21 minutes ago
In response to interest about my TIG footswitch timer thing, here's the circuit I used. If you Google "TIG footswitch and timer" you should find the page on my website about it. Text below is copied from there.
The layout is pretty simple. A large wooden plate is mounted on the top of a box with a hinge, and this triggers a microswitch inside when pressed. Inside, there's a 12V power supply and the timer PCB. This is based on a 555, and is adjustable up to about 100ms. The 555 output drives a relay, whose normally-open contacts go to the trigger switch connection on the welder. The timing resistor is a 220k potentiometer, and the capacitor is a 470nF polypropylene. This gives up to about 110ms maximum on time.
Brief comments on the circuit. It's basically just an edge-triggered monostable circuit. R1 and C1 provide some filtering to eliminate the effects of contact bounce in the trigger switch. C2, D1 and R2 differentiate and clip the voltage on C1 and provide a short (few 100ns) pulse to trigger the 555. The output from the 555 drives a 12V power relay from a microwave oven using a BC337 transistor. D2 provides protection agains inductive kick from the relay coil.
As a further refinement, I added a DPDT switch (the one with the yellow wires in the photo) which connects the microswitch to either the timer circuit or in parallel across the relay output contacts, and hence directly across the welder trigger contacts. This allows the footswitch to be used in two modes - timed, where a press of the footswitch initates a brief weld pulse, and continuous, where the weld is on for as long as the footswitch is pressed down.
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