Servo refers to an error sensing feedback control which is used to correct the performance of a system. Servo motor is a DC motor equipped with a servo mechanism for precise control of angular position. Servo motors usually have a rotation limit from 90° to 180°. Servos are used for precision positioning in robotic arms and legs, Remote Controlled toys like RC helicopter, airplanes and cars. In short, servos convert electrical commands from the receiver back into movement.
RC Servos basically come in three different sizes (micro, standard, and giant) to accommodate the type of RC models they are being used in. All RC servos have a three wire connector. One wire supplies positive DC voltage (usually 5 V) . The second wire is for voltage ground (0V), and the third wire is the signal (control) wire.
The servo motor can be moved to a desired angular position by sending PWM (pulse width modulated) signals on the signal (control) wire.Usually, a pulse of width varying from 1 millisecond to 2 milliseconds in a repeated time frame is sent to the servo for around 50 times in a second. The width of the pulse determines the angular position.
For example, a pulse of 1 millisecond moves the servo towards 0°, while a 2 milliseconds wide pulse would take it to 180°. The pulse width for in between angular positions can be interpolated accordingly. Thus a pulse of width 1.5 milliseconds will shift the servo to 90°. It must be noted that these values are only the estimations. The practical range of pulse width is 0.2ms to 2.5ms, and frequency is 20Hz to 60Hz.
RC Servo Tester Circuit Schematic
Now you can build your own simplest RC servo tester! Here is the circuit wired around the popular timer chip LM555, configured as a low-frequency astable multivibrator which can provide good control over pulse width. Remember to power this circuit from a regulated and “clean” 5V DC supply.
Servo Tester Prototype
- Prototype was tested with a “Tower Pro Micro Servo”. Servo test socket “J1” in the circuit is a standard 3-pin male header. In case of any difficulty, you can use 3 flying leads with alligator clips as test probes
- S1 is a “Push-To-On” switch works as a “Test Switch”
- LED 1 is added to indicate the output status of the tester
- Try to use a precision 4K7 trimpot as P1
- Note that, here the control pulses have a maximum voltage near-equal to the input voltage (5V) of the tester circuit. This can be a problem if your servo expects a 3V control signal
- It is good to add a 100µF/16V capacitor, as a buffer, across the 5V DC supply (VCC & Ground)
- Change the values of R1, P1, R3 and C2 if necessary/when demanded by the RC Servo under test. Before testing an RC servo, carefully refer its technical datasheet