Digital Thermometer Design

You would be much better using something like an LM335 rather than a BC108.
Mr Google will find data on it and it is pretty cheap and commonly available.
Del

I agree that the LM335 would be a better choice if accuracy is important.

If you must use a BC108, you will get a slight improvement by connecting base and collector together - leaving the collector floating can add to the noise which is going to be one source of inaccuracy.

Del,

What are you thinking to just blurt out the answer on the first post? There were two or three posts worth of analysis of the constant current, temperature compensation and precision scaling.

Now what - back to 100 level homework questions?

Dave

I think the poor little BC108 or the LM335 would cry a lot at the suggested temperatures of 150-200 Dec C. The LM335 is good for -40 to 100 Deg C, the LM135 will go as far as 150Deg C.

May I suggest a K type Thermocouple instead, much broader range(-200 to 1350Deg C) and they are readily available, or perhaps a J type Thermocouple (-70 to 700 Deg C). There are equivalence tables available on the web to help you with scaling.

Yeah, but you expect me to read down to the end of the whole post!
Del

Sorry - I guess I got caught up in the excitement.

I was speaking of the range. Can I use the LM335 in place of the BC108 in the above circuit? The LM335 seems to have voltage proportional to the kelvin scale (10mV/°K).

The short answer is no.

The longer answer is that neither BC108 nor the LM335 will be functional at the temperature your looking at of 150 - 200 degrees, the LM335 tops out at 100degrees C.

Thermocouples are the go.

I don't look forward to measuring upto 200 deg C. I meant the range be 150 to 200. For instance, -40 deg C to 110 deg C is a range of 150 deg C. I would like to use a sensor for the greatest range possible, but the "weather/climate" temperatures (say -10 deg C to 45 deg C) should be preferably, well within the range.

Digital??!

This is an analog circuit relying on voltage dividers to set opamp gains. ±1ºC may even be ambitious.

The suggested application in the source link makes me think this is a teaching toy at best.

I used the term "digtal" because it has LED seven segment displays - and not with the mercury level reading. I plan to have 4 seven segment displays, showing a three digit number and one decimal value.

Even if the output voltage is proportional to the temperature, I need to convert the analog value to 4 bcd values - one for the 3rd, one for 2nd, one for the 1st and last for the decimal value - then I could use something like IC 7446 or 7447 to display the values.

So if I have 76.9 ^οC - I should have 0000, 0111, 0110, 1001 going to 4 separate 7446s. My trouble is converting the analog voltage to bcd.

You'd need an a to d convertor then a bcd driver.

That makes the rest of the circuit pointless then as any voltage that is proportional to the temperature could be used to drive the a to d convertor directly without that buffering. Just need a reference voltage.

Fluke has a product that very closely mirrors what you are trying to achieve.

It too only claims a 1 degree accuracy with similar range and operating environment limitations as what you are proposing to build.

This off the shelf solution and others like it rely on a DVM to be the display section. Do you want to do that are is your heart set on building the whole thing?

Not all these products are avaliable here and I don't have money to spend.

So, I would prefer to make a analog voltage-to-LED-seven-segment-displays (4 such displays). Basically if the easiest logic is discussed, it would be helpful.

The TC7107A (ref page 10) is a one chip (and a few discrete components) solution that would directly drive your 7 segment display from an analogue input.

Couldn't get any easier.

You can buy "Weather Stations" on ebay for next to nothing (out of ROC) I bought a digital temperature/hygrometer gauge which I use in my Hermit Crab tank for $2 plus postage. It offers temperature accuracy of 0.1 degree i.e. it shows 26.8 Deg C.

If this is a personal/student project then that's cool, lots of other circuits to play with on the web. You might like to look at PIC16F84(88) tutorials and projects.

The PIC16F88 has an onboard A/D serial coms and stuff to help you drive an LCD display as well.

Yes in your Human rated temperature range an LM385 will be dandy.

How is your design coming along jay?