the Graphics_demo.spin object for demonstration. Open it up and save it on your desktop as 'broadcast_demo.spin'. Scroll down to the DAT section and change 2 variables;
Connect 6 feet of wire to your TV antenna input (the center pin), and connect 6 feet to the RCA jack on the Propeller. Turn on the Prop (and your TV). Make sure your TV is set to 'antenna' input instead of 'cable', and tune to channel 3. If you don't immediately see it, use your TV's autoscan function. We're transmitting a little bit under the standard channel 3 (61.25MHz), so your TV might need to scan while it's broadcasting to lock in the signal. Here's a video of what it looks like;
Disclaimer: Check your local laws before experimenting with RF broadcasts — Some bands are pretty open (Lowfer) while others are restricted. The erp of this setup is pretty tiny, though.