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Cheap-N-Easy 433MHz Sniffer

Here's a simple - and cheap - little 433MHz receiver that has many uses. It is ideal for checking that a suspect 433MHz "wireless" device (and there are scads of them) is actually working. It's great for finding out where interference is coming from. But most importantly (we believe!) it makes a great tracker for "fox hunts" and the like.

by Stan Swan

It’s not widely appreciated that the popular UHF telemetry band – more correctly called the ISM (Industrial, Scientific and Medical) band, centred on 433.92MHz, actually covers a generous 1.7MHz between 433.05MHz and 434.79MHz.

Click for larger image
A computer-generated image (mostly created by Altium Designer) of the PC board version of our 433MHz Sniffer. This is the one to build if you want to make it a permanent project!

It’s probably just as well that more than one spot frequency is available, as an army of wireless door chimes, energy monitors, toys, car remotes, garage door openers, backyard weather stations and the like now festoon this licence-free spectrum slot.

Although such LIPD (Low Interference Potential Device) signals normally travel only a few hundred metres (as just a tiny 25mW transmitter power is permitted), its increasing popularity means that in urban areas a scanner tuned to the band may reveal a near-bewildering “African dawn chorus” of beeps, buzzes, pops, whirrs and scratches associated with nearby wireless data.

The ability to monitor local activity on this ever-more-crowded spectrum slice may ease device fault-finding or interference location, yet the cost and complexity of a UHF scanner may not be justified.

Hence it’s with some satisfaction that we present a cheap (~$25), simple and sensitive “433” band monitor.

In an electronics age when almost anything seems possible, such receivers have not normally been available. With increasing band “noise”, every “433” user should have one in his toolbox.

I’ve used mine extensively for wireless data monitoring and device activity checks and find it a near-indispensable “bang for buck” test item.

This recently again showed its worth, with a “no go” neighbour’s 433.92MHz wireless door chime (an Arlec DC149). Although exceedingly efficient (two AA cells last around a year as the receiver spends most of its time on a ~200µA snooze), they use a super-regenerative receiver which, as with all regenerative types, radiates a small RF signal even while receiving.

It was the work of moments to bring the 433 monitor close to it and hear a suitable increase in background noise, ceasing when the receiver batteries were removed.

At the bell push itself, the outgoing transmitter data was readily heard but the fault turned out to be a weakening transmitter battery which as you would expect, reduced range.

And while we were sleuthing, a long misplaced (but still active) “CENTAMETER” mains energy sender was located in a backyard shed electronic junk box!

Design

Our monitor is based around Jaycar’s widely available 433MHz receiver module (Cat ZW-3102). These reliable modules, sourced from Keymark/SpiritOn, sell for around $13 and find much use with 2400bps PICAXE wireless data.

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