Simple Hot Surface Warning Light

Community Projects
1

I’m looking to make/buy a simple hot surface warning light. Light turns on over xx degrees (Something safe to touch) and back off when it drops back down, but can withstand temps of 1000+ degrees if need be. I know to use a thermocouple with an amplifier, however I’m not sure how to actually turn the LED on and off. Any help would be great.

Thanks,

Josh

2

Should be pretty simple. Do you already have thermocouple wire?

Basically you want a comparator circuit with the comparator calibrated to trip at the appropriate temperature and the output of the comparator itself may be enough to light an LED. If you don’t need precision, this can be really simple.

What is the trip point and how accurate does it need to be?

3

I was going to get this thermocouple. https://www.sparkfun.com/products/251 It does not need to be precise. How would I calibrate the comparator?

4

I found this diagram( http://www.simplecircuitdiagram.com/201 … omparator/) , which makes sense, I would simply replace the “buzzer” with the LED. How would I calculate R1 and R2? I know for 2 “Super Bright” LEDs and 12V DC source I need a 100ohm resister but not sure how to calculate the 2 resisters in the diagram.

5

For that circuit, you would also need -12V…

6

I would use a 10k pot in place of R1 and R2 and measure, by some other means, the temp of the hot plate as it warmed up. Then just before/as the target temp is reached, adjust the pot to make the LEDs come on.

7

When I asked how accurate it had to be, I was really looking for a number…

Anyway, the drop dead simple way is with a [bimetallic strip. Maybe find an old thermostat at Goodwill or somewhere and remove the strip.](http://www.novatech-usa.com/Products/Matter/BMST05?gclid=CI7vxYiO6cECFUdffgodp6kArw)

8

Thanks for the replies.

lyndon:
When I asked how accurate it had to be, I was really looking for a number…

Anyway, the drop dead simple way is with a [bimetallic strip. Maybe find an old thermostat at Goodwill or somewhere and remove the strip.[/quote]

As for Accuracy, ± 10 degrees should be fine. (It just needs to light up when its “Hot” so people don’t burn themselves)

Mee_n_Mac:
I would use a 10k pot in place of R1 and R2 and measure, by some other means, the temp of the hot plate as it warmed up. Then just before/as the target temp is reached, adjust the pot to make the LEDs come on.

I like this Idea. For the minimal cost it would take I think I’ll give this a shot.

If not I was thinking about this http://www.amazon.com/DROK-Comparator-T … comparator and just swapping the sensor out for the thermocouple.

Thanks again.](http://www.novatech-usa.com/Products/Matter/BMST05?gclid=CI7vxYiO6cECFUdffgodp6kArw)

9

That board is basically the prior circuit. Swapping a thermocouple in place of the thermistor isn’t going to work. They’re 2 different animals. Thermocouples need specialized amplifiers. You might want to find out the max temp allowed for the thermistor. It won’t be 1000F but do you really need that ?

10

I don’t need it to read 1000+ degrees just not melt or die if it reaches that. The heating Cable http://www.mcmaster.com/#4550t213/=uhsv5m Its going to be reading can get up to 1400 degrees. the Thermocouple was a nice choice because I can have the board hidden away and just the probe near the heat source.

11

While the heating element says it’s rated to 1400 F, I don’t see it ever reaching that temp or anywhere in that neighborhood. What kind of environment is the heat tape going to be in ? A thermistor should be good to 120 C or about 250 F.

12

I agree it shouldn’t ever get that hot. Not sure if 250 would be high enough though. I’ve attached an image of the general idea. It is wrapped in insulation (made transparent to see detail) I could probably mount the thermistor on the arm “near” the heating element. Its not a harsh environment, this arm will be slowly moving up and down on a track. If I were to use the above thermistor, how would I switch on the LED. It claims it sends a “digital” signal out.

13

I’d have to stir a few dormant brain cells but given the dimensions and material (it’s insulating properties) of the heated “tube”, it should be easy to calculate the heater temp given 104 W and some assumed air temp.