This circuit uses two transistors in a direct coupling configuration and directly activates a relay whose characteristics depend on the supply voltage used. The relay's activation current must be a maximum of 50 mA, and the voltage drop across the 10 Ω feedback resistor must be considered. The light source is a small low-voltage lamp, which has its light focused on the phototransistor at up to 3 cm. More powerful sources and optical resources can increase this distance. The 1 k ohm resistor can eventually be changed according to the desired activation characteristics for the circuit. The figure shows the complete diagram of this passage detector.
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