# hey i need some help with connecting a thermocouple to an adc

#### nnaselli

Joined Feb 12, 2011
5
hey i am trying to get a thermocouple to connect to ac adc to show temp on 7 segment displays and turn a heater on depending on the temperature. but i have no idea how to get the thermocouple to connect to the adc. anyone have any wiring diagrams to help me out.

#### wayneh

Joined Sep 9, 2010
16,384
hey i am trying to get a thermocouple to connect to ac adc to show temp on 7 segment displays and turn a heater on depending on the temperature. but i have no idea how to get the thermocouple to connect to the adc. anyone have any wiring diagrams to help me out.
You may find it easier to use a digital thermometer IC such as LM35, which outputs a voltage directly proportional to temperature. A thermocouple changes resistance with temperature, and you need circuitry to convert that to voltage, and making it linear can be a challenge. For simple thermostat applications, it's probably fine. But I really like the LM35!

#### nnaselli

Joined Feb 12, 2011
5
thanks for the advice.can u tell me how to use the lm35 to an adc

#### bertus

Joined Apr 5, 2008
20,454
Hello,

What is the temperature range you want to measure?
The LM35 will work from - 55 to + 150 °C.
For the negative temperatures it needs a negative powersupply.

Bertus

#### nnaselli

Joined Feb 12, 2011
5
i need to measure from 0°F to +80°F

#### wayneh

Joined Sep 9, 2010
16,384
i need to measure from 0°F to +80°F
Well, you could use the LM34 (Faranheit), although it's harder to find than the LM35 (Celsius). The LM34 reads 0V at 0°F and 0.8V at 80°F. It might not be accurate right down at 0° if you power it only with a single supply. Read the datasheet - it'll answer all.

#### nnaselli

Joined Feb 12, 2011
5
i could just convert F to C and use the lm35. because i just need to know when it dips below 70°F which is about 21°C. that is no problem just that i need help trying to get it to an adc to an 8051 microcontroller

#### wayneh

Joined Sep 9, 2010
16,384
Well, if your equipment can use the voltages directly from the LM35, eg 15-25°C = 0.15-0.25V, then you're all set. You just need to power the LM35. Also, I've had better luck when I load its output with a 220Ω resistor, at the meter end. Others here have not needed that, so you may want to just experiment. I also recommend using shielded cable if you need the sensor to be very far from the meter, more than a few feet.