Is it possible to read digital temperature sensors with a PiFace?

It is possible, depending on what you're looking for and how much work you want to put into it.  In the past, I've cheated and use one of the USB cables from DLP Design. Alternatively, it shouldn't be too hard to knock up a circuit using something from Digikey (example only).  The hard part will be finding a vendor that will sell you just one (they typcially come in bulk packs) and won't be outrageous for shipping a single part.

- Tim

Do you need the Pi Face? If you are using I2C there are all kinds of hubs, extenders, repeaters, and expanders at low prices which will allow you to add up to about 120 I2C devices. I was using DS28B20s on a 1 wire system, which has the advantage of longer length. But I understand that with I2C buss, it can be extended out to about 4000pF instead of the current upper limit of 400pF.

well,,, why don't we go up to 600pF....  WTF ???      I have no idea what u guys are talking about,,,

well,,, why don't we go up to 600pF....  WTF ???      I have no idea what u guys are talking about,,,

Th I2C bus has a lot of sensor variety, but is meant for communications over very short distances. The distance varies by way of sensor and wire used, speed desired, how many sensors are on the bus and other variables. So its "length" limit is best not measured in units of distance, but in units of capacitance of the bus. The pF stands for pico Farads.

I am not really familiar with electronics either. I was a biochemist (amongst many other things in Public Health,) and I worked with an electronics tech to interface Apple ]['s and other several mini-computers to lab equipment. I have no formal training, just a smattering of stuff I picked up here and there. You can do quite well with this approach. Having had the experience I did, I now think in terms of what micro controllers can do for me, though not it how. With the support of a group like this, and a willingness to let the magic smoke out of components every now and then (more an ego issue than financial,) you can accomplish a lot, find a lot of satisfaction, and enrich your environment. While I am new to this forum, the people seem willing to take the time to explain a lot to us "noobs."

The effect of capacitance (pF) on data transmission is that it tends to 'bypass' the higher frequency component, thereby rounding off the edges, and changing the timing.

In extreme cases this can result in a ramping of the leading and trailing edge and the capacitor is charging and then discharging.

Some circuitry can handle this, some can't.

Mark