Geneva Thermometer

[alpacaman]

I've been wanting to build a unique thermometer but was unable to come up with a good idea. Then I was on YouTube one day and I came across the Geneva Mechanism. Perfect - it would let me control the position of the thermometer in accurate and stepped increments. Plus it's a cool little mechanical device which would be relatively easy to build. I did find out that the crescent cutout is fairly critical - I had to make it twice. The first time I had it a little too big and the mechanism had a tendency to jam. The second attempt with the smaller crescent cutout has worked perfectly.

I'm using a 08M and a modified RC servo. The modification to the servo is simply replacing the internal pot with two 2.2k resistors and removing the mechanical stops. This lets the servo turn continuously in either direction. A lever switch rides the orange disk. The disk is notched. When the switch hits the notch it triggers an interrupt routine which simply tells the servo to stop turning.

One side of the thermometer reads in degrees F (32 to 104) while the other side is degrees C (0 to 40). The degrees C are in reverse order.

 

[eclectic]

What a lovely machine!

Just a couple of questions if I may.

1. What did you use for the Picaxe/temperature sensor?

2. Are the gears made of wood, as well as the body?

e

 

[alpacaman]

What a lovely machine!

Just a couple of questions if I may.

1. What did you use for the Picaxe/temperature sensor?

2. Are the gears made of wood, as well as the body?

e

Thanks!

The temp sensor is DS18B20 - this is the one used in the manual (readtemp command).

The gears are made from 0.25 inch baltic birch plywood. The rest of it is 0.5 inch MDF.

 

[alpacaman]

What a lovely machine!

Just a couple of questions if I may.

1. What did you use for the Picaxe/temperature sensor?

2. Are the gears made of wood, as well as the body?

e

I know that I answered your questions in a previous post. However, since then I've learned that the 08M was resetting itself - not good. After much tinkering and forum (08M appears to be resetting) discussion, I found out that the DS18B20 and the servo command interferes with each other and will cause the 08M to reset.

Therefore, I'm going to change out the DS18B20 with LM35DZ. The LM35DZ is analog rather than digital and is a LOT cheaper. If I had done my homework I would have used this sensor. Thanks to Dippy for suggesting it.

 

[alpacaman]

Working fine using the DS18B20. Still going to try the other temp sensor.

I tried turning the servo off right after it moves to it's 'home' position and staying off until a temperature change is received. I also added a 1 second delay after the temp reading before turning the servo on. This greatly improved reliability but every few minutes it would still reset.

I ended up adding another 08M (same power supply) - so I have one sensing the temp and sending an up, down, and interurpt signal to the other 08M. The other 08M gets the interrupt signal, checks the up/down signals and moves the servo accordingly. Haven't had a reset yet. Since I added another 08M I took the oportunity to add functionality. Before I had to physically move the chain to 'set' the temperature. I now have up and down buttons to 'set' the temperature. Why do I need to set the temperature? Since the temperature display is mechanical you need to set it at a known position when power is first applied. During the initialization portion of my code I load 70 degree F into the temp variable. Then I move the chain to 70 degrees. To start reading/displaying the actual temperature you then push the set button and it will read the current temperature and move the chain accordingly. On power up I could move the chain in one direction until it hit a limit switch which would then automatically start reading/displaying the temperature. Since I don't expect very many power outages I didn't do the limit switch option - maybe on version 2.

 

[hippy]

"Since I added another 08M I took the oportunity to add functionality" - Every cloud has its silver lining, and I'm glad you've got it working to your satisfaction, albeit with a bit of unexpected faffing around.

 

[alpacaman]

"Since I added another 08M I took the oportunity to add functionality" - Every cloud has its silver lining, and I'm glad you've got it working to your satisfaction, albeit with a bit of unexpected faffing around.

The Picaxe is so versatile and easy to use that we sometime forget that there are limitations. Even though there was some unexpected faffing around I'm still a big fan of Picaxe and in the end it's worth all the faffing around. I've only been using them for a little over a year and I can't imagine how bored I must have been before I found out about them and got my creative juices flowing.