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Low Cost R/C Frequency Monitor

Do you suspect that external interference is causing your R/C model's glitches? Wonder no more -- You can build your own frequency monitor and find out for sure. Best of all, it is a low cost project that nearly anyone can assemble. I did it at a cost of under $35!

Build this little handheld R/C monitor and find interference!

One of my R/C airplanes was getting random glitches when I flew at my favorite nearby park. I wasn't sure if it was due to a receiver problem, or if I was a victim of external interference. Finally, after a fatal interference induced crash, it was time to find out why.

I have a portable frequency counter, but these are useless for this task. I wanted something with audio so I could listen to the RF background noise and check for coherent sounds that would indicate interference. But rather than spend $150 for a handheld VHF ham radio scanner, I decided my needs did not warrant the expense. So, it was time to warm up the soldering iron and build my own.

The task is really not that difficult. All I needed was a cheap R/C Rx (receiver) and an audio amplifier. It turns out that an entry level "park flyer" Rx is the best choice for the job. These tend to have poor selectivity, which makes it easier to spot RF interference. Used ones are often for sale by modelers that have upgraded to something better.

The result of my efforts is RC-Mon, a very low cost R/C frequency monitor and interference finder that I built from readily available parts. You can build one too.

Hunting and Gathering

My first thought was to order one of those dirt-cheap GWS 4-channel Rx's. Instead, I found a used Hitec Feather 4-channel Rx that was only $18. Luckily I had a compatible Hitec crystal for my R/C frequency, so that saved me the expense of buying one that matched my transmitter.

The details shown in this project are specific to the Hitec Feather Rx, since that is what I had. But you can use the basic details to hack nearly any retired R/C Rx for use as an interference monitor. The Rx choice just needs to be on your R/C channel. Let's begin by gathering the parts.

Gather up the parts. You will need the following (see photo on the left):
  • Radio Shack Mini Amplifier, part number 277-1008. $12.95 USD.
  • Hitec Feather Rx with compatible crystal. The Rx can be JR or Futaba "shift."
  • 4-cell (4.8VDC) Rx battery. An old retired pack will do just fine.
  • Female servo connector.
  • 36" long music wire for antenna (not shown).
  • 2.5" long brass tube, slightly larger diameter than the music wire.

Wired for Sound

Start by removing the shrink wrap from the Feather Rx. On the bottom of the board you will find a sixteen pin  IC labeled MC3361. This is the FM demodulator IC. Pin-9 is the audio output, which is where we will connect our amplifier. There are only three required wires: audio, power, and ground.

Please refer to the photo on the right (click it for larger view). Using small gauge wire, solder three wires as shown (AUDIO, PWR-, PWR+). The lengths should be about eight inches long. Strain relieve the three wires by wrapping them once around the big cap on the component side.

Click for larger photo
Option (not shown in photos): For better volume level control, you can install an attenuator on the audio signal. Refer to the schematic on the right. R1 is 22K and R2 is 1.8K ohms. Note: this circuit is not required but offers more pleasant volume levels. Optional Volume Attenuator
Wires for the amp are shown here. The Feather Rx is connected to the Radio Shack Mini-Amp as shown in the photo on the left. Just remove the circuit board from the amplifier to gain access to the solder pads for the AUDIO, PWR+, and PWR- wires. Solder them in place.

Now permanently remove the existing 9V battery connector. We cannot use it since the R/C Rx would be destroyed if it was powered from such a high voltage.

Instead, we will use a 4.8VDC to 6.0VDC battery pack to power RC-Mon. I wanted to use an old 4-cell R/C battery I had, so I soldered a female servo/battery cable to the BAT+ and BAT- pads. Now it can mate up to the battery pack.

The Mini-Amp's Input jack is a shorting type, so we must modify it. Take a wood toothpick and slide it into the Input jack and force the shorting contacts to open. Note: Do NOT do this to the Ext Speaker jack.

Once the contacts are forced open you can trim the toothpick so that it is flush with the outside of the jack. Place a small amount of adhesive to hold it in place.


Toothpick details

All that remains is to add a sturdy whip antenna. Begin by drilling a hole in the back cover, as shown in the photo on the right. Using epoxy or CA adhesive, glue the 2.5" long brass tubing in place.

Pinch me.

Mount the antenna holder as shown.

Note: Before mounting the tubing, slightly pinch it to help retain the 36" long music wire that will be used as the whip antenna. Please see photo on the left.

Cut the Feather's existing wire antenna to five inches long. Solder the end of it to the base of the brass tubing. See photo on the right.

By the way, the Feather Rx fits nicely in the old 9V battery bay. In case it matters, changing crystals is an easy affair.

Solder the Rx to the antenna holder.
Eye Safety! Protect your eyes! As a safety precaution, the tip of the 36" long music wire antenna is fitted with a craft store bead. A drop of adhesive holds it in place. As an alternative, you can also bend the end into a loop. This little detail will help prevent serious eye injuries. See photo on the left.

When it is time to use RC-Mon, the whip antenna is merely stuffed into the brass tubing. The pinched region should hold it snug, but still allow removal for convenient storage.

Check over your work and ensure that no assembly mistakes have been made. Then carefully lay the Feather Rx into the old 9V battery bay and re-assemble the mini-amp.

Using some velcro strips, mount the 4-cell battery on the back of the case. Please see the photo on the right.

What is nice about the big external pack is that charging is very easy and the extra mass helps keep the monitor upright when it is used on a table top.

Battery fits on the back.

What's that Noise?

Now that everything is assembled, it's time to plug in the 4.8V battery. Adjust the volume to something comfortable. At this point you should hear white noise. It sounds like rainfall or a running shower. Click for audio demo of white noise.

White noise is ideal. This is an indication that there is no interference. Any clicks, pops, or coherent noise, is a symptom of interference of some sort.

You can also check to see if your frequency is clear of other R/C transmitters. A constant buzzing sound is a good indication that another transmitter is in use. The range of the RC-Mon is nearly a mile in unobstructed areas, so it is very effective for this. Click for audio demo of PPM R/C transmitter.

You can determine the strength of the interference by removing the whip antenna and rechecking the noise level. Just walk around and follow the path to the loudest interference.

By the way, if you run into a situation where the monitor is silent, or nearly so, then that can be an indication of a strong RF carrier that just happens to not be modulated. That will still interfere with your model, so be cautious. Frankly, sometimes silence is not golden.

The Small Print:

All information is provided as-is. I do not offer any warranty on its suitability. That means that if you build and use this device, you will do so at your own risk.


If you have technical questions or comments about this project then please post it on the rc-cam project forum.

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