Temperature detector cct

[Slaw]

HELP!!! I need a three level sensor circit using LM35 and op-amp 741 that has an output of 39 degrees celsius when hot, 37 degrees when okay and 35 degrees when cold, whilst connected to leds and a buzzer....HELP PLSSSS

 

[skimask87]

...and "PLSSS" makes it all that much more enticing...

https://www.ti.com/lit/ds/symlink/lm35.pdf

EDIT: Sorry, 4 S's. "PLSSSS"

 

[cowboybob]

Welcome, Slaw!

Take the output of the LM35 and feed that to the input of a couple of 741s configured as a comparator.

Configure the comparator with a lower range equal to the LM35's output at 34 degrees, an upper limit of 39 degrees and the hysterisis equal to voltage difference between the two (which can be very tight or very loose).

Take the outputs to the two 641s, run them, individually, each through a its own diode, tie the diode outputs together and you'll have an "on/off" signal ("on" at 35 degrees and "off" at 39 degrees) to trigger whatever you want to trigger.

 

[Slaw]

Thank you Cowboybob, wld try that out and skimask87...lol...I didnt realise d amount of S' s

Another problem though is how do I make it waterproof cos its for a baby bath thermometer.

Thank you....guySSSS

 

[skimask87]

Don't they make those things by the millions and sell them at Walmart?

(the answer is yes...)

 

[Slaw]

Yes they sell baby thermometers(dats not all d project is about) but that isn't really the answer to how I can make it waterproof...b nice skimask87

 

[skimask87]

PLSSSS...dat...d...b...
wow LOL OMG FTIO

 

[cowboybob]

Thank you Cowboybob, wld try that out and skimask87...lol...I didnt realise d amount of S' s

Another problem though is how do I make it waterproof cos its for a baby bath thermometer.

Thank you....guySSSS

I'd suggest encapsulating the LM35 (and, say, 3-4" of it's three leads) in some shrink tubing that you've stuffed with something like "Shoe Goo" (or it's equivalant) before the shrinking step. As you might suspect, it's a messy project.

This will reduce the response time of the LM35 somewhat (thereby increasing the hysteresis) which can be compensated for by decreasing the hysteresis zone of the 741s comparator circuit (the "ON" portion).

IMPORTANT! Since you're using this for bath water, electrical isolation from the water is, of course, essential.

<EDIT> I should point out that the comparator I've been referring to is called a "window" comparator.