Parts

• Ehdis® High Current Starter Relay 500 AMP DC 12V 4 Pin SPST Car Auto Start Contactor Double Batteries Isolator Control On/Off Switch

Relay

Schematic - Push Button Fire

Schematic - Microcontroller

The Build

I ended up using a Sonoff SV since it had everything I needed. Instead of having to use a transistor to switch the 12v, I used the build in relay.  The Sonoff SV also has 3 GPIOs available which I used for a RGB LED. It also exposes the GPIO-0 (button) which I used for the foot pedal. The foot pedal made things so much easier.

I originally powered the SONOFF SV off of the battery but found that it would cause the SONOFF to reboot sometimes. So, I ended up powered it with a dedicated 12V 2A power supply.

I programmed the SONOFF micro-controller (ESP8266) to open the relay for a number of microseconds in order to perform the weld.

The time required to weld really depends on your battery source. I noticed that when the battery was running low, it took more time to get a good weld.

I programmed the micro-controller to perform the following tasks:

• welding would be performed using 1 of 4 durations (25, 50, 75, 100ms)
• pressing the foot pedal quickly would send 12v to the solenoid for the currently set duration.
• pressing the foot pedal for at least 2s would switch to the next duration. 
• each duration had an associated color (LED):
  • blue - 25ms
  • green - 50ms
  • orange - 75ms
  • red - 100ms

Observations

With my batter fully charged, a 25ms weld did a good job most of the time. For some welds, I increased the weld time to 50ms.