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Description: The SparkFun Continuous Rotation (CR) Servo Trigger is a small robotics board that simplifies the control of hobby RC servo motors. When an external switch or logic signal changes state, the CR Servo Trigger is able to tell an attached servo motor to move from position A to position B. To use the CR Servo Trigger, you simply connect a hobby servo and a switch, then use the on-board potentiometers to adjust the start/stop positions and the transition time. You can use a hobby servo in your projects without having to do any programming!

When we introduced the original Servo Trigger, we mentioned that it could be reprogrammed to be more useful with continuous rotation servo motors. But reprogramming the firmware is somewhat tedious, and users asked for a Servo Trigger preprogrammed with the continuous rotation logic. With this little board you will be provided an easy way to deploy continuous rotation servos into your projects!

The heart of the CR Servo Trigger is an Atmel ATtiny84 microcontroller, running a small program that implements the servo control features designed for continuous rotation servos. On board each of these CR Servo Triggers you will find three potentiometers: “A” sets the position the servo sits in while the switch is open, “B” sets the position the servo moves to when the switch is closed, and “T” sets the time it takes to get from A to B and back.

Compared with a servo motor, the CR Servo Trigger board draws very little current, roughly 5mA at 5V. Be sure to note that if you’re using the CR Servo Trigger to control your motor, the absolute maximum supply voltage that should be applied is 5.5 VDC. Additionally, the SparkFun CR Servo Trigger is designed to make it easy to daisy chain boards – you can simply connect the VCC and GND pads on adjacent boards to each other.

Note: This idea originally came from our friend in the Oakland area, CTP. If you see him, please give him a high-five for us.

SparkFun CR Servo Trigger Hookup Guide


  • Recommended Voltage: 5VDC
  • Max Voltage: 5.5VDC
  • Current Draw: 5mA
  • Control Continuous Rotation Servos
  • Three Control Settings
    • A - sets the position the servo sits in while the switch is open
    • B - sets the position the servo moves to when the switch is closed
    • C - sets the time it takes to get from A to B and back
  • Easy Control with Potentiometers
  • Configurable Input Polarity
  • Configurable Response Mode
  • Compatible with Analog Servos
  • ISP Header Pins Available for Reprogram


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Customer Comments

  • Experimenting with this product, I discovered that it only moves about half of the range on the servos I have. Looking at the github code and testing with writeMicroseconds(), I discovered that the firmware was compiled with the SAFERANGE flag, setting the pulsewidth range of 1000 to 2000 microseconds. This seems a bit restrictive considering that the arduino defaults on its servo library are 544-2400. So be aware that if you want more range you’ll need to recompile and reload the firmware.

    see: https://github.com/sparkfun/Servo_Trigger/Firmware/ServoTrigger/ServoTrigger.c

  • I need to be able to update position commands to a continuous rotation RC servo via software. It’s surprising to me that a commercial solution (with position feedback) doesn’t seem to exist. I have experience with steppers and DC motors as well, but I prefer a servo due to compact packaging and absolute position feedback built-in.

    It seems like this board is great if your desired position (or two) rarely change, as it’s updated by potentiometer. I suppose I could replace the board pots with a voltage command, but I’d prefer not to as this will eventually be part of a STEM package for kids and the rest of the project is complex enough.

    Anybody got any ideas or solutions? Maybe a simple absolute encoder add-on for a servo (which I haven’t found)? I only need ~1° accuracy, and max rotation speed is ~1RPM.

    Thanks, -Bryan

    • An Arduino or the Red card clone by Sparkfun, or a Pololu controler, can be used for your purpose. I have worked with continuous rotation servos myself. It was for a project of a CNC stand. Unfortunately, I did not find any servo having a 1 degree acuracy, even digital ones which are more expensive and precise than analog ones. Instead, I plan to replace them with stepper motors. It is said they have that kind of accuracy, and I see that everybody uses them for CNC machines.

      Edit : if you use servos with feedback, then they must be the servos that are not the standard ones for hobbyists (5 or 6 volts, weight 10 to 50 grams, and are linked with 3 leads only), but the ones working with 7.4 V or 11V or more and 5 leads.

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Related Tutorials

Continuous Rotation Servo Trigger Hookup Guide

May 26, 2016

How to use the SparkFun Continuous Rotation Servo Trigger with continuous rotation servos, without any programming!

Servo Trigger Programming Guide

May 26, 2016

Looking under the hood of the Servo Trigger -- using the development environment and some finer details of the firmware.