Grant Dean

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10 years, 7 months ago.

Analog In PWM Out to control rc servo

I am trying to figure out how to move a servo motor when a light sensor is connected to a analog pin. I am using a infrared line tracker that has Vcc in, Gnd, and a Vout witch is the analog signal to the mbed Controler. What I would like it to do is when the sensor has light in front of it the PWM output will move a servo motor and stop when it reaches its limit of 180 degrees. I have been able to control the the input with the line tracker sensor to turn my led on. But to control a rc servo motor has been proven to be more difficult for me

3 Answers

Jon McPhalen
10 years, 7 months ago.

I believe the mbed is a 3.3v device so it would be unwise to put 4.5v directly into it. Create a divider with two 10K resistors; connect the junction between them to your ADC input. Your threshold now becomes 2.25v which is safe in a 3.3v system.

Accepted Answer

include the mbed library with this snippet

#include "mbed.h"
PwmOut servo(p24);
int main() {
    servo.period_us(20000); //20ms period, typical for analog RC servo
    int servopulsewidth=1500;
    servo.pulsewidth_us(servopulsewidth); //centers the servo. Usually a RC servos range is from 1000 to 2000us.
    while(1){
        servo.pulsewidth_us(servopulsewidth++); //increments servopulsewidth with 1
        wait_ms(20);
    }
}
posted by Pankaj Mishra 27 Aug 2018
Tomas Johansen
10 years, 7 months ago.

If you posted your relevant source code, and perhaps a bit more specifics about your servo problem then we will be able to help you.

A RC servo is really simple though, here is an example code with a fictionary light sensor

RC servo

#include "mbed.h"
PwmOut servo(p24);
int main() {
    servo.period_us(20000); //20ms period, typical for analog RC servo
    int servopulsewidth=1500;
    servo.pulsewidth_us(servopulsewidth); //centers the servo. Usually a RC servos range is from 1000 to 2000us.
    while(1){
        if(lightsensor==1){
            servo.pulsewidth_us(servopulsewidth++); //increments servopulsewidth with 1
        }
        wait_ms(20);
    }
}

Hopefully this will guide you to how to use a RC servo.

Grant Dean
poster
10 years, 7 months ago.

Thank you so much I think this is what I could be looking for. I will try it our for sure. one other question I have and you might be able to help me out, say I wanted the analog pin to read 4.5v from the sensor depending on how I power it. but I don't want it to put a out put on the pin till it reaches that voltage.