As shown, 555 and R1, R2, D1, C1, etc. constitute an astable multivibrator. When the power is applied, the vibration starts, and the starting frequency depends on the charging and discharging time.
Infrared light-emitting diodes and infrared receiving tubes in the circuit are generally paired, because the infrared wavelength range is quite large, roughly 760~1500nm, and different models have different light-emitting areas.
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