| RF Information |
A while ago, I started a conversation with a random person at a local surplus shop. We got to talking about RF transmission, and he came to admit that he was the head of engineering for a local radio station. The best advice he could give me with respect to my RF TX problems was "to add a little L and a little C" to the circuit. I soon gave up on building my own systems...
Simple Pulse Transmission
Many people simply want a way to trigger something remotely. A way to fire a catapult, for instance. The easiest way to do this is by purchasing a cheap RC car from your local Fry's (or equivalent) and butchering it. For $10, a decent transmitter and receiver pair can be purchased. Simply use the motor terminals as the output.
Drawbacks to this system: kids with similar RC cars will set off your device and the link itself is prone to false signals, as no encoding system is used. A better, yet more complicated method is to use an RF TX/RX pair made for data transmission and using encoding chips.
Data Transmission
The first step towards data transmission is obtaining a good TX/RX pair. This needs to be of a higher quality than the simple RC car system described above. I use a 432.8 MHz Velleman pair. It doesn't appear on their website for some reason, but is available at Tech America for around $20 per pair. 18 mW isn't bad for short range communication.
Now to modulate the RF signal. I use Holtek chips, as they are readily available and cheap.
I have experience using two pairs of Holtek encoding and decoding chips. The 12E/12D pair transmits 4 bits of data with 8 address bits. The 640/648 pair transmits 8 bits of data with 10 address bits. The 64x chips are larger, more expensive and slower than the 12x chips, and I only use them when I need to send 8 bits of data per transmission.
Holtek chips are superior to simple on/off RF broadcasting as they actually encode data, and verify it 3 times before doing anything with it. This results in a nearly flawless communication channel at speeds limited only by the TX/RX pair.
| | |