Basic servos can be found
for less than $10
Servos are wonderful little devices! Not only do they provide an inexpensive way to activate mechanical linkages through radio control, but they can be hacked to provide inexpensive drive units for small boats and cars with their self-contained motors and electronic speed controls (ESC's), and they can serve as part of a simple on/off power control switch.
Basic servos can be found
for less than $10
A servo is a little gear-motor, ESC, and a position sensing device in one
package. Rip the guts out of the case, and you'll have:
a) A lead from the Rx… it carries +5 volts, ground, and a signal wire.
b) A circuit board.
c) A small 5v motor, which may be soldered directly to the circuit board.
d) A pile of gears connecting the motor to the servo output. Bin these.
e) A potentiometer (pot, or variable resistor) connected to the circuit board
with 3 leads, and mechanically connected to the servo output.
Click on images for a bigger view
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| Step 1: A few screws loose, and things start to open up. | Step 2: Pressing on the two shafts popped the board out of the case. Don't pry directly on the board. |
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| Step 3: Using desoldering braid to wick off the solder, makes it easier to pull off the motor. | Loose parts. |
First trick in hacking a servo: Disconnect the pot from the servo output.
Normally, as the pot moves with the servo output, it produces a varying voltage
which is read by the circuit board, telling the servo that the output has moved
to the desired position (or not). See Schematic 1. When you disconnect this little knob
from the gear train and the servo output, leaving it in it's
centered position, and then command servo movement at the radio, the circuit
board never detects any movement, and thus keeps telling the motor to run.
So now center the stick, servo is happy, goes nowhere; forward stick, and the motor kicks in and
just keeps buzzing; reverse stick, ditto the other way. You'll want to put a
drop of glue on the pot to keep it from drifting.
Hack 1a: If the Pot itself doesn't suit you, you can replace it with a pair of 2.2k ohm resistors- both tied to the center tap's lead, opposite ends of the resistors to the pot's "outer" leads.
Another nice feature- most servos have a soft start, which means that a little
bit of stick will give part throttle… other than that, it's pretty much
off/on.
Next hack: The little motors that comes with servos may not suit you… they have tiny little short shafts. Any small 6v toy motor will do instead, as long as you don't load it heavily- the servo's ESC isn't designed to deliver much current! If the motor is connected to the circuit board with wires, no problem to swap them. If the motor is soldered directly to the board, you'll need to work a little more carefully. You can get "solder braid" from Radio Shack- braided copper wire that you heat with a soldering iron while pressing onto the board's soldered joint. The braid sucks up the solder, and you can pull the motor loose much more easily.
If you decide to keep the servo motor, no problem. You can leave the motor on the board- that's what I did on my plastic landing craft. And you can get tiny little plastic u-joints to press onto various miniature shaft sizes, metric and English, from Northwest Shortlines- catalog page at http://www.nwsl.com/Catalog/cat-UJoint.pdf
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| Servo motor still attached to board. Motor can is pressed into plastic structure. | Twin drive in landing craft. |
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| Servo motor set in Revell Trawler | Close-up of NWSL U-Joint on servo motor's 1.5mm shaft. "Horn" is sized for 1/16" prop shaft, which is only slightly larger than 1.5mm. |
It can be frustrating to go from the receiver's output signal, which is a pulse varying between 1 and 2 milliseconds every 20 milliseconds, to ON-OFF. Sometimes modelers will set a cam on top of a relay, and by artful use of the control stick, will move the relay to various positions to engage the horn and other on/off devices. Sometimes I find myself with a single free channel and a single function to control, and I'd just as soon not set up cams and brackets and switches. One example- I like to use the Traxxas TQ3 2-1/2 channel radio for simple models. This AM radio has a toggle switch on the third channel for shifting gears in a two speed gearbox using a servo... so it's full stroke one way or the other. For a hacked servo, this means either one motor lead or the other is energized, and the other one will be grounded. So instead of using a cam-operated switch, I pull the motor out of a servo, and wire in a relay. I know the servo can't supply the current needed to drive, say, a smoke generator, but it can operate a relay's coil, and the relay can switch the load.
Just make sure to return the coil ground to a true ground, not the other motor lead, or it will always be activating.
Questions? e-mail me!
©2005 Pat Matthews