Servo-Checker JR / Futaba (2.7V)

Check your servos manually

This circuit shows a Servo-Checker for JR / Graupner / Futaba-Components (positive impulse).
The ZPD2.7 (Zener diode 2.7V) at the output limits the positive servo signal to a maximum amplitude of approximately 2.7 Volt (Futaba receiver output voltage).

final servo checker

The final servo checker in action

  pulse period is approx. 21 ms

Pulse repetition period is 21.2 ms
(spacing of the two vertical markers)

powered by 4 AA batteries

Fully packaged circuitry including 4 AA batteries

  quite dense packaging

Quite dense packaging

The first timer IC 555 creates a negative trigger signal of approximately 45 Hz (a trigger impulse every 22 ms, DO NOT exceed 50 Hz for proper servo function). The second timer IC 555 modulates a positive square signal from 0.9 ms to 2.1 ms every 22 ms, a method commonly known as pulse width modulation (PWM). The modulated pulse width provides the full information for the exact servo position, and allows a total swing of 140-160 degrees of the servo head (see pictures below).
M is a trimmer to adjust the middle-position (the default pulse width of 1.5 ms), the main input for the servo control is the potentiometer P, which modulates the pulse width from 1.0 ms to 2.0 ms.

Note: Since the servo draws quite some instantaneous current (what might cause some power supply droops), ensure that there are sufficient capacitors to support the supply voltage (at least 100nF & 10uF). Otherwise the power supply droops slightly affect the timer circuitry: the pulse width may temporarily vary slightly, resulting in temporarily jerky servo movements.

The green and red rods are corresponding to the signals on the oscilloscope below.

A screen-shot of the signals in the simulation. The ZPD2.7 limits the output voltage to 2.7 Volt.
The two markers are 22.5 ms apart (T2-T1), the first pulse on the screen has a width of approximately 1.0 ms, the second one a width of approximately 2.0 ms.

left position
  center position
  right position
left servo end position

left servo end position:
pulse width 0.975 ms

  servo center position

servo center position:
pulse width 1.5 ms

  right servo end position

right servo end position:
pulse width 2.07 ms

AC signal capture with OSZI
Qualitative signal acquisition using the freeware OSZI for Windows 95. The pulse period is correct at about 22 ms.
At the time I have built this circuit, I did not have a native hardware oscilloscope, so I used the soundcard of my PC.


Last updated: 06.08.2006

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