I need to power the LM35 with at least 4v, so the 3v3 doesn’t work. Will this scematic work out?
There is a small risk that the output voltage of the LM35 could rise over the 3.3v limit of the imp input pin, so best play safe and take it through a 220 ohm resistor and a Schottky diode to clamp it close to the imp 3v3 line. Just in case the LM35 is faulty!
By the way, remember that if you need to measure negative temperatures you will need a negative voltage supply as well. Some web pages are not clear about this, but the datasheet makes it very clear that the output voltage needs a negative bias to allow it to go below zero for temperatures below 0C. Don’t try feeding that in to your imp!
It’s only for temps in the range 13-90 (my heater). So it would definently give my imp problems with too high voltages, since the LM35 maxes out at 6v output.
How do i determine what schottky diode to use?
By the way. Is the two GND on the board allready connected internally? If not, will it not cause problems for the readout?
I have tested an IN5817, my workhorse, and found that at 1milliamp forward current it has a voltage drop of just 0.11 volt. This will clamp the voltage at 0.11 volt above the 3v3 rail, which should be safe enough.
All of this is just for safety, in fact the LM35 should only give 0.9 volt output for 90 C so if there is no fault it will work fine. The Schottky is just for safety in case things go wrong, like putting in a fuse, to protect your imp in case things short out. You could just test the LM35 output with a voltmeter, and if it is sensible then forget the Schottky and just feed it straight to the imp.
[ The other common use of a Schottky, to clamp coil / relay connections to rail, is not optional - these will always give massive back emf spikes which need absorbing ]
If you are going to start buying components, why not just buy a TMP36, which can run down to 2.7 volt and covers -40 up to 125 C. And it costs next to nothing. I know you wanted to use your old components, but the new version matches the imp a bit better.
i have bought TMP36… and tested it yesterday. Hell those things dont work! At least not the one I tried. It might have gotten fried when i soldered, i’ll try with a new one tonight. But the main reason is the accuracy is much better on the LM35.
I actually also have an AD22103 laying around. This is one hell of an expensive trans, but it’s very accurate. and it works for 3v3
http://www.analog.com/static/imported-files/data_sheets/AD22103.pdf