Ambient temperature sensing

"Jason S" bravely wrote to "All" (01 Dec 05 21:12:24) --- on the heady topic of "Ambient temperature sensing"

JS> From: "Jason S" JS> Xref: core-easynews JS> sci.electronics.repair:350096

JS> Hi,

JS> I'm looking for a circuit that will represent temperature ranges up to JS> 60 degrees (celcius) using at least 3 LED's or transistors as the JS> outputs for each range, but I DO NOT want a digital display like a JS> 7-segment or LCD though. JS> I want to be able to utilize these outputs to control others parts of JS> my circuitry (perhaps like voltage calibration).

JS> Example: JS> LED 1 (green) = -10-20 degrees, JS> LED 2 (yellow) = 20-30 degrees, JS> LED 3 (red) = 40+ degrees,

JS> I am open to all suggestions. JS> Thanks in advance.

J,

You might try crossposting to sci.electronics.design.

Your circuit needs a voltage reference and a window comparator. The voltage reference is used with a temperature probe and the window comparator will divide your output into 3 ranges. With some cmos gating you can readily light 3 separate leds.

A*s*i*m*o*v

... I worked hard to attach the electrodes to it.

Jason, You could try the LM34 and LM35 series of temperature sensors from National Semi. There output is linear with temp. 10mv/degrees F for the LM34. The LM35 is 10mv/degrees C. The output can then drive a few comparators with the appropriate trip points.

-- Tony Marsillo Nutmeg Repair

How about an analog bargraph IC? Otherwise a quad comparator chip could be setup to do this, IIRC the LM329 is such a chip.

Hi thanks for the reply. The quad comparator wouldn't really work, because only 1 'output' LED/Transistor needs to be lit at any given time, not more than 1. The analog bargraph would probably work, but would need to be able to set it up as 'Dot' mode (probably easy to do), then I could use logic gates to provide filtering from each of the outputs to produce my final 3 or 4 output ranges. However, any ideas on how to hook up a temperature sensor to it? I found schematics based on voltage measurement, not I know nothing about temp sensors and how I would hook it up with this type of circuit. Can you point me in the right direction?

Thanks, J

Use a darlington transistor, for silicon the leak current doubles every 6 degrees celcius,and the leak current of the first transistor multiplied twice by your current gain should be around the Milliamp range.Up to about 100 degrees,works nicely. Add 3 comparators to check for voltage level across a suitable resistor,you could use a quad IC 741 type for that.

A quad comparator has four outputs, there's four comparators, seems like it would do exactly what you need.

On Thu, 1 Dec 2005 20:45:56 -0500, "Tony Marsillo" put finger to keyboard and composed:

I'd suggest an LM35 plus an LM3914 bar graph driver.

-- Franc Zabkar

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