I've had a wireless interface to this working for a while, but want to reduce it to as small an add-on as possible. Got some ESP-01's which seemed like the right size. All they need to do is route between WiFi and TX/RX.

Got a small burner from Amazon, loaded a simple script, and it works great! WiFiManager (Arduino library) is awesome. When you first run it, it offers a captive portal for selecting a router and saving credentials. After that, it always connects via that access mechanism, and then runs your script. If the router isn't available, it goes back to captive portal mode.

When I tried wiring an ESP-01 stand-alone (i.e. without the burner board), I mostly couldn't get it to boot, though sometimes it would randomly work.

I had EN and RST pulled high, but that wasn't sufficient. I believe I found the problem though :) Normally, to pull multiple inputs high, I'd run a single resistor to Vcc and tie all the inputs together. I think one or more of the inputs temporarily goes into output mode (and drives LOW) during boot, and, since it's tied to other inputs, they are driven low too, which, in the case of EN, is a bad thing.

There was some testimonial evidence that tying EN directly to Vcc sometimes helps. I suspect this may be the reason: it may have worked in cases where multiple inputs were tied together and pulled up.

Anyway, it boots every time now, which is great. Gonna use a SP3T power switch to drive the main board (T2) and then the ESP-01 (T3). So middle position is board only, 3rd position enables the WiFi. So no extra power draw in position 2 :) In position 3, I'll use the power going to the ESP to flip a pair of MUXes and connect the ESP's Tx/Rx to the AT328 (and disconnect the FT231).

Plans...