# Temperature Sensor

Discussion in 'Sensors and Actuators' started by OZwurld, May 2, 2013.

1. ### OZwurld

1
0
May 2, 2013
So i am trying to build a temperature sensor that turns on an LED(blue) at temperatures greater than 20 degrees..and if the temperature is less than 19 degrees the circuit should preferably turn on a green LED.

1. i managed to build the circuit for the blue led but have ni idea where or how to put in the green LED
---------------------------------------------------------------------------------------------------------------------------------------Note that the blue LED represents (warm enough temperature) and the green LED should represent a heater that will heat up the temperature to greater than 20 degrees
------------------------------------------------here is my current cct for the blue led----------------------------

http://i1307.photobucket.com/albums/s582/ozwurld/pic1_zps7169b77b.png?t=1367509736

Thank "U" . . .

2. ### duke37

5,364
772
Jan 9, 2011
You will need to reconnect the op-amp as a comparator.

Remove the resistors on the amp
Place a potentiometer from ground to 6V with the slider connected to 741-.. This will provide a temperature set point.
Put a diode (1N4148) and resistor in series with each led and connect one between the amp output and ground. Put the other between amp output and ground, with opposite polarity.

You may wish to provide some positive feedback to give a dead band so that the output does not dither. This will probably be necessary if you are using the output to drive a heater.

3. ### duke37

5,364
772
Jan 9, 2011
I recherged my brain cell overnight.

A simpler way of connecting the leads is to place them in antiparallel and put a resistor in series, then connect between 741 output and 0V.

Each led will protect the other from excess voltage.
You will be stuck with a similar current in each led which may not give a similar amount of illumination.