Summary of Tiny UHF Tracker Transmitter
This article describes a low-power, 915MHz SAW-stabilized OOK RF transmitter designed to track objects up to 400m. It utilizes two ultra-low power oscillators to activate the transmitter every two seconds, achieving an average current consumption of approximately 180uA. The circuit is built on a custom PCB using surface-mount components and requires specific assembly techniques like hotplate soldering and conformal coating to ensure stability and longevity in varying environments.
Parts used in the Tiny UHF Tracker Transmitter:
- SAW stabilized oscillator
- Two low frequency ultra low power oscillators
- Whip antenna
- Jumper resistor
- NPO capacitors
- Ceramic inductor (approx 22nH for 433MHz)
- Button cell battery
- Battery holder
- Matching circuit (optional for 433MHz)
- EAGLE schematic and BOM files
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.
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.
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.)
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.)
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 Oshpark.com, 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.
Read more: Tiny UHF Tracker Transmitter
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 Oshpark.com, 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.
Read more: Tiny UHF Tracker Transmitter
- What is the maximum range of this tracker?
The setup achieved a range of up to 400 meters. - How often does the transmitter activate?
The first oscillator activates the second one every two seconds or so. - What is the average current consumption of the circuit?
The average current consumption is about 180uA. - Can I adjust the frequency to 433MHz?
Yes, by changing the inductor to about 22nH and adjusting the SAW element jumper. - What type of capacitors should be used in the RF area?
You should use NPO capacitors for the RF area. - How do you assemble the PCB components?
Apply solder paste to pads, place components, heat the board on a hotplate until liquid, then cool it down. - Why is conformal coating necessary?
It protects the circuit from contamination and humidity, as high resistance values in LF oscillators are easily detuned by touch. - Where can I download the PCB layout files?
The EAGLE files including the schematic and BOM are available via a Google Drive link mentioned in the text.
