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The new TSSP4056 proximity sensor from Vishay "determines distance by varying the carrier frequency used to drive the emitter between 30 kHz and 54 kHz, thereby finding the threshold frequency at which the sensor just detects the object." Its data sheet says very little about the function. What is the science behind this? What is the relationship between modulating an IR beam and distance?

The device functions out to 2 meters. Can this range limit be extrapolated from the frequency range (30 to 54 KHZ)?

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Have you read the application note? It explains how you vary the path gain to determine the relative distance to a detected object.

First, they talk about varying the transmitter power while operating the receiver at a constant gain.

Second, they talk about varying the transmitter frequency, taking advantage of the fact that the receiver gain varies with frequency.

Is that enough detail for you?

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