Help random Data in 433mhz reciver - Page 1
Forum section: electronics / projects

author: Isad
author_rank: Frequent Contributor
timestamp: May 12, 2016, 12:59:20 pm
content: Good day evryoneI am trying to recive some data from a 433mhz reciverand well when i power the device it start to send random data out i ma not transmiting anything with the remote it cameyet its still giving somthing.If i transmit somthing it will pick it up and after i stop it will start its usall behaviur.Its using a max1473 for reciving the data and the transimter is a standart 433mhz transmiter.

author: Kjelt
author_rank: Super Contributor
timestamp: May 12, 2016, 01:44:55 pm
content: That seems normal, you are not the only person in your surrounding that transmits on that frequency, so everyone near you that has a car dooropener, windowshutter, airco, or whatever that is remotely controlled will disturb your receiver. So the software should know the to be expected protocol and filter on that specific pre-amble and expected codes and drop everything else.If you want to test in a clean surrounding you should build a faraday cage around the test setup.

author: Buriedcode
author_rank: Super Contributor
timestamp: May 12, 2016, 01:59:16 pm
content: As Kjelt said, its quite normal for a superhet AM receiver to just 'pick up noise' when a transmitter is not transmitting.Also of course, the 433/434 band is extremely common for keyfob control apps, wireless power meters and wireless weather stations (or just outdoor temp sensors). These applications transmit at very low duty, so whilst your receiver will probably pick them up, it'll be a few every minute.The way this is often handled is with channel coding, repeated packets, and relatively long addresses (>3 bytes). The two most popular forms of coding used on these is pulse width modulation and manchester coding. In the PWM with each bit broken into 3 chips - a '0' is two high, one low, a '1' is one high, two low. This ensures a transition at the start and in the middle of every bit, so you just need to sample the incoming stream at 3 times the data rate - preferably more of course. In a similar way to UART's sampling the start bit at 16 times the datarate, then delaying by 8 times the datarate, you can sample at 12, then every 3 to ensure you see a transition. It is also a great way of ignoring minor glitches that are inevitable in RF systems.If you're not abiding by any particular protocol then this gives you the freedom to try anything, but a system I did using 8-pin PIC's a while back was similar to how 'keeloq' works, although I sent a preamble to set the receivers AGC and dataslicer, followed by a fixed low period, then a sync, address, data, crc etc.. The receiver will pick up 'crap' until the transmitter starts, which will then train the rx, meaning the fixed low period will be quiet and the receiving micro can look for the sync byte(s).Whilst it seems rather involved it can be very robust, and cheap way of sending control data. As often 433/434Mhz systems are simplex, and low datarate (control on/off) often packets are repeated 3 times, so the receiver can do a majority vote on the data. With a long enough address, and perhaps CRC it reduces the chances of random noise being flags as a valid packet negligible.I believe Roman Black has an excellent article on using cheap RF systems (I realize yours is probably quite a good system) and forms of channel coding:http://www.romanblack.com/RF/cheapRFmodules.htm