Introduction: Tiny UHF Tracker Transmitter

Picture of Tiny UHF Tracker Transmitter

This is a little circuit that could be used to track an object up to 400m.

It is essentially an SAW stabilized OOK modulated RF transmitter. The modulation is done with two low frequency ultra low power oscillators that activate the transmitter every two seconds for a short period.

With the setup shown here I got up to 400m range. Current consumption is about 180uA average so it'll work for a couple of days with the little button cell. Frequency 915MHz.

Step 1: Circuit

Picture of Circuit

The first oscillator to the left activates the second to its right every 2 seconds or so. The second oscillates at about 800 to 900Hz. Its output signal modulates the RF transmitter which is essentially just a SAW based oscillator with some of the RF energy coupled to a whip antenna.

The adjustment of the RF oscillator can be tricky but works fine with the components shown here. The jumper resistor over the SAW element allows the frequency to be adjusted near the SAW fundamental frequency, then the jumper is removed and the circuit will oscillate at the SAW frequency.

The lower you go in frequency the easier this adjustment will be, so you could go for 433MHz for example too. The component to be changed would be the inductor then (about 22nH).

Use NPO caps for the RF area. The type of the inductor is not critical, I used ceramic.

The circuit would actually benefit from a buffer stage or a matched antenna output, but frankly I didn't fell like investing more time in it. :-) If you want to experiment, I added a pic with a matching circuit for 433MHz that worked pretty well, The inductor for the oscillator changes to about 22nH in that case.

(If you click on the image twice and then on "original DIY file" just below the low resolution picture it will open in hi-res.)

Step 2: Build

Picture of Build

Building it requires a hotplate and solder paste or a soldering iron with a fine tip and steady hands.

Make your own PCB layout or download mine from here: Google drive link These are EAGLE files, Schematic and BOM are also included.

Upload the .brd file to your favourite cheap PCB manufacturer, I used, will take two to three weeks and then:

1. Put solder paste on every pad a component will be placed on

2. Place all components

3. Heat the entire board on a hotplate and wait until the solder paste liquifies

4. Remove the board form the hotplate , let it cool down

5. Flip the board around and solder the battery holder on it

6. Solder the antenna wire into the hole

7. Important: Put some conformal coating or silicon etc on the component side. This will protect the circuit from contamination and humidity. The LF oscillators use pretty high resistance values, which means they are easily detuned if for example you put your finger on it.

Step 3: Range and Stability

Picture of Range and Stability

The RF frequency is SAW stabilized so shouldn't drift. I did not test the circuit in extrem conditions, but it worked fine from room temp to minus 15C.

Range was about 400m line-of-sight ( does that make sense in this case? :-) )

You can play around with the antenna length and also try to increase the ground area adding some conductive material to the GND pin of the battery holder for example. The short green wire increased the range in my case.

Step 4: Receiver

Picture of Receiver

The receiver is comprised of a YAGI antenna, an adjustable attenuator and a RTL-SDR receiver.

The RTL-SDR dongle is connected to a cellphone that runs a paid app called RF analyzer. It's not expensive.

If you mount the antenna on a car for example the dongle could be connected to a Windows PC though, and there is free software available for Windows.

The YAGI antenna design came form here:

There are many other designs on the net and you could also buy an antenna.

The RTL-SDR dongle comes from here:

It's an incredible versatile and very useful gadget for the occasional RF Hobbyist, AND its price is unbeatable.

The attenuator is made of a shielded box with three DPDT switches and attenuates 10dB for each stage. Use small resistors and short connections. Its performance at these high frequencies I didn't feel like evaluating but it attenuates a good amount and that's all that counts. I didn't use any particular website for this part so you have to look this up for yourself. Search for How-Tos of RF attenuators with resistors.

Step 5: See It in Action


WannaDuino (author)2018-01-09

LOOK my friend also very cheap and READY to use with an app on your phone, you can see track movement, where the watch was in FOOTPRINTS on a map, you can set boundaries, and MUTCH MUTCH MORE.


and look soon on my website how i even make it smaller to use it on smaller and all for FREE

i help others i love OUR COMMUNITY and OPEN SOURCE


WannaDuino (author)2017-12-21

sorry to be rude, but to walk around with that GIANT ANTENNA setup, would not work.

if you can make the receiver as small as the telephone oke, but this is nothing compared with the transmitter i already have and receivers for my drones, called RSSI work amazingly and i use also GPS in a watch style cost 14USD and works by an app, so in that case you work backwards in time instead of forward?

