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# Thread: How to build your own thermal regulating film processor - MAKE magazine

1. ## How to build your own thermal regulating film processor - MAKE magazine

I found this online and thought i'd share

http://make-digital.com/make/vol31?p...c.gif%2F#pg125

2. Neat!

3. "Traditional photography is fantastic."

If people are doing these sorts of film-related home "Heathkit" projects today using Arduino boards,* then traditional film photography can survive on the user side.

Somebody should point the potential post-Kodak film suitors to this as a sign of market enthusiasm. Even if they don't read the article, they'll see that Yashica TLR and maybe read that first sentence.

Ken

*I've designed and implemented Arduino projects for commercial products. It's a cool little platform. In fact I've got a couple of unused Uno boards upstairs right now. Hmm...

4. A bit of a bodge, that...

5. I think this is great!

6. This DIY sous vide rig can be used for the same temp-controlled water bath:

7. Having taken some time to look over the design, here are some problems I see with it:

The temperature measurement is not very accurate. It uses LM35 temperature sensors feeding the ADC input of the Arduino. The LM35 has guaranteed accuracy of 0.5 degrees C, and outputs a voltage of 10mV per degree C. The temperature range of interest is around 20-40 degrees C, so the output voltage will be in the range of 0.2-0.4V. The ADC is a 10-bit device (1024 steps), and he appears to be using the default analog reference voltage, which is the +5V power supply (no call to analogReference() in the code). Now, this is a problem, as the power supply cannot be considered a precision voltage reference - its value will vary. But even if we assume that it is exactly 5.0 volts, we are using a very small range of that 5 volts. Each step of the ADC corresponds to approximately 5mV, which is half a degree. And yet, he goes to great lengths to avoid self-heating in the LM35, which is 0.1 degree or less. The schematic does not show any signal conditioning on the LM35 outputs, so noise could be a factor affecting the precision of the ADC readings.

Using the internal 1.1V voltage reference would improve things. The accuracy would then be on the order of 1mV per step, or 0.1 degrees, though I would still not consider this a precision voltage reference (nominally 1.1V, but can be between 1.0 and 1.2V per spec). However, the temperature control loop is still a problem. The heater consists of nichrome wire embedded in fire cement. This has a certain amount of thermal mass and will continue heating (i.e., raising the temperature) of the water bath and developing tank after the heater is shut off. Just how much is unknown without actually building it and trying it (or doing a whole lot of math that I don't care to do), but the control loop simply waits until the actual temperature exceeds the desired temperature before shutting off the heater.

8. Using the internal 1.1V voltage reference would improve things. The accuracy would then be on the order of 1mV per step, or 0.1 degrees, though I would still not consider this a precision voltage reference (nominally 1.1V, but can be between 1.0 and 1.2V per spec). However, the temperature control loop is still a problem. The heater consists of nichrome wire embedded in fire cement. This has a certain amount of thermal mass and will continue heating (i.e., raising the temperature) of the water bath and developing tank after the heater is shut off. Just how much is unknown without actually building it and trying it (or doing a whole lot of math that I don't care to do), but the control loop simply waits until the actual temperature exceeds the desired temperature before shutting off the heater.
I've read of people using Auber PIDs with temperature probes that can read in 0.1C (so they say), all connected to heating elements via a relay. The PID apparently "learns" by repeated cycling how much voltage to apply and for how long, in order to hit the desired temperature. I've priced these, and the PID and probe are around \$60, while the heater is under \$10, the relay being similarly priced.

Perhaps that would be a good alternative?

9. I've also found another link for the same project here: http://makeprojects.com/Project/Monk...1#.UEv3uELWGlI . Perhaps this is the latest version of the article as it made reference to the Make magazine article as well.

10. I'm actually considering building this, but I have no knowledge or experience with doing a build like this. Has anyone else tried this, that can write s more simple step-by-step guide for idiots? I need a better method to do color, but on first glance these builds seem intimidating for someone with no experience in doing builds like these.

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