sheilding timer from welder noise?

Eric R Snow wrote: (snip) > So there will be two pots mounted to the front

If you are talking about aluminum foil, it is very hard to make good connections to aluminum, because of the thin oxide coating that aluminum forms. If you can find some brass shim stock, it can be cut and soldered into a neat box lining or cover that will be much easier to effectively ground. Ideally, the wire shields should connect to the box at the point where the wires pass through, so that they are an extension of the box shield. You may need to stretch the cable braid around the back of the pots (or make brass cans to contain them), if the pots are made of plastic.

There is only a small part of the timing circuit that is sensitive to noise. Unfortunately, that part includes the pots. But it sounds like your approach has at least a fair chance of working. A lot depends on how the welding conductors are routed, with respect to the timer.

What is the problem you're having?

Not turn back on? Erratic timing intervals?

Thanks John. The pots do have metal cans. And it just so happens that I have a couple rolls of brass shim stock. I like the brass idea much better. It will not only make a neat enclosure but will be durable. It turns out that there is a removable plate the fills the space where the timer would go and I'll just make up a brass cover out of .005 shim stock and screw it to the plate. Eric

Try a .1 microfarad ceramic cap directly across the power supply jack. That would be easy to try out, just slip it under the power cord wires in the screw terminals.

You already have a 100 mic electrolytic from the 555's V+ pin, so that's good.

No problems yet. But I don't want any either so it's best to start off right. Thanks, Eric

Eric R Snow wrote:

Hi, Eric. You've gotten a lot of good advice from the other posts. If there's a safety issue, or if you find you're getting unreliable results, you might just want to hunt around in the parts bin for a small 12VDC DPDT relay, an NPN Darlington transistor capable of dissipating a couple of watts, and a 10K pot and do something like this (view in fixed font or M$ Notepad):

| SW1 | | T | --- |VCCo-o---o o---o----o---------. | | | | | | | | +|1000uF | | | || | --- | | '---||----' --- | | || | |/ | CRY1 o-------| TIP101 .------o | | |> | | .-. | | | 1K| | | CRY1 --- Timer | | | | --- Contact | '-' | | | | | | | .---o | '------o | | | C| | | .-. C| | '->| |10K C| | | | | | '-' | | | | | === === | GND GND (created by AACircuit v1.28.5 beta 02/06/05

When you press the momentary pushbutton, the base of the darlington is pushed to 12V, which turns on the relay (Ve = 10.6V). Once on, the relay will stay on until the voltage across the coil drops to about 25% to 35% of nominal. The discharge rate of the cap will be determined by the 1K resistor plus the setting on the 10K pot. Since Q1 is a darlington, and your relay will be pulling less than 100mA, so the NPN darlington base current won't be much of a factor. Of course, once the relay turns off, time's up.

This might be a good fallback position. This timer will give you from less than 1 to several seconds. It should be fairly consistent over a reasonable temp range. The biggest problem is that the time delay is susceptible to change with vibration -- this may be a problem in an automated machine. But vibration is much easier to deal with than electrical noise.

Good luck Chris