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R/C Electronics FAQ

Most of the questions people ask me about R/C Electronics center around test equipment and troubleshooting. This page contains my answers and opinions for some of the most common questions. I take no responsibility for the accuracy of the stuff below, and I accept no responsibility for any trouble or damages that might result from you using this information. I also urge you to take any and all recommended safety precautions when working with electricity. All that said, I sincerely hope that this FAQ is useful.

Do I need an oscilloscope for R/C?
An oscilloscope is one of the most useful pieces of equipment you can have on any electronics workbench, and so it can be a valuable instrument for R/C troubleshooting. While very useful, most R/C hobbyists will never really NEED an oscilloscope. On the other hand, if you ever get into advanced R/C electronics or robotics, an oscilloscope should be considered essential gear. Some examples of advanced R/C electronics projects that beg for a scope include building any of the following: flight recorders, auto-pilots, fly by wire controllers, horizon identifiers, servo rate changers, and servo phase lock circuits. If you have to make a choice between that tricked out R/C helicopter and an oscilloscope, then go for the helicopter -- If money is no object, then get them both!
How can an oscilloscope be used for routine R/C work?
Having an oscilloscope on the bench can fundamentally change the way you approach even simple tasks:
  • On my bench, I have a DIY R/C servo tester that is actually designed to be operated in concert with my oscilloscope so that I can actually see the servo signals while troubleshooting.
  • When matching capacitors across dual motor R/C vehicles (like the E-Maxx), I don't resort to 1920's impedance matching circuits like many advocate. I hook the system up to the scope, and tweak the trimmer cap until the impedance vectors visually cancel each other out!
  • When doing deep battery recycling or capacity testing, I can hook the charger/battery combo up to the scope in data acquisition mode and see a graph of the voltages in real time as the operation is under way.

In addition to making routine tasks less difficult, a scope can transform many almost imposable problems into simple exercises. For example, one of the most common reasons for receiver glitching in battle-bots is 75MHz RF injected onto servo signals because the receiver is intermittently picking up too strong of a signal when the robot and receiver are in just the wrong positions. To diagnose such problems, we simply hook up the servo output to the scope, and watch the screen as we slowly move the bot around. When we find the problem spot, we strategically place some aluminum foil in the bot chassis and the problem disappears. Doing this without a scope is almost impossible. Even worse, without a scope you might think you have solved the problem, but you can't really know for sure.

What is the best oscilloscope for R/C work?
If you are going to focus on low frequency signals like R/C control lines, then a 20MHz scope is fine. If you are going to troubleshoot the RF electronics, then you need a scope with at least with twice the bandwidth of the radios you use -- 100MHz for 27MHz TX/RX, 200MHz for 72MHz TX/RX and 5GHz for 2.4GHz TX/RX. Scopes beyond about 500MHz are quite expensive, and are probably well beyond the price any R/C hobbyist wants to pay. Two channels are a must, but four are better when dealing with complex receiver problems. A digital unit is better for looking for glitches and for data logging, but an analog unit is better than nothing. A portable unit is very nice to have in the field. The FLUKE 12X is an excellent choice for a portable oscilloscope on the slow side (20MHz), and it has the additional advantage that it will also serve as a DMM. For a dedicated bench unit, a low end Tektronix (1000 or 2000 series) or Agilent (3000 series) are more than enough.

I have a Tek 2024 on my bench.

What is the best DMM for R/C work?
The brand is relatively unimportant, but the feature set is. Some useful features:
0.1% basic DC accuracy:
Some R/C gear is very particular about voltage levels -- the 2.7V inputs on some 3rd party gear for FUTABA receivers comes to mind.
TrueRMS AC voltage readings:
This is a must for measuring the complex pulsed DC and AC signals that some systems send to motors.
A 10A current range:
For small R/C stuff, a 10A range can be used for current measurements; however, be careful as many R/C motor circuits can blow the fuse on a 10A meter. For higher currents, see the question later about clamp meters.
Measure frequency and positive pulse width:
It is important that the meter measures positive pulse width and not just "duty cycle" -- this is a VERY useful feature for working with servo signals.
Thermocouple temperature measurement:
This can be a great aid for diagnosing various engine problems on nitro cars, and heat sink problems on electric vehicles. They also are great for measuring battery heating during motor torque, burn-in tests, and monitoring recharging lithium batteries after a crash to avoid fire and explosions.

Grabber test leads are a great help when trying to clamp onto stuff inside a model. I like the heavy duty ones from FLUKE and Pomona. Micro grabber test leads used for delicate electronics work come in handy too, but it is easy to melt them on high current wires or hot leads.

I regularly use several meters: FLUKE 8808, FLUKE 289, FLUKE 189, FLUKE 117, and a Simpson 260.

