Servo - June 2010

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by Dennis Clark

I’ve received a ton of questions in the last month or so and I want to try an experiment to get you folks going on your projects. Since there is only so much time in the day, I’m going to try to find some of your answers with “off the shelf” solutions if at all possible. We’ll see how this works, but let me know if you prefer more DIY solutions for the future. With that in mind, let’s get started!

Q. I am looking for a simple circuit to drive a hobby servo and want to be able to set endpoints on either side of center for the servo to stop at.

I am going to use this to move a lens barrel (focus) on a TV camera lens, so it will have a gear on it. The barrel has mechanical stops so I need to be able to set endpoints with this servo. Thanks for your help as I have searched and searched, and found stuff thats close, but no cigar.

— Erik, Waterbury, CT

A. Erik, I’ve worked on this for a bit and it all depends on what you want to spend. I’m sensing that you’d like something that you either can build easily or can assemble from a product that you can buy. I have not found anything that you can use that will allow you to set endpoints using a simple analog pot and another pot for the servo position. Creating your own firmware solution that allows you to read three pots can be done, but getting the resolution that I think you will need will be challenging.

There are two parts of this that could use solving: one is how to control the hobby servo; and the other is how to move the gear on the lens. Let’s look at the servo controller first. To get your own DIY project going, I would suggest that you get a serial servo controller — one that will allow you to set endpoints and control the movement speed of your servo. It used to be that such controllers would be very spendy indeed — not so any more! Take a look at the Pololu Serial Servo Controller ROB-08897 at www.sparkfun.com. This little guy (see Figure 1) is inexpensive (under $18 US) and can be interfaced using a simple serial protocol. But what will it talk to? How about the easy to use and easy to buy (read inexpensive) Arduino? SparkFun sells a variety of Arduino boards, but you can shop around and find your own favorite form-factor and price range. Arduino’s are often found for less than $20. A SparkFun Arduino board is shown in Figure 2. The SparkFun website has some comments attached to this product that points you to source code examples for programming this board. You can work from these examples and add potentiometers to the board to set your position and set hard stop limits for your servo. Program your Arduino, wire your pots to the board, and mount the whole thing into a project box.