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Simple, Cheap 433MHz Locator Transmitter

So you made last month's 433MHz "sniffer" receiver and now fancy some adventurous tracking? Here's a versatile PICAXE-08M controlled transmitter, based around Jaycar's ZW-3100 companion 433.92MHz ASK (Amplitude Shift Keying) module. While relatively short range, this transmitter makes a great model plane or model rocket locator and/or tracking beacon - something we?ve been asked about numerous times.

by Stan Swan

Using last month’s “sniffer” receiver and simple wire antennas, line-of-sight (LOS) ranges of 1km have been achieved from this simple transmitter, falling to several hundred metres when light vegetation and wooden buildings obscure the propagation path.

Better receivers and antennas (perhaps a UHF scanner and Yagi) could significantly extend this range.

The transmitter module

Jaycar’s “always works” ZW-3100 433MHz transmitter module has long been recommended for simple wireless data links. We’ve used it before for assorted wireless projects and although rated at only a few milliwatts (meaning it’s not going to blister paint on nearby buildings!), this module gives a good account of itself, especially when elevated and feeding a decent antenna.

Although they’re essentially a slow (300-10kbps) data transmitter, pulling the module’s data line to the supply voltage via a 10kΩ resistor means capacitively-coupled audio tones can be sent instead.

Although more sophisticated and powerful 433.92MHz offerings are now appearing (and are under consideration for a possible future article), these can be very demanding to configure!

PICAXE driver

The legendary versatility of the PICAXE-08M allows beeps, simple tunes, Morse ID, or even sequential multi-tone (SMT) Hellschreiber to readily modulate the transmitter. Deep sleep periods can be included as well, greatly extending battery life – perhaps an important issue for a homing beacon.

Power supply

The transmitter module is normally rated for just a 3V supply, although some data sheets indicate 6V may be used. To remain on the safe side, we’ve supplied it via a 3 x AA (ie ~ 4.5V) PICAXE-switched control line.

Click for larger image
This Altium Designer diagram is actually much larger than life size. The 433MHz transmitter module is mounted flat to make the smallest package possible – this necessitates removing the four pins on the module and soldering direct to PC pins. Note the pins are offset: there’s a wider gap on the "ant" side.

With such a supply, under 4V will normally be on the TXC1 positive. Use of a 4 x AA holder and a dummy cell allows versatility for use of weary batteries or lower voltage rechargeable cells as well. A ~ 100mAh lithium coin cell may even be suitable but duty cycles will have to be very low to prolong battery life in this case.

The circuit shown uses about 10mA but if powered on for (say) just a few seconds every minute the average would drop to under 1mA. Alkaline cells of 2000mAh capacity may thus last hundreds of hours, translating to perhaps months of beacon service – a key benefit when trying to locate a device before batteries run flat.

Solar power could even be considered but a model plane or rocket lost in dense vegetation may naturally mean little solar charging occurs.

The method of assembly is not critical and could be built on solderless breadboard for trials, then transferred to the tiny PC board as shown here if weight and size is an issue.

A homing beacon

Googling “lost model plane” returns all manner of heartbreaking tales relating to searches for downed radio-controlled planes. These models may be worth thousands, especially FPV (First Person View) types that carry a video camera aloft.

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