Why do my 433 MHz receiver and transmitter modules not work reliably?

Question

Background

To learn about radio, I started with the simplest module I could find: a simple 433 MHz receiver and transmitter module pair, where you can use RadioHead ASK to bit-bang a signal on the MCU.

I purchased the most widely available module pair on eBay UK. The receiver has the marking RX470C-V01 and the transmitter has no markings. Curiously, this module pair is not the most common on eBay US (so it doesn't feature heavily in videos and tutorials on the Internet).

The most commonly used 433 MHz module pair demonstrated on the Internet actually seems to be the pair with the markings, FS1000A and MX-RM-5V 080408 (e.g. Using Inexpensive 433 MHz RF Modules with Arduino). This seems to be the most available pair sold on eBay US.

Problem

I've been struggling with reliability the RX470C-V01 module pair (the first photo in my question). Often the receiver doesn't get the message from the transmitter. Taking a look at the received signal on the oscilloscope, it doesn't match the transmitted signal very well (some waves are longer/shorter/missing when comparing the two). Reliability degrades further when separating the transmitter and receiver by a few feet. I have tried making my own antenna out of wire and using various external SMA antennas, but this didn't improve reliability and sometimes made it far worse.

The particular eBay listing I purchased from did not specify which antenna was for which module, so I made what I thought was a good guess and soldered the wiggly end of the short antenna to the RX module with two holes. To me, it just looked like that was the right way round. I'm not exactly sure why it has two holes, so if someone could explain this for me, that would be helpful. The longer antenna, I connected to the transmitter (one hole/via for the antenna).

What didn't help, is that the shorter antenna is often shown in eBay listings with the receiver, which tells me that many sellers don't really know either (hardly surprising).

I'm wondering: Why is the end of the shorter antenna wiggly? Why does the the receiver module have two antenna holes/vias?

Edit:

As requested, here is the waveform. I took a picture of the beginning and the end. It's generated by RadioHead ASK, which bitbangs on IO. There are two MCUs and 4 modules; each MCU has a TX and RX module. The first MCU is ESP8266, and the other MCU is an ATtiny84 using an external 8 MHz crystal.

ESP8266 TX waveform start:

ESP8266 TX waveform end:

ATtiny TX waveform start:

ATtiny TX waveform end:

Answer 1

Consider alternatives

"How much do you value your time" is something that maybe you should ask yourself.

-- Andy aka

These cheap radio modules are not particularly reliable and don't come with a datasheet. They're also very sensitive to any noise on the 433MHz range, so in terms of a learning experience it can be intermittent and counterproductive (unless you want to learn how troublesome cheap modules can be).

Lesson: When buying a part, always make sure there's a datasheet. If you want a reliable part, order from a reputable electronic components supplier (i.e. not eBay or a hobby site).

For anyone who wants to learn how to use radio, a more modern low cost module (e.g. LoRa, RFM69, etc) would most likely provide a much more productive learning experience.

For 433Mhz, I found that LoRa, while designed for long range, worked really well for short range, and it took almost no time to implement. Don't let the SPI interface put you off, LoRa is really easy to implement with libraries, e.g. #include <RH_RF95.h> (RH_RF95) from the RadioHead library.

///                 Arduino      RFM95/96/97/98
///                 GND----------GND   (ground in)
///                 3V3----------3.3V  (3.3V in)
/// interrupt 0 pin D2-----------DIO0  (interrupt request out)
///          SS pin D10----------NSS   (CS chip select in)
///         SCK pin D13----------SCK   (SPI clock in)
///        MOSI pin D11----------MOSI  (SPI Data in)
///        MISO pin D12----------MISO  (SPI Data out)
RH_RF95 rf95;

Simple example code can be found in the Arduino IDE menu (File > Examples > RadioHead).

You can also visit the "RF Transceivers" category of your favorite electronics components supplier (probably under something like "Wireless Modules & Adaptors > RF Modules"). It's a bit pricer than eBay, but you get the benefit of quality control and datasheets.

If you do insist on purchasing from eBay, prices for LoRa modules are almost the same as the less reliable 433MHz modules (though almost certainly they're cheap knock-offs with no quality control). Actually though, I have found that eBay is sometimes much more expensive than proper electronics components stores.

Swap the antennas

If you absolutely must use the RX470C-V01 or similar...

With the lack of a datasheet or any useful documentation, I resorted to digging around on the Internet. It became apparent that it's the receiver that actually needs the longer antenna. I sifted through a few eBay listings for the RX470C-V01 and eventually found one that states which antenna is for what module.

After swapping the antennas (as shown below), I found that reliability improved (but it's still not bulletproof).

Caveat: It is possible that some other factor that changed my environment, and I'm aware that 433MHz is quite a busy frequency.

Answer 2

I've used exactly these modules for the simplest possible remote control: I wanted to switch on a solid-state opto-coupled mains relay (to illuminate an "On Air" lamp in a radio studio). My approach was very crude, running an NE567 oscillator at the transmit end, at about 2 kHz, and then using another NE567 as a tone detector at the receive end. When the output of the '567 at the receiver went "low", I energised the LED inside the solid-state relay, switching mains to the "On Air" lamp. Both transmitter and receiver were run from 7805 regulators to ensure frequency stability.

This simplest approach worked flawlessly over about 17 metres and through two walls!