Latest Relay Sensor

See the faq:

FAQ: List of devices with dry contact input from external sources

I would note, however, that “cheap” and “reliable” don’t always go together. And in particular, the entire smartthings platform does not guarantee reliability.

If this is an absolutely critical application, I would not use SmartThings for it. And the company agrees: see their official product usage guidelines:

Data accuracy and consistency from SmartThings sensors, including those provided by SmartThings directly, resold by SmartThings, or supported by SmartThings, is not guaranteed. Therefore, you should not rely on that data for any use that impacts health, safety, security, property or financial interests.