If there is a biometric screening system for a locked door, can one do biometric screening on shabbat? In this case, this is a completed circuit with running electricity before Shabbos (it’s always running), but it does certain actions (such as unlocking), when it does biometric detecting. Would it be acceptable?
Even if the "listening" circuit is always on, the "action" circuit is turned off until activated. When anything comes near the sensor, it immediately springs into action, searching its database for a match. If the object is a match with its verified fingerprint, it sends a message to the lock to open. That's a lot of circuits opening and closing. Hence, it would be as assur as turning on a light.