Greetings.
Im a biomedical engineering student, been following the forums for quite some time now (quite helpful), but this is my first post since i'm having some trouble figuring out this project i'm supposed to make.

Project description : (I'm implementing it on MultiSim)
Implement a circuit that allows for body temperature measuring with visual LED display.
The circuit should have a potentiometer (denominated PT-100) that will simulate the temperatures, and 3 different coloured LEDs that turn on during different temperature ranges (eg: <37ºc= green, 37ºc<yellow<40ºc,red>40ºc)

Only one LED may be turned on at any given time.
Im also allowed to use resistors, op amps, and transistors.

Now my real question is, where to start. Im considering using transistors , but i'm having a hard time figuring out the "one LED at any given time" bit.

Any insight would be greatly appreciated!
Thank you for your time.


PS: English isn't my first language, so i apologize first hand for any mistakes/bad grammar.

 

By far u are the only one asking help in the proper way, cause all the others asks funny stuff and never comes back.

Ur project is quite easy for most of us but in order for u to learn u have to draw Multisim schematics and simulate it.

We can show you how to do it with opamps and leds.

But first you need to install multisim.

Do you have it installed and is it a working one.

Second, what do you know about OP Amp comparator circuits. Did you study them ?
For this u need 3 OPAMPS. Any particular no. do you have in mind or will LM393 be allowed by ur professor.?

 

Although OPAMPS and comparators use the same symbol and I personnally never experienced a problem when using an OPAMP as a comparator, these two are not to be confused. You can read HERE and HERE to know why.

As R!f@@ said using 3 comparators this can easily be solved.
Have a look here. The site gives some example circuits.

Note also that in order to make sure that only one LED is on you can use the outputs of the comparators to block or enable the other comparators...

 

Although OPAMPS and comparators use the same symbol and I personnally never experienced a problem when using an OPAMP as a comparator, these two are not to be confused. You can read HERE and HERE to know why.

As R!f@@ said using 3 comparators this can easily be solved.
Have a look here. The site gives some example circuits.

Note also that in order to make sure that only one LED is on you can use the outputs of the comparators to block or enable the other comparators...

This could be done by making the output from some of the comparators influence the inputs of others, but this could lead to degraded accuracy.

You might think of how the middle lamp could be made to light only when the middle comparator was indicating, but the top comparator was not.

 

Thanks for the quick replies!

R!f@@
I do have multisim installed, and i've worked with it before, in class.I've also done some OP amp circuits as well.Maybe the problem is that i was too focused on getting it to work with transistors that i didn't even consider OP amp comparators.

I will have a look at the links you provided, and give some feedback later.

Thanks again!

PS: The OP AMP i used during class is the 741-C and LM471 models.I would assume these are the only OP AMP im allowed to use.

 

Ive been doing some research on the links you provided,it's very helpful indeed.
I've also learned that the project must include op amps and transistors.

I'm still "green" when using transistors,only used them once in class,and it was a 1 LED circuit, pretty straightforward.

As it stands, my plan would be to use the potentiometer % scale to simulate the change of temperatures, changing its resistor value in the process,and through the comparators turn on the appropriate LED.

Thing is,where does a transistor find its way in the circuit?
I know their properties and uses,just not quite sure where it fits.

If i could abuse your patience ,a simple schematic would be wonderful!

Thank you

 

The transistor can be used to drive the LED.

U said u have used it that way, right ?
so why don't you draw one and post it.
We can correct it.

First draw one circuit.

A pot varying voltage to a comparator. Comparator triggers at a given voltage. It's output drives a transistor where a LED is the transistor collector load.

 

R!f@@ , after experimenting for some time, i think ive got it.
Its quite simple,as you said,and it is indeed working.Not sure if it's the best way to do it though,since i used 4 OPAMPs instead of 3.
Since the 2nd LED will have a range between 37and40, i had to use 2 OPAMPS to cover that gap.

I would pretty much love to show you the multisim design, but quite honestly i have no clue whatsoever on how to do this.Browsing forums is my specialty, not posting in particular x)
Should i take a printscreen and post it here?

 

When printing screen......U should zoom out all the way to show everything nice and tidy.

Then print screen. Paste it in MS paint. Then save as .png format. Resolution should be 800 by 600.

Next attach it to ur post.
You can see the "Manage Attachments" button when u click "Go Advanced" button.

 

Ok,here we go.Dont mind the numbers,still messing around with those.
And thanks for the tip!

 

Blimey

That's what we are talking about. Put some leg into it and we will push you till you fall .

It's a bit unclear to me...

{ed}
Wait..it's fine....but there is a itsy bitsy problem with the diagram

 

Time for the boring part and figuring out the math and the numbers.Ill let you know how it turned out. Thanks again for your time!

 

Ur circuit is not the way it should be..

U cannot use voltage sources on each comparator.
U cannot connect LED's that way.

Try this.
Instead of Voltage sources use resistors as voltage dividers.
Use ohms law to calculate the VI drop.
And use common Power to Power all the OPAMPS.

Next try putting the LED in series with the transistor collector. Not across CE.

 

I only had a quick look but I think you are powering your op amps off +12V and +12V. This could leave you confused for a long time trying to work out what's going on.

Also, although not really required, I'd drop the pot to a 10k and attempt to simulate a real world thermistor, may get you extra marks if required for building in the ability to use a real temperature sensor. I can provide you with a table of resistances vs. temperatures for a 3k thermistor if you wish.

 

I've replaced the 12V sources by regular power sources.Makes everything much less confusing.Also,i had my leds wired to the 12V sources, and i dont really know why i did that in the first place,must be getting sleepy.
LEDs are now working properly at the voltage associated with the temperatures!

Evilclem,thanks for the tip but i don't think im that comfortable enough with my knowledge of this yet to adventure any further.

Again,thanks everyone for your time.If i have any further questions i'll be sure to let you know!