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SERVO CONTROLLER
UPDATED:.13:02 29 June 2014
OPERATION NOTES

Servos are devices that can place an armature is a specific position. This circuit will produce pulses to control the position of a servo armature.

The circuit below produces pulses between 0.2ms to 11ms. This stretches beyond the range of most servos but will enable specific positoning to be achieved.

The useable range is 0.75ms to 2.25ms

ASTABLE:
Set the Range 2 - (31Hz-850Hz) with VR1 at near minimum, (anti-clockwise)

-SPIKE GENERATOR:
Set the polarity to negative and the duration to a minimum.

MONOSTABLE:
Set the Range jumper to 1 (0.2ms to 11ms)


BLOCK DIAGRAM
CIRCUIT BLOX
EXPLANATION
Astable 1 produces a steady stream of pulses around 30Hz

The output of the Astable is connected to the trigger input of the Spike Generator. This is configured to produce a negative pulse on every high-to-low change in voltage on the trigger input

The output of the Spike Generator is connected to the trigger of the Monostable. Every low pulse triggers the Monostable to produce a high pulse which is sent to the servo.

The Spike Generator is needed since the low (space) time of the astable output, is longer than the required. So the Spike Generator shortens the low time to about 200us, which triggers the Monostable to produce a longer pulse. So the "low" time must be shortened before it can trigger the Monostable. Otherwise the Monostable output would stay high for as long as the tirgger is low - too long!


TEACHING OPPORTUNITIES: DEVELOPMENT
Topics could include:
  • Pulse Width Modulation (PWM) control methods.
  • Radio control systems - RC Cars
  • Servo control
  • Speed control
  • Monostables - generating timed events
  • Calculating time using T=1.1CR

The circuit can be used to rotate or move via linkages, anything mechanical. For example directional antennae positioning, or aiming control for a sweet launcher.

By connecting a Motor Speed Controller to the Servo signal instead of a standard servo, you can create a directional motor speed controller. Take care with wiring on these. They usually require their own supply for the motor and feed 5v to the servo +V connection. Threrefore the 5-6v connection to the above circuit can be omitted.

PROJECTS:


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