Is a "voltage detector" a safe way to tell if a high power R/C circuit is energized before I touch it?
NO! Well, at least probably not. Most "voltage detectors" and "voltage testers" only detect AC voltage. In fact, some only will detect 60Hz voltage like that from wall outlets in the US. So, in general, such testers don't work. Even if yours chirped near your R/C craft, you should not trust it unless the manufacturer of the device expressly states that it works on low voltage, DC circuits.

If appropriate safety precautions are taken, a good quality DMM may be used to directly measure the voltage across such high power terminals. What standard safety precautions? The same precautions industrial electricians use on high energy circuits in factories. Do some research.

Note we are talking about VOLTAGE above and not current. Your meter can most likely NOT measure the current through high power R/C circuits. For these circuits a DC clamp meter is a good choice. See the next question.

Can a "clamp meter" be used to measure the high currents driving my brush-less motor?
Most clamp meters only work on AC voltages, but some will work with DC too (many hall effect devices for example). If the meter will work with DC voltages, it will be clearly displayed in the product specs as a "DC Clamp Meter" -- and it will COST MORE than a similar AC clamp. This is a much safer option than using your meter for current measurement, and it is also nice be able to not break open the circuit for the measurement.

Clamp attachments for DMMs work just fine. For example, the FLUKE i410 is a good DC clamp adapter that can deal with 400A. For lower current, something like the FLUKE i30 is a good choice. Another option is to use a device with a clamp integrated right into the meter like the Extech 400A or DC400.

For high power circuits, I use the FLUKE i400 for AC and the FLUKE i410 for DC.

Is a "current shunt" a good way to measure the high currents driving my brush-less motor?
While current shunts may be used to measure high R/C currents, a clamp meter or clamp adapter for your DMM is probably a safer choice. It is also generally easier as clamp meters do not require the circuit to be broken.

That said, if you insist on using a current shunt, they can be used effectively and safely; however, one must be very careful. For starters, make sure the shunt is rated for the current you need to measure. One thing that catches many off guard is that they use the "transient" current limit listed on the specification sheet instead of the "continuous" current. This is a dangerous mistake. Always make sure the continuous current rating is 2x higher than what you expect to measure. Secondly, use a shielded and isolated unit with banana jacks in and out -- not one of the bare metallic ones with screw terminals. Finally, follow standard high energy circuit safety precautions -- like making sure the circuit is not energized when you put the shunt into place.

What kind of servo tester should I get?
A servo tester is a very useful bit of equipment. You can make your own with a simple circuit, make something more complex, or buy one ($3-$25 depending on features). Things to for include:
  • Direct read out of the pulse width of the signal
  • Direct read out of the frequency of the signal
  • Pulse width should be adjustable from 250 microseconds 3000 microseconds
  • Frequency should be adjustable between 20Hz and 80Hz
  • Pulse width should not change when frequency is adjusted and the frequency should not change when the pulse width is adjusted.

Note that if you have a meter that measures positive pulse width and frequency, then you don't need the first two items in the list above. Also note that generic test equipment may be used as a servo tester too -- i.e. a pulse generator, AWG, or power supply combined with a DMM or oscilloscope.

Do I need all the stuff on your bench to effectively troubleshoot?
Absolutely not -- you would just be throwing away your cash. Much of the stuff on my bench isn't for R/C work. For example, the signal sources and counters on my bench are not very useful for R/C work at all. Even the equipment that I do use for R/C is not cost effective if all you want to work on is R/C electronics. For example, my oscilloscope and DMM are serious overkill if all I used them for was R/C troubleshooting.
Can I build my own transmitter?
You probably can not build a standard 72, 75, or 27MHz transmitter yourself in a legal way -- the FCC rules are quite strict. That said, you can build a R/C controllers and receivers using commercial transmitter modules. These modules range from simple, analogue radio units to sophisticated digital devices like Zigbee modules.
When I look at my receiver output I see huge "fuzzy" tops to the pulse signals. When I zoom in they look like high frequency (around 70MHz) sine waves added to the pulse train. What is this?
You are probably seeing direct RF interference from your transmitter, or signal bleed from an over stimulated radio section in your receiver. You can see some pictures of this here. The most typical cause is your transmitter being too close to your receiver. Move it several feet away, and retract half of the antenna.
What is this magic trick I saw you do at the track where you were "listening" to servo signals?
Servo signals are pulse trains in the low frequency audio range (20Hz to 80Hz, with 45Hz relatively "normal"), and they can frequently be picked up with an "inductive probe". You can find probes near the telco test equipment at places like Altex and Fry's. Avoid the expensive filtered probes as they may filter out the signal -- get a cheap one. I use this trick all the time. With some experience, you can learn to diagnose many servo signal problems by ear. This is much faster than probing around with an oscilloscope or DMM.
© 2009 Mitch Richling