You can buy it here,

GPS type: GPS tracker
Screen size: 0.96" LED
Communication Module: GSM / GPRS
GPRS:Class 12 TCP IPbuild in GSm MODULE
GPS Senstivity: -159 dbm
GPS Positioning Accuracy: 10m (2D RM)
Gsm Positioning Accuracy: 50-200mm
Speed Accuracy: 0.1m/s
Maximum Altitude: 18000m
Stand-by time: 100 hours
Battery Capacity: 400mA

How do you use RSSI and a GPS watch to find a lost drone?

with an app that comes with the watch

the RSSI is already in EVERY transmitter these days, it is a home beakon that beeps and give you live data on screan if you are close or the wrong way,

BUT your receiver needs to have it too, but mots of them have it with our hobby becauzzz of the HIGH cost of our drones

TomTechTod (author)WannaDuino2017-12-21

You are comparing apples to oranges. Yes it's backwards in time you are right, the point is to use a single miniature button cell and a PCB size of what I showed though. If anybody is willing to post an instructable for free how to built an GPS based transmitter that size I would be happy to see it. :-)

Your link doesn't lead to an actual product.

ErnestD11 (author)2018-01-04

Hello can you send me more info about price, to email
Full complekt system

cheezedog (author)2017-12-23

Hey, i'm totally out of my element but this might be just the thing I need for not loosing my Disk golf driver when I have a wild throw. I actually been thinking about thing just like this to help me but I was thinking earlyer about those UHF tracker they make for finding lost keys... but found those have been a bit too expensive and nobody could tell me if they actually work on a disk. I was looking for something that is really light, and has good distance to its transmission since a disk throw can travel some 400 meters or so.. I do like to know if you can maybe answer some of my questions to help me better understand if this is a good solution to my issue.

The disks i'm using range in weight from as little as 170 grams to a little over 200 grams.. I could add a little bit more weight. (a few grams) but obviously the more I add the worse it will for me to be able to throw it. just how much does this weigh as a unit? Do you see this being something that can be light if required?

The reason I need this is I will probably loose my disk in heavy brush...The brush can get really thick... and I had it where I walked over a disk not to notice it... so how close would I have to get to be if heavy brush reduces the transmission distance? I normally have a good idea about where it went but brush can really obscure a disk so what I'm hoping is I can just point this and find in what general direction I need to walk to to find the disk. Can I use more battery power to boost the signal? would a bigger Antenna on the transmitter help? also would having the antenna only on one side of the disk be a issue?

I plan on hot gluing the UHF tracker to the center of the disk and then spiraling the antenna round that so that the antenna does not unbalance the disk. Can the antenna be made longer, and be bend in a spiral shape?

Finally, could I make the receive bigger to catch the signal better, I see myself just making a new disk bag out of receiver frame and holding that in front of me to find where the disk landed.

I really appreciate if you found the time to answer these questions and maybe do another demonstration of this units capabilities and limitations to better understand how it might be applied to use.

TomTechTod (author)cheezedog2017-12-23

The unit as shown weighs 1.26g including battery. A bigger battery and some modification to the circuit could increase transmitting power yes. The disk cannot be conductive, that would severely degrade antenna performance, they are made out of plastic right?

Through thick brush you are good for at least 100m with this setup I`d guess, didn`t try it out though.

When getting near the transmitter you might have problems determining an exact direction and your object is relatively small so that would be something to experiment with.

Since you already know where the disk landed (more or less) maybe an audible signal would be better, with a little buzzer that makes a beep every 1 or 2 seconds, but what could be the range of that? Maybe about 10 to 15m? Just a thought.

cheezedog (author)TomTechTod2017-12-23

Even at 2-3 grams is not going to burden the throw of the disk at all so I could actually afford to even put a luxury item like a power switch into the design to save battery power. (the battery switch would be needed to be set so it turn on if the centrifugal force of the throw could jar it though..))

Disk are non conductive plastic. the more expensive they are, the more durable and easier to grasp onto they tend to be.. I am just a little worried would I need to put in a second antenna for both sides of the disk to send data out. Could I maybe just use a metal foil sticker of some kind as the antenna? that would also be a good way adhear the device to the disk.

Its good to know the size is maybe a issue... I already am using vary brightly colored disks, but because the course are not maintained vary well the grass is more then long enough and thick enough that the disks disappear into them. If all this can do is point me in the right direction, it already will help me out soo much.

I am willing to try a auto-able noise thing too.. the only issue with that being It have to be reasonable well defined. I can off as much as 50 meter from my throw since I have to sometimes blindly throw over a hill towards a target. Being able to wand the air and say, "its this way!" seems like a much better option over trying to listen for a noise.

