antenna / feeler
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antenna
A long, thin sensory organ on the head of insects and other arthropods, used for touching, smelling, or tasting.
Als sich der Käfer bedroht fühlte, zog er sofort seine Fühler ein.
When the beetle felt threatened, it immediately pulled in its antennae.
The noun 'der Fühler' (antenna/feeler) is derived directly from the verb 'fühlen' (to feel). So a 'Fühler' is literally an organ with which a living creature can feel or touch something.
Die Schnecke tastet mit ihren Fühlern den Weg ab.
The snail feels its way along with its feelers.
The idiom 'die Fühler ausstrecken' (to put out feelers) is very common. It is often used with the preposition 'nach' (for/toward) to name the target of the exploration.
Vor der Investition streckt die Firma ihre Fühler nach potenziellen Partnern aus.
Before making the investment, the company is putting out feelers for potential partners.
You can almost always recognize the figurative meaning by the verb 'ausstrecken' (to stretch out/put out). The literal meaning with animals, on the other hand, is often used with verbs such as 'haben' (to have), 'einziehen' (to pull in), or 'bewegen' (to move).
Der Schmetterling hat lange, dünne Fühler.
The butterfly has long, thin antennae.
sensor
'der Sensor' and 'der Fühler' both have to do with perceiving or detecting something. A 'Sensor' is a technical component that detects things such as light, movement, or temperature. A 'Fühler', in the meaning given here, is a sensory organ of insects and similar animals. It is used for touching, smelling, or tasting. The words can be confused because both take in information from the environment. Still, in these meanings they are not simply interchangeable: in technology, you say 'Sensor'; for insects, you say 'Fühler'.
Die Ameise bewegt ihre Fühler, während sie den Weg sucht.
The ant moves its antennae while looking for the way.
Der Sensor der Lampe reagiert, sobald jemand vorbeigeht.
The lamp's sensor reacts as soon as someone walks past.