The Phillips FM320 UHF CB has the facility to be locked to a particular channel. This is controlled by the existence or absence of a number of diodes on the circuit board. I’ve been looking into this for a telecommand project I’m working on. I wasn’t able to find the information of how to do this anywhere else, but it was pretty straight forward to figure out.
This post describes how modify the FM320 radio to set the nominated “NOM” channel.
Here’s a blast from my radio past, the Phillips FM320 UHF CB radio. I’ve dusted one off from the slowly collapsing box of radios left over from my farming days. My plan is to use one as the receiver for my DTMF pump switch.
As a kid I had always been encouraging Dad to consider radios for the farm. I was a regular reader of CB Action and it seemed like the perfect excuse. He never succumbed to this until the advent of UHF CB. We had seen them at one of the field days, and organised for a Phillips representative (a German guy called Hans) to come out to the farm and do a demonstration. Hans arrived with a wind up mast that he set up at the house and wound up to the same height as our TV antenna. Then we drove around the farm to see what the coverage was.
I’m in the process of replacing a windmill on the farm with a solar pump. The windmill, carefully adjusted, can keep up with the water consumption at the house and troughs a couple of kilometers away. It isn’t perfect; sometimes the house tank overflows and we either water the garden or go down and turn the windmill off for a few days. Sometimes the tank sounds a bit empty and I drive down to the windmill and adjust it for a bit more flow.
The solar pump is not so analogue. In order to lift the water the height needed, the centrifugal pump is oversized in relation to the actual flow required. The pump controller can be turned down to some extent, not not enough to avoid overflowing the tank and wasting water. Some system is needed to turn it off an on remotely. My current plan for this is to use DTMF tones over UHF CB radio.
Dual-tone multi-frequency signalling (DTMF) tones are used widely in telephone systems. The frequencies are chosen to be easily transmitted in the narrow voice bandwidths and using two tones for each key allows for a small amount of frequency drift as well as good rejection of noise.
The frequencies were originally chosen far enough apart to use analogue bandpass filters for detection, so it’s possible that someone smarter than me could implement a simplified Fourier transform that would run on the Arduino. As I need this to be robust, I rejected that idea and decided to use an MT8870 Integrated DTMF Receiver. Continue reading