If you watch this video... you can see just how bad a disk golf park can get... when you screw up your thrown you go into thick garbage and can't find anything in that kind of stuff.

TomTechTod (author)cheezedog2017-12-23

Wow that's some distance. For the antenna other conductive material could be used too, sure, like copper tape for example. You only need one antenna. The receiver antenna is pretty big, I dont know if you want to walk around with this on a regular basis though. The only advantage is its low tech, small and inexpensive

cheezedog (author)TomTechTod2017-12-23

Actually the receiver could be built into my disk golf bag. I normally carry around at least 6 other disks, a water bottle,small plastic rain poncho for if I should get caught out in the rain, and a towel to wipe off the disk should it hit the mud...((This is vary minimum most people carry anyway as far as disk golf is concerned..)) so I need something to carry it all in anyway. building the receive to fit around the bag would not be a huge issue. The bonus would be that now I also have a dedicated spot for a phone to sit. :-)

TomTechTod (author)cheezedog2017-12-23

Ok, so I just did a test and removed the antenna and left only the receiver dongle and attenuator connected. The range drops to about 10m max. The signal gets considerably stronger depending on distance to the transmitter so an additional switch to disconnect the antenna or an additional attenuator would probably work at short distances. Determining direction is then solely done by walking around and observe signal strength/beep vs background noise strength.

WannaDuino (author)TomTechTod2017-12-27

micro SMD switch i got loads of them very small 2mm by 0.4mm

cheezedog (author)TomTechTod2017-12-23

Wow.. this might be the ticket then.. Now I gotta learn how to make electronics.. :-)

WannaDuino (author)cheezedog2017-12-27

USE GPS locator 14 USD with an APP sir

cheezedog (author)cheezedog2017-12-23

Holy cow, I am sorry... i keep forgetting to add links and make sure they work. :-(

cheezedog (author)cheezedog2017-12-23

Neilyboy (author)2017-12-21

This is fantastic. I just ordered some oshpark boards. now to order some components! Thank you so much for this! I want to try hooking one on one of my cheaper drones, flying it out aways and trying to recover it! Well done.


michaelb2 (author)2017-12-21

Seems a natural for very small drones that get lost easily.

Cliffystones (author)michaelb22017-12-21

or big dogs that wander off!

WannaDuino (author)michaelb22017-12-21

That is called RSSI and is already YEARS in our transmitters and

JimG178 (author)2017-12-21

Too many abreviations. What is SAW? OOP? WTF

TomTechTod (author)JimG1782017-12-21

SAW surface acoustic wave

OOK on/off keying

WTF no idea :-)

jtechian (author)2017-12-21

Nice work. I was wondering about adding a MAR1 amp on the output to boost range a tiny bit and make this more useful. Would I use the same choke on the output of the mar for stable power out?

TomTechTod (author)jtechian2017-12-21

Sorry I don't have an answer to your question. You could try to lower the bias resistors of the oscillator first. Also a matched antenna output would probably greatly improve range. If you want to experiment with the circuit I suggest you go lower in frequency, for example 433MHz, makes it much easier to adjust the values. I added an image in the circuit section for a a matching circuit for 433MHz, I found these values to be more or less appropriate for the output filter. L of the oscillator would change to about 22nH.

portoinfo (author)2017-12-21

Congratulations on the excellent project!

Is it possible for you to sell this module ready, with the components?

I am in Brazil and do not have the components to sell here.

TomTechTod (author)portoinfo2017-12-21

Oi, sei que é difícil obter essas pecas no Brasil mas não vendo esses circuitos. Talvez alguém venda um kit um dia? (não vai ser eu :-)) As peças são baratas, não sei se a Digikey manda pra lá, o problema talvez seja taxa de importação.

sgbotsford (author)2017-12-21

I recently saw a story about someone who was losing Amazon packages from their door. Something like this with a longer range would help people set up stings. For that purpose you need more range -- maybe 2 miles. But that would take more power.

What if it made a LOUD (rf wise) beep every 15 seconds, and a lower power one every second.

Also, if these became popular you would want to modulate it so that your tracker could be distinguished from other trackers.

The biggest problem is that you don't know when you want it, so when you do want it it's probably got a flat battery. A much more sophisticated version would echo. E.g. it listens on one frequency, then chirps for a while.

