IR Reflection Proximity Switch

IR radiation from LED2 (modulated by a 1-kHz wave) is keyed by U1 , and Q1 is radiated. Reflected IR energy is picked up by Q3, and the audio signal from Q3 is amplified by Q2 and sent to the decoder. The LED1 lights to indicate presence of reflected IR. LED1 can be the input of an isolator so that a triac or SCR can be controlled.

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IR Reflection Proximity Switch - Circuitos de Electronica