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Multiple Temperature Monitoring/Control System

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RowingMunkeyCU

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Hey guys, first post here and I'm working on a project and trying to determine the changes needed to an existing project to make mine work. I'm not as familiar with electronics as I'd like (I'm a software engineer) so I know I have a lot to learn.

What I'm trying to do is monitor approximately 20 separate temperature sensors (thermistors) and control 20 separate heating devices (FlexWatt heat tape). The sensors/heating devices are to be in a 1:1 ratio. Each sensor/heating device pair should operate independently from the rest.

The project that I'm working from is:
**broken link removed**

**broken link removed**

So the main question is how can I expand the number of sensors that the project is using to the number I'd like to use (~20)?

1. I see that each thermistor occupies 2 channels on the ADC (8 channels)which limits it to a max of 4 sensors. 'Bigger' (40 channel) ADC or multiple ADCs?

2. Since there is a limit to the number of control signals output from the parallel port, what's the possibility of running it over USB and using a demux of some sort to control the heaters?

Thanks in advance!
 
Seems like a nice project for a microcontroller.

Edit the web site you link was a student project and hasn't been updated since 2004. Also claimed a cost of $17,524.15 AU.
 
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USB + micro is nice but if you want a quick change try this.
The parallel port has two unused input pins S5 & S4.
Duplicate the (MAX186 and temp. sensors) two times. The three MAX186s have their digital pins connected together except Dout. Dout2 goes to S5 and Dout3 goes to S4. The software needs to be changed to read in 3 bits at a time.
 
blueroomelectronics said:
Seems like a nice project for a microcontroller.

Edit the web site you link was a student project and hasn't been updated since 2004. Also claimed a cost of $17,524.15 AU.
Most of the cost they attributed to their own labor hours though. The actual cost of the setup was only ~$70.
 
ronsimpson said:
USB + micro is nice but if you want a quick change try this.
The parallel port has two unused input pins S5 & S4.
Duplicate the (MAX186 and temp. sensors) two times. The three MAX186s have their digital pins connected together except Dout. Dout2 goes to S5 and Dout3 goes to S4. The software needs to be changed to read in 3 bits at a time.
Looking for the best option, not necessarily the fastest. Any advice/resources for reading up on microcontrollers?
 
Sure this site, there are lots of PIC based home control systems on the net. If you can wait 6 months you'll be able to build a home control system using a couple of kits I've got coming out.
The kit I'm working on right this moment (Ladybug) is designed to be a home control system with Ethernet & RS485. Adding zones is part of the design.
 
Check my PIC tutorials, you could use CMOS switches to switch multiple inputs to a single analogue input - which is what the MAX186 above does internally, and what PIC's do (the 16F877 has 8 analogue inputs as well).
 
The 1 wire digital temp sensors overcome the need for an ADC and they can be used over a reasonable distance. + a far more modern approach, parallel ports are disappearing.
 
1-wire sensors make a lot of sense. They can be bussed so you can hang several on one I/O port, and you can extend your wire length a reasonable distance, so it is easy to hook them up to a small PIC. Plus the data is digitized at the sensor which eliminates most noise and drift problems that you might face with analog voltages and A/D converters.

There are several vendors doing 1 wire sensors. Here is one:
**broken link removed**

Digikey sells these for about $5.50 each.

TI also has one for about $2. Just search at Digikey for Temperature sensor and you'll find several.
 
Interesting, the 1-wire solution looks like it's a good way to go. Is there an advantage for parasitic vs. non-parasitic thermal sensors?

Also, I've been looking a bit at the SHT11/SHT15/SHTXX series of thermal and humidity sensors. They're listed as 2-wire, is there any way to run one on a 1-wire system?
 
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