Found a use for a crappy servo - RC-Monster Forums

BrianG · **Found a use for a crappy servo -** 03.06.2010, 07:58 PM

I have a few servos laying around that aren't, and will never be, used. They either need gearsets not readily available anymore, or servos that come in radio kits (these are ok for VERY light duty, but certainly not in car use). Either way, it's a waste.

So, I took out the driver boards, did a little wiring, and now can use them to drive LEDs or whatever via CH3 of the radio. A DimensionEngineering picoswitch does basically the same thing, but this is "free".

Step 1: Remove driver board from servo.

Unscrew the case, unsolder the motor terminals, and carefully pull the driver board out. You may need to somehow unclip the pot from the top part of the case, but how to do this may vary servo by servo.

Step 2: Solder wires
Solder a red wire to one of the old motor solder pads (doesn't matter which). Solder a black wire to the servo input ground lead (black or brown wire, depending on make). If soldering to the ground wire isn't possible/easy, there may be a ground trace where you can scrape off the coating and solder there (as I did).

In the pic below, I also soldered a white wire to the other motor solder pad, but ended up removing it later. The pot was bent out of the way to give me more room to work.

Step 3: Replace the pot

Cut the pot off but leave some of the leads behind. You could totally remove the pot, but there may be a lot of surface mount components very close and you could accidentally ruin them with the iron. You will need two equal value resistors that when placed in series, total the pot's value. The pot in this servo is rated 5k ohm. So, I took two 2.2k ohm resistors (common value), soldered them in series (total of 4.4k ohms, which was close enough), and soldered them to the pot leads. I then applied a little hot glue between the resistor legs just to make sure they won't get bent and touch each other.

Step 4: Button up

I applied a little heatshrink to the resistors to keep them from shorting on the circuit beneath them. Then heatshrunk the whole thing.

I can use these outputs directly via a resistor to power LEDs, or attach a 5v relay to the outputs to switch larger voltages/currents. With the pot in the center position (or two equal resistors wired in), I get the following:

Neutral/center: 0v
From about 1/4 forward throttle to WOT: 5v
Any brake/reverse signal: 0v

If you reverse CH3 in the transmitter, you get:

Neutral/center: 0v
From about 1/4 to full reverse/brake: 5v
Any forward throttle signal: 0v

So, you can have something that simply turns on via CH3, or if you use a Y cable off the throttle signal, can have reverse/brake lights.

FG101C · 03.06.2010, 08:35 PM

I would suppose you could make a kill switch for a Baja with this method also. I have the stock steering servo from my Baja that I won't be using, may have to do this. Nice mod.

glassdoctor · 03.06.2010, 09:42 PM

Very cool BG. I might have to make one of these to power led brake lights. Looks a lot cooler than my other option of doing a micro servo + micro switch setup.

I made up some LED fog lights for my SC10 (Jconcepts body with the molded light rack on the roof and front of tuck. I also used a 555 chip to make a set of police flashers. Brake lights would be pretty cool to finish it off.

suicideneil · 04.18.2010, 04:40 PM

Just to follow up on this rather cool tutorial, I located a crappy little plastic geared hitec servo thats been sitting around in one of my tubs forever and decided to sacrafice it in the name of r/c.

After opening up the case I found the POT and motor were on handy little fly-leads, so I cut off the motor and just twisted the wires around the legs of a random LED I had in one of my many tubs of electronics-related stuff ( God bless tearing apart broken appliences ). I plugged it into the rx on the Gmaxx in place of a steering servo, turned it all on, centered the POT until the LED went out, then gave some steering input; hey-presto, works like a charm

Not sure quite what Im gonna power with this, maybe a lighting kit or seperate motor fan, or even a small buzzer/horn. I'll tidy up the wiring and shrinkwrap it fairly soon, just gotta get some wrap large enough.

Many thanks to BrianG for this idea, and all his other crazy little contraptions.

BrianG · 04.18.2010, 04:53 PM

Neil, if you are going to power lights from the servo's motor outputs, you should still use a resistor. The only thing saving the LED right now is the low servo input voltage (minus the H-bridge v-drops), but running LED(s) directly like that will severely shorten their life. Theoretically, the current through the LED is only limited by the wiring: (Vsupply-Vf)/resistance. But since the LED Vf value does change with current, it will work for a while.

What's_nitro? · 04.18.2010, 07:34 PM

Quote:

Originally Posted by BrianG

Neil, if you are going to power lights from the servo's motor outputs, you should still use a resistor. The only thing saving the LED right now is the low servo input voltage (minus the H-bridge v-drops), but running LED(s) directly like that will severely shorten their life. Theoretically, the current through the LED is only limited by the wiring: (Vsupply-Vf)/resistance. But since the LED Vf value does change with current, it will work for a while.

What he meant to say was:

Sorry I couldn't resist!

suicideneil · 04.18.2010, 08:26 PM

Not to worry, I has a cunning plan...

My old lighting kit from the first TXT-1 I built, comes prewired with the correct resistor for 9v power; would I need to change it for ~6v use, or will it be alright as is pretty much ( just not quite as bright )?

I've seen a couple websites with great info/ interactive menus for picking the right resistor, but spending ~£2-3 on a couple resistors and postage seems daft since I tend not to create these kinda things very often.

What's_nitro? · 04.18.2010, 08:31 PM

There's bound to be some great deal on a resistor assortment on eBay if you wanted to stock up for cheap money.

simplechamp · 04.19.2010, 02:53 AM

Don't you have anything like Radioshack in the UK?

suicideneil · 04.19.2010, 05:31 AM

Not locally- anything I order would have to be online; postage would be more than the items worth...

simplechamp · 04.19.2010, 02:21 PM

Gonna try to rig one of these up tonight. I have some 5mm ultra-bright white LEDs I want to turn into a headlight system. They aren't like the fancy, expensive ones used in the really bright flashlights, but they are still pretty bright.

medusa'd maxx · 10.09.2010, 09:50 AM

would this mod power 24 5mm leds in paralell if hooked up to a MMM spektrum combo? doing baja headlights. can i use a single resistor on each group of 12 if i solder all the legs together

BrianG · 10.09.2010, 10:30 AM

It should be no problem if you size the resistor so that ~15mA is on each resistor. 15mA X 24 leds is 360mA.

jpoprock · 10.09.2010, 10:40 AM

Brilliant!

Question, what kind of temp/tip do you use to solder on these types of boards? is it one of those "tweezer" kind? i've seen those, but cant recall how they heat up. sre they irons, or attachments to an iron? i have a typical digital adjustable hi temp model, but have never worked on pcb's..

nice work! thanks for sharing the knowledge!

jpoprock · 10.09.2010, 10:40 AM

Brilliant!

Question, what kind of temp/tip do you use to solder on these types of boards? is it one of those "tweezer" kind? i've seen those, but cant recall how they heat up. sre they irons, or attachments to an iron? i have a typical digital adjustable hi temp model, but have never worked on pcb's..

nice work! thanks for sharing the knowledge!