Weller, weller, weller boom

I bought a dead Weller soldering iron from that auction site. I have always wanted a posh soldering pencil, and now there is more lead-free around I need a soldering iron I can wind the temperature up a bit on. So, another project in waiting is to replace the electronics in the Weller with an AVR, a bit of PID code and a few gizmos.

To show the selected temperature, I dug out some old HP TIL311 displays from my junk box, so I have decided to use those. My first attempts at knocking up a circuit caused a very, very hot 5v regulator so I have decided to go for a switching regulator instead.

And here it is .. I've used this design before, so I know it works, it is reliable and is very efficient so no more burned fingers on the heatsink.



The soldering iron has a heating element, and a temperature sensitive coil of wire. The circuit - culled from many, many sources and kind of joined up into the simplest possible design (since there is not much room inside a Weller soldering iron itself) is like this. I've made a constant current source from a transistor and an LED, and that current passes through the temperature sensor. The small voltage across the element is amplified a bit, and filtered a lot before feeding the integrated ADC in the AVR.

I couldn't decide whether to write the code in C, and use AVRISP to program it, or to do the Arduino thing. So, I have put both styles of connector on the board. I've added three terminals for a pot to control the temperature ..easy enough to read that.

So anyway, the soldering iron is I believe an EC1201 pencil (40W) .. the base was (before it caught fire) an EC2100. According to a few web sites, the sensor is an EC229, with a room temperature resistance of about 21-22 ohms. I am calibrating it using boiling water and the usual cold water / ice mix allowed to stabilise at room temperature. Turns out that the relationship is roughly linear, and is something like resistance = 20.7+(TempInCelcius/12). I might increase the gain of the amp so that I get a few volts into the AVR.

The five pin connector on the solder pencil has two gold pins for the temperature sensor, one ground pin connected to the barrel, and two ordinary steel-coloured pins for the 24v heater.