S2K - a TETRA Detector

Sometimes it is interesting to know whether the Emergency Services are present. Commercial devices like Target BluEye can be purchased. This TETRA detector is an attempt to build a similar device.

With the introduction of TETRA (Trunked Terrestrial Radio) the possibility to eavesdrop on communication from governmental agencies disappeared. However, due to the standardized nature of the TETRA system as used by the government, communication takes place in a specific part of the radio spectrum. Downlink (base station to mobile station) is 390-395 MHz, uplink (MS to BS) is from 380-385 MHz. TETRA is similar to GSM. This means that the MSs are in a network. Because the network must know if a terminal is active it sends a SYNC signal to the BS every 4 seconds. While this message is brief it can reliably be detected:

This waterfall display over a duration of 16 minutes shows the SYNC messages of a TETRA terminal every 4 seconds. Because the sweep is also 4 seconds the signals appear in a more or less vertical line. This takes place somewhere within the 380-385 MHz frequency band. Amazingly, this is also the bandwidth of an analog TV channel. So I took a TV PCI card (Hauppauge Win TV GO) and removed its tuner can. I replaced the TDA9800 demodulator chip with an AD8307 logarhythmic amplifier and observed its output:

This is a typical SYNC message. At 0.5 ms per division, the message is about 2 ms. By then I already kludged together code to tune the FI1216 tuner to the correct frequency and drive a LED bar with 8 LEDs:

A working prototype is not enough. I created a PCB for the drop-in board with the AD8307 and the main board for the tuner. Found parts at an electronics supplier and got a load of the tuners. Still to do:
- Implement threshold (THR) and brightness (BRT) buttons
- Make PCB
- Fit it in a boxie
- Audible alert (uSD/PIC based)

I'm still a ways off series production but it looks most definitely feasible:


Previous update: 22 December 2014

Fixed the brightness adjustment. Now only the threshold adjustment must be implemented and I'm a bit stuck with that. I only have the outline:
long press THR -> blink display, threshold setting mode
short press THR -> lower threshold (min 2 LEDs)
short press BRT -> higher threshold (max 8 LEDs)
long press THR -> done setting new threshold, write to EEPROM

So I completed the PCB. Just a few jumpers on the single sided board. Changed to it a new boxie (Hammond 1591XXBSBK). Only thing left to add is the additional two pin header for the optional sound player next to the ICSP header. This PIC/uSD card combo plays a raw WAV file for the alarm. There are many designs floating around the internet. Another option is to include a beeper. The threshold (when I've got it working) only emits a brief beep over the 3.5mm socket:

You may notice that I left ample copper on the board. Etches faster and shields better. And to my horror I had to flip the board. The LEDs now go the other way and the buttons are reversed (THR is now right). But if the board is mounted upside down then all wil be fine. Parts (from Mouser, not Farnell because their site sucks) are in, let's see how it goes.

Updated 6 January 2015

Dropin Board Done!

Got the wrong developer for my photoresist boards (NaOH). From a friend got the right stuff, KCO3 or something. Anyroadmap, while the photoresist all but flew off the board with the NaOH the new stuff was more gentle and kept the exposed film. Needed just 15 seconds of UV radiation. Once developed, I plonked the PCB into the heated FeCL3 solution for about 15 minutes. After cutting the boards and removing the photoresist with the developer this is what I got:

The board is about 1x2 cm. Note that pin #5 is not soldered. It's not connected to anything. And now for the main board!

Updated 9 January 2015


I got fed up with the problem with the tuner dimensions so I fixed it. Also figured out how Eagle CAD handles polygons. Very confusing... I also bit the bullet about producing the boards by a third party. Looks better, almost no extra cost but once the hassle of the design is done easy going:

Still nagging is the "layer abuse" by the elliptical pads for the grounding tabs of the tuner. No idea how the PCB maker will handle that. We'll see.

No work done on the firmware. The threshold setting doesn't work and I must dive into it to figure out why. Maybe over the weekend...

Updated 14 January 2015

Firmware done!

Finally got the firmware going with all the bells and whistles. OK, the whistle is just a measly "pip" but it does the job. And it can be upgraded to a custom message with a dedicated WAV player or a built-in beeper. The TODOs that were left (EEPROM and threshold setting) are completed so I released the final version.

But now I'm stuck with the hardware. Fucked up all photoresist boards that I had due to bad exposure timing. Totally forgot what it should have been and by the time I remembered all boards had gone phut. I'm getting too old for this shit... Anyway, ordered new boards and if they come out OK (and actually fit) I can do the outsourcing of manufacturing. And finding a supplier for the tuners.

Update 19 January 2015

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