I am using and emergency stop in an application. It is a push button with a twist release. I have the main circuit connected to the normally closed side and a detection circuit wired to the normally open side.

When the stop is activated, the normally closed switch of course opens and remains open (until I twist release the stop).

However, the normally open switch closes only momentarily. Do emergency stops exist in which the normally open switch will remain closed until twist released. If so, what keyword should I look for in searching for such an emergency stop?

  • 1
    \$\begingroup\$ That's unusual (I think). Are you sure? I think I remember that most emergency stops can have 2 circuits normally closed that latch open. You can use that for your detection circuit. What's downstream? \$\endgroup\$ Jul 15 at 17:24
  • \$\begingroup\$ This one only has one normally open circuit (the button has for pins, two for the normally closed switch and two for the normally open). The normally open circuit is 5V from an Arduino, the NO switch and a 10K pull-down resistor. This is is wired to a digital in pin of the Arduino. \$\endgroup\$
    – Siva
    Jul 15 at 17:47

2 Answers 2


I speculate that the N.O. is not momentary. If it truly is then read the data sheet to find the truth about any limit switch.

What I think is happening is the "wetting current" and/or the "sealing current" is not being met by the detection circuit.

Some switch manufacturers will specify these two parameters, but not always. You may have to modify the detection circuit to draw several tens of milliamps to maintain the contact.

Search for these parameters.

Switches also suffer from "bouncing" on contact or separation. It will ultimately settle to closed on contact. The detection circuit may have todeal with this.

  • \$\begingroup\$ I hadn't thought of the wetting/sealing current for the switch. I forgot to also mention that the detection circuit does pick up the momentary closing of the NO switch. It also doesn't bounce. Would this change your thinking about the wetting/sealing current? \$\endgroup\$
    – Siva
    Jul 15 at 17:53
  • \$\begingroup\$ @Siva All switches bounce even if it isn't visible. There may be enough force on the first contact, but not on the others. This would not change my ensuring there is enough wetting/sealing current.. \$\endgroup\$
    – RussellH
    Jul 15 at 18:08
  • \$\begingroup\$ I will try to increase the current in the detection circuit to a few tens of milliamps (basically power it from a different source that can provide more current and use a smaller pull-down resistor), and see what happens. I will post back when I have been able to try that. \$\endgroup\$
    – Siva
    Jul 15 at 18:15

When a latching-type, E-stop, mushroom push-button switch is actuated, the 'NC' as well as the 'NO' contacts remain latched till the switch is reset with a twist of the mushroom button.

It's apparent that the 'NO' contact is defective and needs replacement.

Should it be a low voltage / current / contact tarnish issue, a relay with gold-plated contacts would solve the problem.

enter image description here


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.