+ and - ==> +

[Calebay]

Hi

I need some help. I am about to build a simple Circuit that can measure temp. It will be connect it to a analogue meter. I like it to measure from -30 to +100C. the meter has to show zero at zero degrees
(my circuit will put out 0 volt a zero degrees and 10mV per degree depending on how I adjust the gain)

when temp gets under the freezing point I want the circuit to light a Diode and then make the meter go up as the temp raises. it has must be analogue. pref. using opams if possible. guess I just need to make something simple that will put both positive and negative DC out as positive DC.

anyone who know what I can do?

tnx

[Carondimonio]

Hi,

freezing point of what? Brine?

Cheers,

Bruno

[richie00boy]

I think it's obvious enough what the freezing point is. Can you post or link to your schematic? It's not clear if it will give out negative voltages.

[Calebay]

I have not completet the schematic yet but when it is finished it will put out negative voltage below the freezing point(zero degress) also 10mVolt per degress did that help any?

[Carondimonio]

It' s obvious what the freezing point is.
My point is: how do you know you have reached the freezing point of the liquid you' re measuring?
Example: pure water has a freezing point of +0.01 °C, any impurities present in the water will change (actually, lower) the freezing point.
Now, either you know in advance the EXACT composition of the liquid of which you' re measuring the temperature, or you have to rely on some other measurements (for example, conducibility) to recognise the state change.

Cheers,

Bruno

P.S.: Being professionally involved in winter road safety, I know a thing or two about freezing point...

[AudioFreak]

Carondimonio,

For the purpose of this exercise, it is clear that freezing point = 0C. Anything else is irrelevant. Now let that be the end of it and move on.

[moamps]

Originally posted by Calebay
[B].... guess I just need to make something simple that will put both positive and negative DC out as positive DC.

anyone who know what I can do?...

Use a precise full wave rectifier.

Regards,
Milan

[Calebay]

a precise full wave rectifier?

do they exist? will it put the same voltage out as it get in?

[moamps]

Originally posted by Calebay
[B]a precise full wave rectifier?
do they exist?

Of course.
Check the LM3914 datasheet or use Google (for example:http://www.ibiblio.org/obp/books/soc...put/opamp8.pdf).

Regards,
Milan

[richie00boy]

Yes and as you won't be feeding it a fast signal it should work pretty well. Google for "precision full wave rectifier".