Home > Circuits > RC Servo Tester
Modified:21:42, 22 October 2013
DESCRIPTION
Servo control is very common with radio control and uses PWM to control the position of the actuator. The standard is pulses every 20ms varying in width, with 1.5ms pulse in most cases being at the center of the travel of the actuator - like a neutral position. There is some variation but generally the useable range is 1.25 to 1.75ms. There are some devices that accept a further range so this circuit will provide from 0.75ms to 2.25ms pulses to control and test most servos.

Below is a block diagram of the circuit. The astable is set at 50Hz whose output is shaped into a short spike to trigger the monstable which in turn generates a pulse that can be adjusted.

An example of the standard type of RC servo.

CIRCUIT DIAGRAM
CIRCUIT FUNCTION

The whole circuit can be made from one logic IC - 4001. Although the CMOS type was used, you can use 74HC series instead, but you may need to adjust some values.

The astable is formed by IC1a/b and is fixed at a frequency of 50Hz. This provides a square wave pulse of even mark/space ration at the output of IC1b.

These pulses are shortened by the pulse shaper formed by C2/R3/D1. This differentiates the pulses to produce brief spikes. The diode prevents the negative spikes, generated by C2 when the astable goes low, from damaging the NOR gate inputs.

The final section is the monostable. This is triggered every time the spike generator produces a high pulse. The output of IC1c goes low and since C3 is empty, then so does the input to IC1d, thus producing a high at the output, being the start of the PWM pulse. As C3 charges, the input to IC1d rises until it reaches the logic threshold for high and sends the PWM output low - being the end of the pulse. Again D2 clamps the excess voltage appearing across the input of the gate to +0.7v.

With the values shown, the range can be varied using VR1 from 0.75ms to 2.25ms. This should allow correct operation of most servos.

Blue trace = IC1d input
Red trace = PWM output

Minimum setting

Neutral or mid-point setting

Maximum setting

A suitable power supply for the servo-tester is shown right. The regulator should be a 1Amp type or above (7805/78S05) and may need a heatsink if operating from supplies greater than 9v.

C5 should be a tantalum type at 35v or more if possible and both capacitors should be positioned as close to the regulator as possible on the PCB.

Designed and Written by Phil Townshend 2009
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