Gathering Information

While my plan involves several stages, I'm doing some extra work at the beginning to gather information about all the stages. This will allow me to save time later, because if it's not going to work to turn my shower knobs with servos, I'd like to know before I order servos and hook them up to my knobs.

I first started with a temperature sensor. I chose the TMP36 from Adafruit, which is an analog sensor that costs $2 and measures from -50C to 125C with 1/10 degree precision! Awesome. Wonderfully, it comes with all the code and instructions you'll need in order to use it with an Arduino. I consider this to be feasible.

Next, I'm gathering information about the servos. This document describes in a general way how to size a servo motor to your application. Brush or brushless motor? This site seems awesome, and it tells me useful stuff about servo motors! Ok, so I think I'd actually prefer a stepper motor, and once again Adafruit has one that might work. But how do I know? The motor is rated for 28 oz-in of torque. Will that be enough to twist my shower knobs? Let's find out...

I've got a little scale that measures up to 24 oz. The knob on the shower is a cross with arms about an inch long. So if I press the scale against one of the arms and it turns with 24 oz or less, that means the motor should work!

After testing, the answer is... nope. Rats. Next, I've found an awesome video describing the characteristics and differences between steppers and servos! It sounds like a servo is what I want, because they are designed to operate within a single rotation. I tested my shower knobs, and I only need to go between 0-180 degrees, so a servo should work great! They also are much easier to operate than steppers because you don't need a stepper driver, and you can just hook them directly up to an Arduino. Ok, so I'm looking for a servo after all.

I found one servo that's rated for 70 oz-in. That seemed like a lot, but I wanted to be sure. So I had an idea! I'd tie a ruler to my shower knob and push it with my little scale. Then I'd multiply the distance from the pivot point by the number of ounces of pressure on the scale needed to move the knob. So I did it! Unfortunately, twisting the knob from the off position took about 100 oz-in, and moving it while on took about 70. So it looks like I'll need a pretty strong servo. They're often rated in kg-cm, so I need one that's about 7 kg-cm. Hopefully it'll be less than $30.

I've found one rated for a whopping 24 kg-cm, but it's $79. I found a few others that either were too expensive or too cheaply made or otherwise questionable. Finally, I found this one for $31, which is acceptable but still a little too expensive if I want to sell this idea as a product. I'll need two of them, and with the addition of the Arduino it'll cost over $80 in parts. I may just go with this to save time, but I'd ultimately like to use an inexpensive motor but with additional gears, which would provide the same torque at a much lower cost. If I could get the total cost close to $20, I think I'd have a product that at least a few people would buy. I remember Kelsey saying she wanted one at the party last night.

Stay tuned!