I am mostly done with the code at this point,
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/xsmiley.gif.pagespeed.ic.R8GFI-pF6f.png)
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Here is the schematic:
1. mcu is a 28-pin PIC - 16F1936 specifically. But the code should compile, with minor modifications, for other 28-pin chips, like 16F886, or 18F2xK20/K22. You just need 6x2 consecutive output pins to drive the led, 1 adc pin and after other IO pins. With some modification, it should work on other mcus as well.
2. The 4-digit, 7-segment led sits on top of the mcu. No current limiting resistor. Current consumption is about 60ma between the led and the mcu.
3. The code drives Hakko cores with PTC sensors (A1321). With some modification, it can be made to work with cores with thermalcouple sensors (A1322 or T12).
4. Temperature sensing is done via the PTC, calibrated to Hakko clones: the resistance curve in the code is purposefully reduced by 10ohm. A trimmer (RV2) is built in the circuit to allow additional calibration specific to your own cores.
5. The code drives either common anode or common cathode leds. Jumpper attached to RC2 tells the mcu if the display is common anode (high) or common cathode (low).
6. User settings / input done via an el cheapo encoder (with a built-in switch).
7. Unit works in both Celsius or Fahrenheit, user selectable.
The driver is designed to be cheap. Depending on your parts box, it shouldn't take you more than $20 to build it.