LM35 LED Temperature Bar

[Englewood]

Hello,

I have a couple of projecs on the go.

I have a LM35 and i want 3 LEDS to run at different temperatures IE.

Blue LED AT 10C
Green LED AT 30C
RED LED T 60C

The output from the LM35 is 10mv 10C.

Im struggling to make this circuit, i hve found one using a Arduino but i dont want to use one

 

[esp1]

hi,
Look up LM339, its a quad comparator IC.
You can set the threshold levels for 4 different temperature values.
E

 

[Englewood]

Can i do it without a LM339?

I want the circuit to be as small as possible

 

[esp1]

I want the circuit to be as small as possible

This image shows the dimensions for a SMT LM339, I do not think you can get much smaller.

How small are you requiring.?

 

[Englewood]

Good Point.

Do i use this with the LM35?

 

[esp1]

Yes, the LM35 would be connected to three of the LM339 inputs

You said:
Blue LED AT 10C == 100mV
Green LED AT 30C == 300mV
RED LED T 60C == 600mV

These would be the SET point voltages to the other input pins of the LM339.

Note: use the outputs of the LM339 to sink the open circuit current of approx 10mA , via a current limiting resistor and LED, for the ON state of the LED.
Arrange for the Blue LED to be ON, when tamb <=10Cdeg
Green LED to be ON when tamb => 30Cdeg
and the Red LED to be ON when the tamb => 60Cdeg

Do you follow that OK.?

 

[d123]

Hi there!

Another simple device is the LM3914/LM3915 (same device all things considered). It's called a Dot Bar Display Driver (used for LED bar graphs). There are loads, have a look for one that suits your goals if the idea appeals to you.

I used this with the LM35 to make a bar graph LED thermometer, it's relatively simple to set the "temperature range" using a couple of resistors.

You could scale the voltage reference/temperature range of the 3914 (or 3915) to use, for example, the first or second, middle and last outputs for your three LEDs, and I would imagine could leave the other outputs floating; you can even get away with using no current limiting resistors on the LEDS, but not recommendable.

Using this device you can really simply set all three to be on at 60ºC or one by one, bar graph or dot mode. It is very easy and low parts to use, but like all these things the devil is in the detail of calibrating it right, which can take a bit of fiddling, a trimpot or two (or a potentiometer/variable resistor) would make this easier than using 2 fixed resistors to set the scale/temperature range.

Being adventurous, if you want grossly minimal (I don't recommend it!), and if you have a liking for maths and experimentation you could calculate resistor values based on the voltage drop of each LED you have ( they're not all the same in this respect), the LM35 output, and the power supply voltage, and the current you want each LED to draw; if 5mm green or red LEDS from a dim 5mA to a worryingly bright 20mA; blue LEDs are higher current, some are maybe 30mA, and in practice I found some blue or white LEDs need a larger value resistor to avoid being dazzling - but in the end it has a lot to do with the current limiting resistor you opt for each LED.
The disadvantage, if it is one, to this approach is that at 60ºC all three would be on; and another is that it might be hit-and-miss re temperature signalling at those specific temperatures.

 

[KlausST]

Hi,

LM73. Sot23- 6pins, IIC controlled, comparator inside, thresholds programmable.

Small, all inside.

Klaus

 

[D.A.(Tony)Stewart]

Hello,

I have a couple of projecs on the go.

I have a LM35 and i want 3 LEDS to run at different temperatures IE.

Blue LED AT 10C
Green LED AT 30C
RED LED T 60C

The output from the LM35 is 10mv 10C.

Im struggling to make this circuit, i hve found one using a Arduino but i dont want to use one

Your spec is incomplete and needs to include overlap , variable intensity and /or <= and >= with thresholds or pulse rate interpolation.

otherwise it is ambiguous, ie. all 3 LEDs on at 60C or not

 

[Englewood]

Im thinking of using this circuit.

I have simulated the circuit in Proteus but dont seem to work get the CPU error :-(.

 

[KlausST]

Looks ok.

Klaus

 

[Englewood]

I have built this circuit on breadboard but the LM324 gets hot very very quicky

 

[KlausST]

Hi,

Don't be afraid of capacitors. Use them. ;-)

On every IC VCC and on every signal input of comparator.
I also recommend to use high ohmic positive feedback to generate a small hysteresis.

I suppose the LM73 heating comes from oscillations. Use a scope to check on this.

Klaus

 

[betwixt]

It should run cold, either it is wired wrongly or oscillating. If the problem is oscillation try adding capactitors as Klaus suggests. Wire 100nF capacitors with short wires directly across the supply pins of the sensor, the LM324 and the CD4532. I would also add anopther capacitor of say 100uF across the power source or you risk oscillation of the power line as the LEDs draw current. If the supply drops as the different LEDs turn on, it will alter the reference voltages from the potentiometers and may change the LEDs again cyclically.

Brian.

 

[Duresh]

try resistors between LM324 outputs and 4532 inputs.

 

[Englewood]

Okay, ill give it a try.

The LM324 just gets hot instantly. Ill see what the caps will do