This board is based on the 555 Timer in Astable configuration and integrates an SSR based in the commonly availible BTA25600B (for the spot welder) or the BTA12600B (for the bag sealer), isolated from the timer with an MOC3052 opto-triac.
I made both, smd and tht versions of the boards.
Made a quick thru-hole prototype, totally forgot the capacitor in series with the microswitch
and had to add it directly to the switch terminal.
That capacitor make sure that if you mantain the switch pressed close, the timer won't keep output in a high level, making the welder never shut down and keep outputing current.