Appendix C - Using Servos with the Quasar
The Quasar 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). With a trap
door, it’s altitude-independent; once you pop the door, the parachute is going to come 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 Quasar’s servo
support is ideal for these scenarios.
We strongly recommend that you power the servos with a separate battery. This prevents any
motor-startup current spike from affecting the computer side, and possibly resetting the Quasar.
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. Some micro-
sized servos may also operate from a 1S Lipo. If you’re not sure what battery to use, ask your
local hobby shop; they probably know more about R/C servos then you’ll want to hear.
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.
The QUASAR has special logic-level servo outputs, these are labeled SERVO on the board.
They are designed to connect to the SIGNAL line of the servo (typically a white wire). We
recommend that you install a 330 ohm resistor in series between the servo control line and the
QUASAR output; this protects both the servo and the QUASAR from possible noise glitches.
The GROUND wire of the servo(s) (normally black) should be connected to the “-“ side of the
servo battery, and also be tied together to the GND pads on the QUASAR; we have conveniently
provided extra pads labeled GND next to the SERVO outputs.
The POWER wire of the servo (normally red) should be connected to the separate servo battery.
DO NOT connect it anywhere on the QUASAR!
We also recommend that you connect a 220uF to 470uF electrolytic capacitor between the
POWER and GND leads of the servo. This helps filter out the motor noise so it won’t get to the
Quasar’s logic. Be sure to get the polarity correct; the “-“ side of most electrolytics is marked, so
make sure that lead goes to the GND lead.
To Next Page
Loading...