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Appendix C - Using Servos with the Proton
The Proton was designed to modulate standard PWM hobby servos, for use with non-pyrotechnic
deployments. Typically, these will be some kind of "trap door" mechanism that's released by
pulling out a pin. If you go very high, over 30,000', pyrotechnic charges lose much of their
efficiency, because the reduced air pressure requires a larger amount of generated pressure to
achieve the same force against the nose cone (or AV bay, for a drogue). Black powder charges
also have a harder time lighting too. With a trap door, it's altitude-independent; once you pop
the door, the parachute is going to come out, especially if you have some kind of spring
mechanism to push it out.
Also, some very large rockets do not lend themselves very well to pyrotechnic deployments, and
minors cannot legally use pyrotechnic deployments. For example NAR TARC teams are
allowed to use multiple deployments, but they cannot use pyrotechnics. The Proton's servo
support is ideal for these scenarios.
The Proton provides support for servos on CH1-CH3, in the Drogue and Main modes. This
means that you can only use a servo "on the way down", which is pretty typical of what people
want to do with them. You can not use a servo with Airstart mode, since it's not designed for
triggering on descent.
There are three dedicated pads on the Proton for the CH1-CH3 servo connections, at the lower-
center of the board next to the deployment terminals. To connect a servo, you need to first
identify the 3 leads on the servo:
Power – Provides Positive battery power to the servo (usually red)
Ground – Provides Negative battery power to the servo (usually black)
Signal – PWM signal input to the servo (usually white)
If you're not sure which lead is which, check with your servo manufacturer's support web site.
Not every servo has the leads in the same order, or using the same colors, but they all use the
same three leads.
To use a servo, you will need to provide power to the servo, and connect the Signal and Ground
leads to the Proton. Typically, the Signal lead will connect to the Servo output of the Proton,
and the Ground lead of the servo (which should be connected to your servo battery's "-" lead)
should be connected to a ground point on the Proton; the "G" pad of the Servo pads is ideal for
this. We recommend that you connect a resistor in series with the Servo-Signal lead to help
prevent noise... 220 ohms is ideal. Note that the servo pads and their holes are smaller than the
other pads, they're designed to take a 1/8W resistor directly.
For most applications, we recommend that you use a separate battery to power the servo, rather
than using the same battery that your Proton is using. This prevents any servo motor "noise"
from getting back into the circuitry of the Proton. Most hobby servos are rated for 5V
nominally, however they work great with 6V and many will work with a single 1S 3.7V LiPo
battery too. Check the specs on your servo for the appropriate power. In general, you should
use whatever battery you would if the servo was going into a R/C model; most of the time a 600
mAH 4.8V or 6V NiCad, NiMH, or LiFe pack will suffice. If you're not sure what battery to
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