> Here, let me outline two possibilities for a system:
>
> I'm going to assume 12 temp sensors, 12 controlled devices for this
> example, you can scale that to what ever you want.
>
> 1)
> Output:
> 12 I/O lines from the PIC go to NPN transistors, which drive relays
> for the fans and pumps, unless the current requirement is under
> 100mA, in chich case, the transistor itself drives the devices.
> Software PWM is used to control the voltages on the fans and pumps.

100mA for a fan or pump is a bit optimistic i think :D
Would the NPN transistors also work for AC pumps? (don't think so)
Hmm, maybe i should check if the pumps i plan to use are AC or DC...

> Input:
> All 8 A/D lines are used for reading 8 current based temp sensors.
> The PIC takes its analog input between the temp sensor and a 10K
> resistor.  This takes only 1 PIC pin per sensor.
> I2C is used to interface to a 4 channel A/D converter for the
> remaining 4 temp sensors
>
> 2)
> Output:
> Several I2C DACs are used to interface to small power transistors,
> which feed the required current to the fans and pumps at 0.7V less
> than the DAC output.
> Input:
> SMBus temperature sensors are fed through an I2C hub to allow for
> more than 8 of them, and to convert the SMBus voltage levels to I2C
> levels. Each temperature sensor requires a connection to +5V, 0V,
> SCL, and SDA.  Several PIC control pins are used to change the
> selected channel, thereby preventing bus contention.
>
> Conclusions:
> Mix this up as much as you want.  Remember that with I2C, you have to
> be careful about bus capacitance.  You may need to use special I2C
> trancievers to handle any long runs.

I don't think there are going to be any really long runs... the distances
will be pretty short (haven't really measured yet) :)

> My recommendation?  Use the input from 1) and the output from 2)

> Here's what I'd do:  Use some kind of analogue output temperature
> sensor, and connect it however you were going to, then read those
> ouputs with I2C A/D converters.  From those outputs, calculate what
> you need to, then run the outputs to I2C D/A converters, which run
> their ouptuts into power transistors.  Connect the power transistors
> thusly:  Base to DAC collector to 15V(12V maybe), emitter to
> fan/pump.

Hmm, yea, that looks like the easiest... And with multichannel A/D and D/A
chips this should be pretty easy & clean. And if I would use thermistors,
then I could keep the I2C bus pretty short to avoid problems (although
longer wires to the thermistors will give a worse accuracy afaik).

> And you still have all your PIC inputs left except for the 2 for the
> I2C interface.  With that, you can use a small PIC, just as long as
> it supports I2C.  Though, you *could* use software based I2C on a
> smaller PIC, but it's a bit trickier.  But you said you were thinking
> of an 18F PIC, I think, and I think that most of them support I2C.

Yea, I basicly wanted to save a few pins so that I have some spare ones for
some buttons/a keypad and a LCD... Now I still have to find a nice, small,
18F PIC (28 pin DIP or something like that was what I originaly wanted to
use...)

> Another protocol to consider is SPI.

Hmm, i don't really know that much about I2C (just the basic theory, haven't
actualy used it yet), but i know even less about SPI, IIRC it's a 3 wire
interface...

Going to think it over one more time, select the components to use, find me
a decent PIC programmer (which i can use from linux... Wouter's programmer
looks nice, maybe I should ask him if i'm allowed to port his software to
linux) and lets get busy! Be sure to expect me back with some more
questions/problems soon :)

Thanks, you (and others on this list of course) have been very helpful :)

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