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Mar 04, 2013, 10:42 AM
Registered User
Build Log

Using an old servo to turn LEDs on/off with TX

The following photos show how to modify an old (or new) servo to enable you to switch LEDs on or off using a free channel of your transmitter.
1a. Remove bottom part of servo (four screws).
1b. Cut off or unsolder wires to motor / remove motor.
2. Solder capacitor to plus and minus solder tags in place of the motor.
3.Replace circuit board in the casing, cap goes where the motor was before.
4. Glue circuit board to case - be careful or you will strip the copper tags from the board...
5. Solder resistor to plus lead of cap - the other resistor lead goes outside the servo casing.
6. Solder long wires to resistor end and to minus lead to cap.
7. Glue it all into place.
8. Attach LED to the two wires.
9. Connect servo to a free receiver channel or servo tester.
10. Toggle switch or joystick on transmitter or servo tester.
11. Adjust var. resistor on top of servo ( the small axle/pin sticking out of the casing) to enable the LED to be switched on/off as required

With fine adjustments of the var.resistor it is possible to get the LED to flash on and off.

You can solder an additional resistor in series with the first one to suit individual LEDs ( blue, red, rainbow etc.).

If your soldering skills are not that good you can cut the wires leading to the motor and solder "extension" wires to them. Then reassemble the servo and attach the cap, resistor and LED externally.
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Mar 06, 2013, 03:58 PM
RC beginner
nice how-to. specially the photos. however the cap serves no purpose except to heat drive transistors. BIG TIME! i will say its refreshing to see somebody decide to leave the pot in. too many feel obligated to replace it with fixed resistors which is not smart. and doing it your way makes the mod 10x easier too.
Mar 07, 2013, 02:25 PM
Registered User
Thanks for your comments Dave. After reading your remark about the cap I did some more testing with the following results:
Using an red LED the cap is not absolutely necessary.
Using an rainbow LED (as I did initially) the LED will not change colour unless a cap is included in the circuit.
Looking at the motor terminals on the circuit board with an oscilloscope, you will see that the voltage is not constant but is a square wave impulse. The cap smooths this out so that the rainbow LED can operate correctly.
I have tested the above with four servos, each of a different make.The results were the same.
I have also had the circuit with rainbow LED and cap running for over two hours on a servo tester ( complete on/ off cycles). I cannot measure any increase in circuit temperature and when using the circuit on a model the slipstream will provide additional cooling. As long as the resistor has the correct value ( my rainbow and blue LEDs draw approx. 10mA and are easily seen in the air at dusk) there should be no overheating problems. Id welcome any more comments or thoughts on the subject...
Mar 07, 2013, 04:50 PM
RC beginner
no, overheating would not be an issue. probaly not even detectable with a finger. i did a quick calculation for reactance vs frequency and rise time for tp9 servo and cap wastes 20x more power than led alone. still we talk milliwatts so dont really get hot. maybe the fet warms up.

i dont know what you mean by rainbow. ive used some two way bi-color leds but no experience with those more complicated 2 wire multi color. got a link? anyway ill take your word the cap has a function there. no matter what its a cool project. i love using these tiny servo boards as micro-escs.
Mar 09, 2013, 03:39 PM
Registered User
Hi Dave (and others)
Ive just been searching on the web for you for stores in the NY area that sell electronic components (i.e. LEDs). Apart from Radio Shack (useless) no luck. So I tried web sites in the U.S and Canada. Again not much luck. This amazed me as we can get them everywhere in Germany. They are one of my students most loved electronic "toys". However I did manage to find two sources:

(20 rgb LEDs for 3.80 USD

(100 RGB LEDs for 23.99)

If you are searching yourself, look for two pin 3mm or 5mm RGB LEDs, they come in two sorts - quick colour change (quick flash) and slow colour change.

Connecting two in parallel and placed next to each other they will start in sync but after a while one will "get ahead" of the other and you will get some really nice colour mixes.
Have fun!
Last edited by Tech Teacher; Mar 09, 2013 at 04:11 PM.
Mar 11, 2013, 09:16 AM
RC beginner
aha... not controllable colors but just sequencing. i do have some of those after all. pens and novelties that change color automatically. now i see why the cap is important. they are actually tiny mcu in the pak along with the 3 leds and so need a stable dc supply.
Mar 12, 2013, 09:14 AM
Registered User
jhaywood's Avatar
I am Interested in turning white LEDs on and off. Is the cap needed for this?
Mar 12, 2013, 02:58 PM
Registered User
No,its not - but be careful not to use a white LED which has a high power rating. The ones I use have a turn-on voltage of 2.5 volts and when used in series with a resistor of 270 ohms draw 10mA at 5 volts. They can easily be seen at dusk on a model plane in the air.
Mar 12, 2013, 03:30 PM
Registered User
jhaywood's Avatar
I do plan on using 8mm white LEDs. I will be trying to find the brightest I can to be seen during the day as landing lights on 1/3 scale plane.
Mar 12, 2013, 04:17 PM
Registered User
Ive just been looking at some data sheets for 8mm LEDs.

This one for 8mm "Super Bright LEDs":
The LEDs draw 20 mA @ 3.6 volts

This one for 0.5 Watt LEDs draws 100 mA max at 3.8 volts.

The next step was to measure the current for one of the motors that I had taken out of a servo. I measured between 100 mA and 250 mA (with my finger pressed against the gear wheel). That seemed like a lot so I measured the current taken by a normal 9g servo built into one of my planes. Result: 6 mA when not moving, average of 150 mA when working normally and 240 mA when blocked. Summing up: it would seem that the transistors/FETs in the servo circuit can manage quite a bit of current. If I can find a suitable LED in my collection Ill try to hook it up to a servo board to see what happens :-). If anybody out there is feeling brave and gives this a try let me know.....
Mar 13, 2013, 10:13 AM
Registered User
A simpler method with no disassembly required, is to plug a brushed ESC in to a receiver channel (or possibly connect with a Y-cable) to drive the LEDs. If you control this channel from the transmitter with a knob or a slider, you can vary the brightness.

Jim R.
Mar 13, 2013, 08:52 PM
RC beginner
in other words just like the servo boards. maybe simpler but 10x heavier and a LOT more expensive. servo boards are historically the best way to control leds for those w/o great fear of soldering iron.
Mar 13, 2013, 09:00 PM
Registered User
jhaywood's Avatar
I have a 2amp brushed esc that doesn't weigh hardly anything.
Mar 13, 2013, 09:14 PM
RC beginner
still 10x more than micro servo board. last one i weighed came in at .15 grams. these are really tiny. and 2amps is overkill for a led.
Apr 06, 2013, 02:29 PM
AMA16634...Just Me
Just found this thread and it looks like what I've been looking for but please go easy on me guys as I'm not an electronic whiz. If I understand correctly this is an on/off DIY switch using auxillary channel that will supply recv power to led's. Want to keep things as light as possible for headlights on a cartoonish E-powered park flyer. Commercial units are ~$20 but I've got most of the parts on hand and I know which end of a soldering iron gets

I have on hand both HXT500's and some Dynam 7gram servos to choose from. Some clear 3mm (only two required) led's and a few various resistors. I can pick up the rest locally.

Would the cap be necessary? And am I going the wrong way with this?



Below is what I think is correct......
Last edited by Brner; Apr 06, 2013 at 02:51 PM. Reason: Added Diagram

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