Pick an ESP8266 version. The minimal requirement is one GPIO input, so any will work. I used a Nodemcu board (flashed with Arduino SDK and using Visual Micro for coding). The ESP12e/ESP13 dev boards and the Nodemcu board have a voltage regulator built in, but there are cheap AMS1117 power modules to use for the chip versions.

The output jack for the trigger signal uses an open collector on the tip (blue wire in RatZapper). My first version used a PNP to pull a GPIO high. The second version just uses a 10K pullup (didn't try using just the internal pullup). So it is easy to use either a 2.5mm plug or tap off the jack terminal.

I've been powering my two for several years with 5Ah 5-cell NiCD packs. One pack died last year so I'll be switching to 2S LIPO at some point. I'll probably add a 6V regulator to be safe. There may be some 3K+ mAh packs that will fit in the battery compartment. For now, I am trying an old 6V 2A wallwart. The RatZapper gives a green light with it and I could trigger it manually without total protonic reversal.

There's room in the control box lid to add a DC power jack if desired. For one, I added a jack and made a Y-cable with JST connectors, one for power in (wallwart) and one for the ESP (input to a voltage regulator or VIn on Nodemcu). For the minimal version, I'm using 3-wire servo connectors, which gives power, ground, and signal to the ESP. For both, I just used the unswitched power wires.

The first version has a couple PCB relays with internal diodes and 3904s and more disco LEDs, because I had them and they fit. The nice thing about the ESP is that the hardware can be repurposed with a flash.

Software Notes:

• The code implements a simple server, emailer (POSTing to a nodejs server running on openshift), and interface logic for the trigger. I may switch to using SPIFFS once I write a persistent config library for some other projects. But there's not much else interesting to do. I may add ntp time lookup and periodic status updates to thingspeak because it makes it easy to tell if any resets occur.
• The web page just polls with an ajax json request to /api, which returns a simple json object (hostname and trigger status).
• The trigger logic requires a minimum trigger pulse width (1 second) and enforces a delay between triggers.

Hardware Notes:

• I haven't done anything with the analog input yet so I will probably experiment with a voltage divider on it to periodically measure the input voltage to check battery status.