TomTechTod (author)sgbotsford2017-12-21

2miles... probably would exceed permitted output power. Good idea though. This here totally experimental, in other words, a toy, or a starting point for something better. A professional (sellable) solution is probably GSM/GPS/Bluetooth based, but passing government regulations is probably one of the biggest hurdles. There are already good solutions out there but they should be more customizable/configurable to allow for longer standby times or that they can be adapted to particular situations. There is bluetooth but it relies on other people having their cellphone enabled for that, there is GSM but battery runtime is very low and then there is the monthly cost. Possibilities for professional solutions are endless, I'm just playing around with this stuff though. (as the title said, it's low-tech) Implementing an accelerometer is another feature they could consider, a tracking device that doesn't move also doesn't need to transmit that often, would save more energy.

m0j0TX (author)TomTechTod2017-12-21

I don’t know much about it, but doesn’t APRS have GPS reporting built in?

The challenge would be getting it into a small low-power package.

mikelgeelen (author)2017-12-21

Hi Tom, Excellent instructable!! I am looking for a small transmitter-receiver that let me determine the distance between them. Could the signal strength on your receiver give a distance with +-25cm error in a range of about 10m? Thanks.

TomTechTod (author)mikelgeelen2017-12-21

Hi, no that's definitely not possible with this circuit. The signal strength will vary with distance yes but also with any object in the path or nearby that causes reflection/absorption of the RF signal. The idea behind this transmitter/antenna is really only to determine direction, not distance. If you need distance up to 10m my guess is ultrasound would work pretty well, but that wouldn't really work indoor through walls. RF time-of-flight measurement is surely possible but not an easy task at all.

m0j0TX (author)2017-12-21

Very cool project!

Have you calculated the total output power of the transmitter?

In the United States the FCC (Federal Communications Commission) regulates radio usage. In particular, Title 47 CFR Part 15 regulates unlicensed transmissions.

I am not familiar with all of the details of Part 15, and this is probably low-power enough to be exempt, but people should be cautioned that if they try to increase the range of the transmissions by increasing the power, they may inadvertently break the law.

The frequency 915 MHz is in the 23cm band, and in the United States it is legal for licensed amateurs to transmit on that band, although there may be some geographical and power restrictions.

TomTechTod (author)m0j0TX2017-12-21

No, nothing calculated. Good comment though. Of course, when building any "intentional radiator" legislation should be observed. The circuit draws about 4mA while transmitting.

studleylee (author)2017-12-16

Hi this is very well done and I like your layout. Add mention that it is an Eagle schematic and layout. The schematic is hard to read, please make/add a finer resolution one and a parts list.

TomTechTod (author)studleylee2017-12-17

Thanks. When you click on the schematic picture twice it will open in a new window then there is a link just below the picture named "original DIY file", click on it and the full res pic is available. I don`t know how to make the full res picture show in the instructable itself.

studleylee (author)TomTechTod2017-12-17

Hi Tom, I did just that,and got the largest, but even zooming the pin numbers,refdeses,part numbers are badly pixelated. Can Eagle print to pdf? just add that as a file. Reason is: I want to make some :-)

TomTechTod (author)studleylee2017-12-20

Schematic should be better now.

studleylee (author)TomTechTod2017-12-20

Thanks Tom!! and again beautiful soldering and layout. Very compact!!!
Do you use a stereo microscope?

TomTechTod (author)studleylee2017-12-20

Yes. One of the "cheap" ones. A headband magnifier would do though for 0201 packages.

alexrecycle (author)2017-12-20

Hi, really interesting little project with some.. few hundreds possible uses..
But if I may I'd like to point out something you mention and I'm always be wondering about: how to use a hotplate -you suggest in this project- or else -indeed what else can be used- to make precision multiple soldering on a circuit board.
I mean the concept is very simple but the actual realization, well,
I guess is a good example of a Columbo's egg: simple enough, perhaps, when you know but virtually impossible when you don't..
I think it would make a great instructable, would you agree?

TomTechTod (author)alexrecycle2017-12-20

Yes it would make a good instructable probably. For now you can check out my other instructable with the little FM transmitter, the whole process is shown in the video.

It is really easy once you get the hang of it. All SMD components can be soldered this way but is really useful for the ones that have pads underneath like DFN packages or chips with middle ground terminals etc. Instead of a professional hotplate you can probably use a stove top. I did do it in a little toaster oven (the ones with the front door). The only thing to be careful with is the time you leave it in there. Components usually have a recommended soldering temperature profile that can be found in their datasheets. But it doesn't has to be complicated, as soon as the solder paste liquified / components are soldered on the board remove the PCB from the hot environment. That works best of course on a hotplate or stove top because you don't have to lift the PCB and risk shifting the components while the solder is liquid.

DanM290 (author)2017-12-16

Nice! You should enter this in the Coin Cell Challenge contest over on Hackaday.

nqtronix (author)DanM2902017-12-17

I second this!

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