Irony problem

It seems to me that everyone is focusing on the wrong part of Royston's request.

Making a variable heat iron is rather simple. The problem lies with making a readout of the temperature.

royston dsouza wrote:

Is there a way to find the temprature (without a microcontrollers) and how many watts equals to 800F(to use with lead-free components like RPi)

According to Ben Heck's explanation of thermistors and thermocouples in the Home-Brew Solder Reflow Oven 2.0 (6:25 in) suggested by Lupe Lopez. Whether you are measuring the resistance of the thermistor or the voltage of the thermocouple, in order for the readings to be converted into temperature data, the only practical solution is to process it through a microcontroller chip.

Unless someone knows how to modify a $10 car thermometer with 3 digit 7 segment display, to display higher temperatures than 125c. The highest thermometer circuit I can find is 150c. Those use a LM35 or similar. The only other way I've found is to use a LM3914 chip in this video https://www.youtube.com/watch?v=JQzWSN9K3_c or similar type circuit. (circuit diagram link is in videos description) He mentions modifying indicated temperatures with a potentiometer.

Maybe replace each LED with a complicated circuit that activates a sequence of the 7 segment LED display for each number. This will limit the number of preset temperatures you will be able to display though.

One possibility would be to use a heat resistor. Just as we use electrical resistors to bring high voltages down to useable levels you can use a heat conductor like a strip of copper connected between your heat source and a heat sink. The heat flowing from the heat source to the sink will produce a gradient along its length. Pick a spot along the heat conductor that is within the temperature range of the sensor and attach the sensor there. Then it is a matter of calibrating our sensor reading to actual tip temperature and having the micro processor print out the adjusted temperature. Now before I say Voila' I have to say that this is more a thought experiment that a prctical solution. For one thing the heat resistor could adversely heat sink the tip unless it was very low mass. This would require more current to maintain tip temperature. The added sensor and heat resistor on the solder iron would perhaps make it clumsy and difficult to use. I would expect the data from the sensor would only be accurate in a very narrow band of temperatures unless there was a lot of processing being done on it to compensate for heat losses at different temperatures due to conduction and radiation. Temperature of the sink would have to be brought into the calculation also since, on a small poratble item like a soldering iron, it would surely not be stable. And the list goes on.

John

Yeaaaaaah.... That's not helpful at all. The temperature sensor would read the lower temperature. You would have to keep viewing a conversion chart or keep thinking about the difference on the other side of the heat resistor.

Temperature Sensors, at least the ones without internal processing, change resistance or output a voltage. They do not output a temperature per se. The processor maps the range of voltages that are output on to a range of temperatures to produce an output. As long as there is a correspondence between the range of sensor outputs to actual temperatures we can call it anything we want. Look at our ability to display the temperature as degrees Fahrenheit, Celcius, or Kelvin. All different but talking about the same temperature. In the case I described with the heat resistor our sensor would sense 100 C but our display would display 300 C. There would be no need for any conversion chart.

John

Sure, but you're impeding it's ability to read a temperature. You go from reading the temperature of the flame to reading the temperature of the pot you're heating. In this case the flame is the iron and the pot is the leads or anything you are soldering. That's like guessing the volume of a stereo from outside your house. All you'll end up with is, going from "that radio is loud" to "is it on?".  Plus dealing with environmentals, an air conditioned room will affect the temperature probe more so than if it were on/in the heat source. Even if it were encased in the buffer.
And with the delay of heat transfer from your heat source, the best you can hope for is to measure your holding temperature.

The solder would flow on the tip and damage the thermistor. So i planned about mounting it to the barrel.

The solder would flow on the tip and damage the thermistor. So i planned about mounting it to the barrel.

I would have hollowed out the inside of the tip and mounted the thermistor inside.

I hope you get it working with a thermistor eventually. I've decided to go with one of these. REX C100 Dual PID Digital Celsius Temperature Control Controller Thermocouple | eBay

It uses a thermocouple. But for it's purpose, it's may end up being worth it to me.