Digital Thermometer Circuit with LCD Output

[AKspartan]

Hello,

I'm looking to create a digital thermometer using an LM35 and an LCD screen. I'm a sophomore chemical engineering student and this circuit will be used as part of an open-ended project for one of my introductory engineering classes. The project goal is to "create or improve upon an existing technology to improve your daily life." This thermometer will be used to create a coffee cup which can measure and display the temperature of its liquid contents. This will not only allow the user to not only avoid scalding their mouth with piping hot coffee, but also allows them to enjoy their coffee at their most preferred temperature.

My knowledge of circuits is limited - my physics course just finished going through Ohm's Law, Faraday's Law, Lenz' Law, etc. We're currently working on different types of circuits (RC, RL, RCL, etc.) and current sources (DC/AC). Anything beyond this is foreign to me. I come here not asking to have the forum experts simply hand me what I'm looking for, but to use the helpful posters here to guide me in the right direction and to learn how this thing works.

The number one priority on this project is the minimization of cost (the constraint is that the final product must cost no more than $20 in materials). With that in mind, here are few questions to get started:

Is the LM35 appropriate for this sort of application (being frequently exposed to these liquids)?

What sort of analog-to-digital converter will I need?

Can anyone recommend any distributors who sell simple/cheap LCD screens?

How does one translate the analog input into an input the LCD can display as a temperature? I understand that it has to be converted to digital, but beyond that what needs to be done?

I appreciate any and all comments and help.

Thanks,

AK

[beenthere]

Your easiest way out is to use a DVM chip like the ICL7106 - http://www.intersil.com/products/dev...asp?pn=ICL7106

The only thing remaining is an amplifier to change the output of the LM35 so it scales into engineering units. Use something modern like a TL071 and be guided by our Ebook - http://www.allaboutcircuits.com/vol_3/chpt_8/5.html

[rjenkins]

Search for 'digital panel meters' - these are complete modules with a DVM chip and LCD or LED display.

With that and the LM35, you would need minimal components to do the complete project.

[AKspartan]

Is this how the circuit will look, generally (minus resistors)?



Can anyone recommend a suitable amplifier for this?

I found this digital panel meter, will this work alright?

Also, how does the LCD/DVM work? I understand that 0 V will produce a 0 reading - what reading constitutes a full movement? Does it simply display whatever the voltage drop is between its two terminals? So my goal here is to have a voltage differential of 0 V when the temperature of the liquid is 0 deg C (which is a given), and a voltage of 100 V when the temp of the liquid is 100 deg C?

I think I have a decent grasp of how this works, but I don't really understand how the amplifier works so I don't understand how it affects the circuit.

[AKspartan]

I have no problem figuring this out through trial-and-error but I'd greatly appreciate some guidance so I don't waste time/money on the wrong parts.

[eng1ne]

Where in the world are you?

[rjenkins]

That meter should be fine.

It comes set for nominally 200mV full scale (1999) so 0 - 100mV would display as 0 to 1000

There are usually jumpers to set where the decimal point is shown and to add a selection of symbols to indicate what is being displayed (eg. V, A, C etc.).

Set the decimal point to give a single digit and you 100mV signal will display as 100.0

You can re-scale the display using resistors on the DVM module, so you could make it read whatever you want, like showing temperature in Farenheit rather than centigrade (but you would lose the decimal place to show over 199.9).

[Bychon]

The LM35 outputs one millivolt per degree. The panel meter registers millivolts with no amplifier and no modifications. Read the data sheet for the LM35 and the panel meter and follow the instructiuons about where to connect the wires.

[AKspartan]

Originally Posted by Bychon

The LM35 outputs one millivolt per degree. The panel meter registers millivolts with no amplifier and no modifications. Read the data sheet for the LM35 and the panel meter and follow the instructiuons about where to connect the wires.

I knew I was overthinking this... thanks.

[AKspartan]

OK, finally got all the parts I need. How do I hook up the LM35 to the digital panel meter? The digital panel meter is straightforward - there are +/- terminals for the battery and for the input and output. The LM35 on the other hand has 3 terminals... documentation labels these at +Vs, GND, and Vout.