cheap and simple way to measure temperature

[Heckler]

I am designing a simple PIC electronic POV (persistance of vision) project for the Boy Scout "Electronics" merit badge.

I would also like to include the ability to measure and display temperature. The "display" part is no problem as it will use a POV matrix.

It is the "Sensor" part I need help with. I am considering the DS1820 One Wire sensor, but would love to hear from this group's vast bank of experience. Low cost then simplicity is importaint.

Thanks in advance.

Dwight

[mackrackit]

I like the LM34. It comes in C or F.
Here is an example
http://rentron.com/PicBasic/LM34.htm

[malc-c]

My current project used 4 x DS18B20's

http://www.picbasic.co.uk/forum/showthread.php?t=12712

All you need in addition to the DS18B20 itself is a 4.7K resistor and obviously some means of displaying the temperature it reads - either an LCD display or serial to PC and use Hyperterminal. If you chose the LCD screen, then estimated cost would be less than £25 for the sensor, resistor,LCD, case and (depending on the PIC) the PIC too.

[Acetronics2]

I am designing a simple PIC electronic POV (persistance of vision) project for the Boy Scout "Electronics" merit badge.

I would also like to include the ability to measure and display temperature. The "display" part is no problem as it will use a POV matrix.

It is the "Sensor" part I need help with. I am considering the DS1820 One Wire sensor, but would love to hear from this group's vast bank of experience. Low cost then simplicity is importaint.

Thanks in advance.

Dwight

Hi Dwight

Could you tell us the temp range and precision + resolution you look for ???
Also if you have an ADC aboard your PIC.

Sensor selection will be greatly simplified ...

BTW ... using a OneWire device with some POV Display could be really interesting ... but, it's just me ... ;)

Alain

[Heckler]

I am just looking for ~2 degrees F accuracy. I just want to measure ambient temp. in the range of say 0 to 100 F.

Here is a link to the base kit that I am trying to modify. http://www.chinchillasoft.com/Projects.aspx

This kit is not my design. But I wanted to add the temp sensor to make it more interesting and useful for the kids.

Is it true that the DS1820 does not also require the PIC to have an external precision voltage reference?

It appears that the LM34 does require the external voltage reference. Is that true?

This is a pretty neat kit that shows off the CNC router software on his site also. He uses it to route the PCB instead of etching the board. The kit is only $10 US and that is in the price range that kids can afford for the merit badge. If I could add the temp sensor for say $2 more then I think that would be a neat addition.

Thanks all
Dwight

[mackrackit]

VDD can be used as a reference for the LM34.

[Heckler]

hey Mack,

But if VDD is a 3 volt lithium coin cell whose voltage decreases over time, will that make the temperature also become less accurate over time?

Although, even if that is the case, that may be an acceptable amount of drift for this type of project.

Thanks I'll have to break out the breadboard (DANG!... Now the fun starts:))

Still open to suggestions, though.
Price is really the main factor for my needs.

Dwight

[mackrackit]

I wonder how long a project like yours willrun with a coin battery?
Will the coin also be running the motor?

[Heckler]

I wonder how long a project like yours willrun with a coin battery?
Will the coin also be running the motor?

The Coin Cell only runs the PIC and the 7 LED's.

The project is designed to simply be waived back and forth in the air to view the POV message. Or better yet most people will attach it to a paint stir-stick (a thin 1" wide by 12" long woden stick) with black tape or a rubber band, then with a 16d nail poked through the stir stick (or some orther sort of handle attached to one end of the stick) twril or spin the thing around to view the message. It actually works quite well. The kids can use their imagination and come up with other ways to spin the thing around to properly display the POV image (I lump myself in that KID category any time I am working with electronics, at least I always seem to have fun melting solder.)

Dwight

[ardhuru]

When I made my POV propeller clock I was also tempted to include temperature display, besides the time and the calendar. What gave me second thoughts was the fact that the PCB would be spinning all the time, and therefore the temperature being displayed was unlikely to be exactly the ambient.

Are you sure your POV stick, while it is being waved, would also not influence the sensor's readings?

Just a thought...

Anand

[mackrackit]

Hand motored... That sounds interesting. They could even put them on the spokes of their bikes.

The LM34 may not be the best option then.

[ScaleRobotics]

Certainly not the cheapest, nor the most simple, but you can pick one up from Future Electronics for just under $10.00. The cool part about this one, is that you could have your POV clock sense a person in front of it, and have it do different things after sensing body heat, like saving your battery life.

http://melexis.com/Sensor_ICs_Infrared_and_Optical/Infrared_Thermometers/Digital_plug__play_infrared_thermometer_in_an_ultr a_small_TO-can_685.aspx

[El_AMPo]

When I made my POV propeller clock I was also tempted to include temperature display, besides the time and the calendar. What gave me second thoughts was the fact that the PCB would be spinning all the time, and therefore the temperature being displayed was unlikely to be exactly the ambient.

Are you sure your POV stick, while it is being waved, would also not influence the sensor's readings?

Just a thought...

Anand


Check how old fashion wet bulb hygrometers work,
http://www.agri-shop.co.uk/whirling-hygrometer-37-p.asp
Spinning all around but still the best readings, so no problem with the winding, probably a bad idea because of line noise if you weren't using batteries, but is not the case.

If you want a really CHEAP thermometer with decent accuracy use a diode as a sensor.
Here is the schematic:
4806

Still working for me at a 0.5ºC resolution, but needs a two point slope calibration, some chip math and a pic with ADC.

Maybe the 10bit ADC will give you a resolution of 2-3ºC, maybe less with oversampling with just the 4k7 resistor and the diode directly to the adc.
Anyone for the test of oversampling?
http://www.darreltaylor.com/DT_Analog/

[fixitjs]

Be careful not to confuse ambient temperature with wind chill. objects will not cool to a lower temperature then the ambient temp, but will cool at a rate of the wind chill. thus, swinging the isplay stick will give you an accurate temp reading as any temperature increase caused by holding it will be released by the air flow accrossed the stick but not lower then the ambient temperature. hope this is helpful.

[mtripoli]

Have a look at the LM94022. Price is $1.27 from Digikey you can get the data sheet there as well (I had a link but Digikey uses scripts and the link comes out bad). It has a couple of options for gain set through pins. The biggest downside to this part is the package; not user friendly for hand soldering.

IIRC the LM34 is a little weird that it requires a negative bias to give voltages below 0 or something, and it only operates down to 5V.

Good luck.