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In the case that there is no obstruction above the infrared transmitter tube and receiver tube, adjust RP1 to make the LED3 on the circuit board light up, and then adjust RP1 in the opposite direction to make the LED3 just go out.
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.