I need to make a homemade toster oven for a college project. That uses a microprocessor. I discovered the max3155 chip that communicates directly from the thermocouple to the microprocessor but I need help with the circuit. I was thinking of using a relay for the on off controll. My element runs off 120 what can I use to lower the voltage so that my microprocessor gets only it's 5 volts but element still having 120 without using a transformer

 

What microcontroller family are you familiar with, and what language do you code in? Why do you want to use a thermocouple?

 

It would be better to use a solid-state relay since they are silent, have a very long life, and are easy to interface to a computer.

You need isolation from the power line to your microprocessor power for safety. A small line-powered switching regulator can do that, such as a wall-wort supply.

 

I just started to learn microprocessors it's been a couple of weeks in school.. So far we been using microcode programme with the pic16f877 and 16f873. I forgot to mention I'm using the thermocouple cause I need to display the temperature on a LCD so that I can change the setpoint up and down with push buttons

 

Microcode Studio is what is called an IDE (integrated development environment.) It is often used with the PicBasic Pro compiler. Does that sound familiar?

At what temperature do you plan to switch the heating element on and off? Are you going to work in Celsius or Fahrenheit?

Unless you are required to use a thermocouple, there are probably better choices. You could use a thermistor in a voltage divider arrangement, or you could use a dedicated temperature sensor, such as an LM34 or LM35.

Are going to interface with your LCD using parallel lines or a serial adapter?

 

what can I use to lower the voltage so that my microprocessor gets only it's 5 volts but element still having 120 without using a transformer

You can't.
At least not on this site. Powering off the 120 volt supply with no transformer is not allowed here because it is not safe.

 

If you don't have, or don't want to build a 5V power supply, you can simply use 3 single cell alkaline batteries in series. The batteries can be whatever size you want (AAA, AA, C, D.) I would use D cells to get the longest life.

 

I didn't think of that.

 

... you could use a dedicated temperature sensor, such as an LM34 or LM35.

A Toster oven will have to get the bread warmer than 200C (approximate) to start toasting. The LM34 and LM35 will be toasted at those temperatures.

 

A Toster oven will have to get the bread warmer than 200C (approximate) to start toasting. The LM34 and LM35 will be toasted at those temperatures.

Researchers have determined that the perfect piece of toast is produced when the bread reaches a temperature of 154°C, http://www.dailymail.co.uk/sciencet...st-2-000-slices-216-seconds-optimum-time.html which is only a little above the max rated temp of the LM35 and LM34. I have never used either one at that temperature, but it would be worth a try.

However, I see your point.

 

Yes we are using the pic pro compiler. And I have to have the heating from 0-350 degree Celsius or equivalent in Fahrenheit . And the thermocouple has to be used cause it is in the specifics of my project

 

If needing to accurately control temperature this way you will need to include some kind of PID loop, if using a SSR this is either on or off, so the loop would need to detect when the set point approaches and start to cycle on/off in order to obtain stable temperature.
If using phase angle (TRIAC) control, you MAY get away without the PID feature but the switching will be electrically 'noisy'
Max.

 

I don't have any experience with thermocouples, so I can't be much help there. Maybe I can help with the code when you get there.

What about the LCD? What size is it, and are you interfacing the PIC using multiple pins (parallel) or only one pin plus power and ground (serial)?