Hi All.

After searching through the forums i can see this is a common problem - but i cant seem to work out what i need to buy to resolve my issue. I have a Velleman USB 8005 board which has an onboard ADC (8 bit) which takes 0-5v

Ive interfaced an LM35, but due to the low outputs i cant get the results i need ( im measuring temps circa 40-60 degrees celsuis ) - im only getting circa .4 to .8 volts for my temps so im unable to get any meaningful results from the ADC.

I see that i can use a 2 resitors across + and ground and take a reference of 2.5v which may increase the output scale by 2.

I also see that i can use an Op-Amp and scale the output from the LM35 by a factor of 5 which would give me the resolution required by both my application and the ADC.

My (rather long winded) question is - what op-amp and components would i need to buy to fabricate a 0-5v op-amp.

The LM35 and the ADC are all being fed from a 5v supply.

as you can tell, very new to all of this and still trying to get good electronics selection purchased to avoid the frequent trips to maplins.

thanks

David

 

hi,
What tempr range do you want to cover.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Hi,

im really interested from 0 - through to 85 degrees C max

regards

David

 

Hi,
From the LM35 you can see that its basic range is +2Cdeg to +150Cdeg.
At +2C the output is zero, changing by 10mV/C over its range.

The problem is, if the OPA are operating from a single supply, common OPA like the LM358 outputs will only go as low as about +0.5V

You can get rail to rail output OPA's, again they will not go to absolute zero output without a small negative bias on the output.

So for 85C max == 85 * 0.01 =+0.85V hence gain in OPA is going to be *5.88.

Assuming a low cost LM358,, with a +0.5V minimum output is equal to about +9Cdeg.

Is this lower limit going to acceptable.?

Do you follow.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Eric,

+9Cdef is fine, im more interested in the 30-85 degrees range, but will have another sensor for monitor outside airtemp and was hopging to use the same op-amp circuit on the other LM35.

For now though, +9 is absolutely fine as my typical readings with the multimeter have been no lower than 29CDeg.

regards

 

hi David,
Do you need a circuit design for the LM35 with a LM358 or equivalent.?

EDIT:
Attached a basic circuit, you could use a higher spec OPA.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

You star!

Could you clarify what the 5V1Z symbol is , and also the 10K0 with the line through it please? Im so new to all this that i dont want to get anything wrong as it takes days for the components to arive in the post!

If you dont mind, could you cast your eyes over http://uk.rs-online.com/web/search/s...archTerm=lm358

and let me know if the LM358 listed there are ok for my requirements?

once again, thank you!

 

You Star!

is it just a basic LM358 i need? Could you also quickly clarify what 10K0 is as it has the line going through it, and also 5V1Z ( a diode ? )

once again, thank you for your quick and thorough advice and help.

David

 

Consider that you powered the OPA from a 12V supply, under fault conditions the OPA could drive the input of the ADC upto about +12V. Although the inputs to the PIC are protected, IMO its not a good idea to connect a 12V low impedance source to the ADC input.
The series resistor and 5V1Z, clamp any excessive output to about 5V.

The 10K with a line thru it is a variable resistor, its purpose is to set the gain of the OPA.
In this way you set a +5V output for the 85Cdeg point, OK.?

Do you follow.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Got it - filling in an order now for the parts.

thanks again