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

 

Your easiest way out is to use a DVM chip like the ICL7106 - http://www.intersil.com/products/deviceinfo.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

 

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.

 

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.

 

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.

 

Where in the world are you?

 

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).

 

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.

 

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.

 

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.

 

This is from the the documentation for the LM35:

How does the configuration of Figure 1 work? Don't you need two terminals for the power supply and another two for output?

Don't understand Figure 2 either.

I would greatly appreciate some help.

 

Ok, I think this is the type of configuration I'll need:

The two ends on the top right marked with +/- will connect to the corresponding input terminals on the digital panel meter, and the bottom wire will be grounded. What I don't understand is how the 5V power supply on top is supposed to be supplied with one wire. I'm assuming a battery is supposed to be used to provide the 5V - how should it be connected to the rest of the circuit? How does the twisted pair affect things?

 

What accuracy do you need for your thermometer, you could use a Dallas 1 wire thermometer.

 

What accuracy do you need for your thermometer, you could use a Dallas 1 wire thermometer.

Accuracy isn't very important (a working prototype is most important), but it's too late to change methods at this point. I need this working 1 week from now.

 

In the last diagram I posted, the the bottom wire end is grounded to the earth (an infinite sink for charge with an unchanging potential of 0 V), and I assume the top end is connected to a constant 5V source. Does this mean that I can instead connect those two ends to a 5V battery and have the same effect?

 

I am working on a temperature measuring project, so I am interested in what you are doing. However, I am not an engineer and so won't be much help, but since you aren't getting much anyway, I'll offer an uninformed opinion. It's probably wrong, but maybe it will help you think.

The native output of the LM35 is 10mV per degree C.
The serving temperature of coffee is from 70 to 80 degrees C - say 75.
Therefore the output from an LM35 measuring 75 degrees C will be 750mV.
The meter you have has a maximum input of 199.9mV.
Therefore, you cannot simply connect the output of the LM35 to the meter and get an intelligible temperature reading of coffee at 75 degrees C; the meter will simply show its max display of 1999.
In order to get an intelligible reading on the display, you will need to install an optional voltage divider in the meter to increase its range to 20V. When you do that, I guess the 750mV input will read out as .750. Maybe that will satisfy your professor?

 

In the last diagram I posted, the the bottom wire end is grounded to the earth (an infinite sink for charge with an unchanging potential of 0 V), and I assume the top end is connected to a constant 5V source. Does this mean that I can instead connect those two ends to a 5V battery and have the same effect?

Did you buy the 5v version of the meter? or the 9v version? (Your link in an earlier post goes to the 9v version.) Either way, I think all you need to do is connect the LM35 ground to the negative on the PS, the LM35 +Vs to the positive, do the same for the meter, and then connect the output of the LM35 to the input of the meter. I don't think you need any other components or wiring. Since you are only going from the LM35 (glued to your coffee cup?) to the display, there should be no problem using a three wire connection; a stereo audio cable with ground should work.

Don't forget that I probably don't have a clue about this; I'm just speculating.

 

Did you buy the 5v version of the meter? or the 9v version?

The meter uses a 9V battery, but the circuit between the meter and the 9V battery is separate from the circuit between the meter and the LM35.

Either way, I think all you need to do is connect the LM35 ground to the negative on the PS, the LM35 +Vs to the positive, do the same for the meter,

Are you saying I should connect the meter AND LM35 to the 9V battery? I didn't know you could do that.

and then connect the output of the LM35 to the input of the meter.

The meter has two pins for input (+/-) and two pins for the 9V (+/-). Would I just connect the output of the LM35 to one of the pins then?

 

I have attached a schematic of how I would do it, but before you connect it this way, check with someone more knowledgeble than I am. I wouldn't want you to ruin your project. Remember, I'm no engineer.

 

I have attached a schematic of how I would do it, but before you connect it this way, check with someone more knowledgeble than I am. I wouldn't want you to ruin your project. Remember, I'm no engineer.

And if you decide to try this, at room temperature, the display will max out until you put in the voltage divider that I spoke of earlier. Of course, to see if the circuit even works, you could put the whole thing in the refrigerator. Anywhere it's less than 19 degrees C should cause the display to show 10 times the temperature. For example, if you can chill the unit to say 17 degrees C (63 degrees F), then the display would read 170 (mV). Of course, remember that all this is just uninformed speculation on my part; I haven't tried any of it.