Fun at 433 MHz

05.08.2011 21:06

A while ago I was invited to participate in Farnell's product road testing initiative. I'm a semi-regular customer of theirs and the idea seemed interesting, so I chose a pair of 433 MHz transmitter and receiver modules to test and review. It's something I wanted to play with since I threw a scrapped weather monitor into my parts bin. The modules arrived last week and I found some time to try them out.

AM transmitter and receiver modules for the 433 MHz band

These particular modules are RF Solutions AM-HRR3-433 and AM-RT4-433. They are hybrid circuits built on a ceramic substrate with SMD components and are equipped with 100-mil spaced pins (something of a rarity these days and quite convenient for quickly testing ideas). They are meant for simple digital remote control or telemetry using 100% amplitude modulation on the industrial 433 MHz band.

This receiver is super-regenerative (they also have super-heterodyne receivers that trade low price for somewhat better sensitivity and selectivity). The datasheet mentions the lack of any tunable components which means they are pretty much usable out of the box. The inductor on the receiver is laser-trimmed - you can see the dark trim line at the top-center of the (larger) receiver module where a laser cut adjusted the length of the coil. The receiver works on 5 V and sinks around 2.5 mA of quiescent current.

As the datasheet promised, the receiver went live at the first try, even on a protoboard with its less-then-stellar RF properties (the thin ceramic substrate did get me worried though that pushing the pins too hard might break it). The manufacturer advises against using protoboards, but apart from that doesn't specify any particular bypass capacitor or layout requirements. Not surprisingly perhaps, since there are basically just four connections you need to care about: supply, ground, antenna and demodulated output. Just to be sure I used a 47 μF electrolyte and a 100 nF polyester capacitors on the supply lines.

My first experiment was with the weather monitor I mentioned earlier. This is very simple telemetry, carrying a few tens of bits in a burst at around 500 Hz. The picture below shows the modulation input to the monitor's transmitter on the upper trace and the demodulated receiver output on the lower trace.

Demodulated RF transmission of a weather monitor.

The receiver will also catch for instance the transmission from my car keys (a somewhat more compressed burst of a few hundreds of bits at 2 kHz):

Demodulated RF transmission of a car key.

Or any number of other of transmissions for which I have no idea where they originate from (although this page gives a few pointers in identifying the devices that transmit them). At least in this residential part of Ljubljana it seems this is a pretty crowded part of the spectrum.

Unidentified transmission on the 433 MHz band

Unidentified transmission on the 433 MHz band

A robust error detection and/or correction certainly looks like a must for any kind of communication here. Basically all those devices are communicating on a shared channel and depend on the fact that other devices only transmit for a small time interval and collisions are rare and detectable.

So to conclude, at £10.31 (a bit under 12 €) the receiver module certainly looks like a good bargain for anyone that doesn't want to get their hands dirty with their own RF circuitry. The TTL-compatible output makes it trivial to interface it to digital circuits and receiving telemetry from wireless devices like my weather station is just one microcontroller and a Manchester decoding routine away. Also beyond tinkering with one-off projects I don't see many cases where you would want to roll your own instead of using a finished module like this.

As you can see I only touched the receiver at the moment. I'll try a few experiments with the transmitter next. Now that I have a verified working transmitter/receiver pair I definitely plan to also check some of my own receiver ideas.

Posted by Tomaž | Categories: Analog


Hey, you are ages behind! Svet Elektronike has been offering these modules for a long time. Have fun with radio on 70 cm band!

Yeah, not to mention radio waves were discovered way back in the 19th century. Talk about outdated technology!

Posted by Tomaž

Very informatve, thanks. (Im designing a 433 MHz locating beacon)

Posted by penguinman

Bought the HRR3 and RT4 for a wireless temperature monitoring solution, the RF link never worked reliably. Manufacturer advised me to buy RF600T but this also failed to work .Look for an alternative.

Posted by RH

One thing is not clear for me. These receivers should have ACG (automatic gain control). Thus the real signals should be surrounded by noise, which I can't see on the pictures. How did you manage to do that?

Posted by hurjup

Hurjup, the output of the receiver module is a digital signal - it's either high (when a signal is present) or low. There's a comparator on the module that compares the analog signal from the receiver to a threshold. That's why you don't see an analog noise floor on the oscilloscope.

Posted by Tomaž

Have I misunderstood something? If I apply a continuous CMOS 'up' or 'down' level to the transmitter (RT4 in my case) can I expect to see the same continuous 'up' or 'down' level at AM-HRR3 output?

Ray, yes I think that is basically correct. There is no AGC if I remember correctly, so a continuous transmission that is above AM-HRR3's preset threshold will generate a continuous "high" output.

However, continuous transmissions might not be legal (there are usually some duty cycle restrictions on the 433 MHz band - check your local regulations). Also, interference might break up the output, so it will not perfectly resemble the input to your transmitter.

Posted by Tomaž

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