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i have a tiny RF 433mHz project with my Raspberry Pi 3 Model B v1.2 and Arduino Pro mini.

My Raspberry works as a receiver and Pro mini as a transmitter.

I have wiried the popular RF receiver MX-RM-5V to my RPi's GPIO.

Receiver and transmitter modules have coiled antennas.

I used the GPIO 5V pin to supply my receiver and logic level converter.

I'm using logic level converter because the receiver works with 5V and GPIO with 3.3V logic.

The problem is that if i use the GPIO 5V pin to supply my RF receiver module the range drops to 10 cm.

When i supply my receiver module with external adapter @ 5V the range increases to 10 - 15 meters.

Raspberry has micro usb 2A adapter. The MX-RM-5V uses 11.50 mA.

Why the GPIO 5V pin can't supply the MX-RM-5V module ?

Is there some component on the raspberry's 5V rail between micro-usb and gpio 5v pin that limits the current ?

How much noise the Raspberry generates on its rails and what kind ? Can this be the problem ?

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All the Pi's GPIO are 3V3.

I assume you mean one of the 5V pins on the expansion header (pin 2 and 4).

5V is 5V, it should make no difference.

Do you have a very weak Pi power supply? Perhaps there isn't enough power to keep the expansion header pins at the 5V level. Measure the voltage.

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  • I have the Pi Hut power supply, rated 2A. The receiver module consumes 11,2 mA The voltage level is 5,23 V right now i have soldered supply wired straight from under micro usb, to bypass the 5v rail protections and regulation. But this seems not the case. 2A power supply suppose to be ok. Maybe AC noise is blocking RF433 some how ? Jan 31 '17 at 18:17
  • @degreestudio You need to measure the voltage at the expansion header pins.
    – joan
    Jan 31 '17 at 18:18
  • voltage is 5.23 V, on header pins Feb 2 '17 at 6:33
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So i made two tests with oscillator and the problem seems not to be the supply.

Test 1: I connected the RX-RM-5V receiver's 5v pin and gnd to the external power supply. Capable going 5A, its laboratory PSU. Then i measured module's data pin with oscillator, and the signal is ok and clean, i saw receiving moment clearly. There is very little noise. The Raspberry was not connected with my module

Test 2: I connected the module to the Raspberry Pi 3 's gpio pins. 5v to 5v and gnd to gnd Then again i monitored the modules data pin with an oscillator. Results showed that the data pin had huge constant 1kHz noise on the data pin. Module is receiving data on much lower frequency than the noise. (300Hz-400Hz) So i assume it picks up noise from pi gpio or something.

I also made a low pass with 700 Hz cut off and this works ok but i had no luck gaining the distance. The data comes to the receiver between 300-400 Hz frequency. Hard to tell the exact frequency. I tried to make a bandbass filter but the high pass side seems not letting the data through with 20 Hz cutoff.

Theres an article on the internet which talks about module for Pi which is basically filter module for RF projects. They have bandbass filter which filters broadband out. So there is something which Pi dont like about RF signals.

Right now im waiting two diffrent receivers from ebay...

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Update:

I mentioned that i measured 1kHz constant noise with oscillator from MX-RM-5V module when it is connected to the raspberry pi 3. When the module received something the noise dropped tp 300-400 Hz.

I found a visual solution and rather simple way from youtube to really see the noise on the data pin. THE LED.

When i connected green LED with a 100 Ohm resistor to the data pin the led is constantly glowing. Its not bright but you see small green light. On this occasion the receiver did not receive nothing. So is was pure constant noise.

Next step was to start messing with Low pass filter. I connected 240Ohm resistor and 1.5 uF cap as low pass between data pin and ground. So i connected the greed LED to filtered voltage and the constant noise were filtered.

Led is blinking by the data signals only when the transmitter sends something to the receiver.

Also, my baudrate in the transmitter and receiver code was a bit high. I moved from 1000 bits/s down to 200 bits/s.

From now, the raspberry can detect data from this module and everything is a lot better.

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