Cheap air pressure switch?

Idiot light OPS are good to 100psi or so; usually NC, trigger at 3psi. VW ones also can be NO to 1.8 bar. JR Dweller in the cellar
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Three bucks for a 100 PSI brake light switch. Only 911 are left. --Winston

Air cylinder, stiff return spring, flag on the cylinder's rod end that pops up in the operator's face when the air pressure overcomes the spring.

What pressure. There are A/C system switches as well as oil pressure switches and even brake light switches that operate using brake pressure. The OP idiot light switches usually turn OFF (NC contacts) at about 5-8 PSI. The A/C switches depend on which end of the system they are on. The high pressure switches usually turn on about 120-140 PSI (and there are adjustable ones) The brake pressure light switches turn on around 100-150 PSI. (Ford, Chrysler)

Perfect! Thanks!

I'll bet that would work fine. It might eventually fail after 5 years or so with the pressure going on/off/on all the time. Buy two. There likely is a less expensive strain gauge type sensor available that would last forever since there is no bellows constantly stretching.
Jon

Umm, these are good to thousands of PSI, my guess is the trip pressure may be anywhere from 50 - 250 PSI on each unit. They may not work in a system with just 100 PSI line pressure.
Jon

The part is spec'ed at "100 PSI", (+- something).
It looks to be a piston - activated device. What might cause the switch to work properly yet activate at a pressure significantly different than 100 PSI?
The piston / cylinder could change diameter by more than a few percent. (Unlikely.)
The return spring behind the piston could change pressure. (Possible with temperature change though we can be pretty sure Tom's shop stays cooler than the temperature commonly found in the area of a car firewall).
Piston stiction caused by 1000's of wear cycles could cause it to activate at different pressures from cycle to cycle. (Possible. The replacement part is not expensive.)
At three bucks one could afford to buy 20 and cherry pick for actuation pressure.
Tom, you can also try the 'low pressure' version. It's not cheap, though. Eighteen smackers, yikes!
--Winston

switch, look around, various pressures. Greg

I'll get a bunch of the $3 guys and test them. Shop air rarely falls below 110, but even at $18 they won't hurt my budget. (which is about $5k over already)

Excellent option if the $3 ones are problematic!

Please let us know how it goes.
--Winston

I'm fond of Nason pressure switches. I needed a SPDT oil pressure switch for my car that operated at 15psi but didn't want to pay an arm and a leg for one. A standard oil pressure switch operates at 3-5 psi, which for me is way too late. The 15 psi gave me the warning I needed, and on the engines I've used them on, the light would barely flicker when the engine was really hot. Got it from my local Carquest that ordered it for me. I think I paid in the $25 range, retail. The thing is solid as a rock and very well built. I was never worried about the stem breaking off from the vibration.
Anyway, the price was really good as well. A buddy of mine repeated what I did, and they sampled him the one he wanted. Link is You pick out the pressure range, connection type, etc, rather than figuring out how to make what's on the market wire up to your existing system. They also have adjustable ones, too.