I've already soldered together the jack for plugging in the RT tip, but now I need to figure out which PIC to use... so I'm going to use this post to catalog what pins I'll need.

Inputs:

• 1 analog - temperature sensor (via op-amp)
• 2 digital - for rotary encoder
• 1 digital - for standby switch (possibly not used until I get a better 3.5mm jack that actually supports this, unless I decide to pull the trick where I check if the tip has connectivity to the stand)

Outputs:

• 1 digital - heater control (to MOSFETs)
• 7 digital - display (using the HPDL2416: 1 write pin, 2 address pins, and 7 data pins, though 3 of the data pins can be driven by 1 pic pin with an inverter)

I'm still undecided as to whether or not I want to expose the ICSP pins, since my plan is to use a DIP socket, and I have a separate programmer socket I can use to program the PIC. On the other hand, it may be much more convenient to be able to simply connect the programmer to the soldering station directly.

Total I/O pins: 12 (or, with ICSP, 15)

Given the above pin lists, we should be able to use a 20-pin PIC. I have a few of these available:

• PIC16F690 - I have two of these, and it's the chip I'm most familiar with.
• PIC18F1220 - I only have one of these
• (PIC24F04KL101 - This one apparently doesn't have any A/D pins, so it won't work, though I have a tube of 5)

Looks like I'm going with the PIC16F690 again. :)

I'd also like to understand the MOSFET output stage used by the Soldering pen project better - especially why one of the MOSFETs shown on the schematic there is N-channel and the other is P-channel. (even though only one part number is shown in the BOM) I've been reading up on using MOSFETs to control heaters, but I'm still not sure if any of the parts I have on hand will be sufficient to drive the tip, and I'm not entirely sure whether it's possible to build a correctly-working output stage using various mixes of P-channel and N-channel parts. (P/P vs. P/N vs. N/N) Luckily, none of this should affect the pin count, so it shouldn't influence the PIC choice directly.