Serial Data Transmission with RF - Improved Hardware and Range Testing

A better receiver is justified. 

The build quality of these super regenerative receivers, like most of the Aliexcrement or Crapbay stuff from China, is woeful and I have little confidence will work on an extended basis. I had two receivers. After getting them working due to manufacturing faults one stopped working again very quickly. The second unit works but appears to suffer from dry joints because the performance is inconsistent.

During development I discovered that the receiving module is a super-regenerative type. I was able to confirm, as expected, that it generates noise on 433.2MHz which prevents multiple receivers from being located close together. But the biggest concern I have is temperature stability. While it doesn't matter so much when located in-side a house, it does have implications for being located outside. As ambient temperatures change, say from 0 Celsius to 35 Celsius, the frequency to which the receiver is tuned will shift. That will reduce the sensitivity since the transmitter is in comparison stable. So not only do receivers need to be physically spaced apart they also need to be in a relatively stable temperature. 

Search for 433MHz receiving modules on your favourite site and there will be plenty to chose from. Get one with a crystal like this one and you should be far better off for the few extra cents.

 

However, not all modules work. The one pictured above was purchased from LCSC. A mediocre datasheet did not help get it working. A typical decoded data stream is shown below:

I gave up trying to work out why these didn't work when the second one I pulled from the packet had an aerial that was swinging in the breeze.  My suspicion is the data rate was too fast for the receiver chip.

I trawled my junkbox and found another receiver module. The MPN was RX520. Like the module discussed above, the output also tended to idle in a high state. So I altered my code slightly to account for this.

Then, in a real twist, I discovered that the for this second superheterodyne module the first pulse following the start pulse is slightly longer than you would expect. Perhaps an artifact of AGC action since logging strong pulses and weaker pulses showed a clear difference. This is not an ideal situation. However, adjusting the 1/0 threshold allowed range testing to begin.

A preliminary range test was made. The transmitter used a vertical antenna, but no ground plane, and was located on the ground. The receiver used a 13cm piece of wire. Neither was ideal. But this combination would work reliably at 35m line of sight when no interference was present. The interference was evidenced by the flashing led cycling on and off correctly for up to 4 seconds, then it would "freeze" for a random length of time before working again. 

On balance I'm satisfied with the result. I have a list of further potential enhancements which might improve the robustness of a link. My first real application may not require them so I'll push on with that application.