simple optical switch

When the phototransistor is stroken by IR light it conducts and the voltage between the 1Mohm resistor(arbitrary) and the phototrans drops from VCC to lower values. When the voltage drops lower than VCC/3 the 555 is triggered and goes high (from 0 TO VCC). The amount of light that strike the phototrans necessary to bring his collector to VCC/3 is
simple optical switch - schematic

determined by the resistor (Vdrop = Icollector * R, so, if Vdrop= 2*VCC/3, the resistance needed to set the threshold on current is R=2*VCC/(Icollector*3). High sensibility phototrans would need a smaller resistor, and weaker phototransistors higher value resistor, you can also use a trimmer to set the on threshold level with precision. The time of phototransistor isn`t critical. The 555 has high current capability and can drive various devices, such as Bipolars, relays, bipolars+relays, mosfets, mosfets + totem pole, or give a logic output (see pic). In case you need to trigger something when the gate is blocked (for example a burglar alarm, or a multistage coilgun) you need to invert the output, which is accomplished using a small bipolar transistor wired in an inverting setup (see pic) or swapping the positions of phototransistor with the resistor, so the voltage will drop under VCC/3 when blocked: The formula to determine the resistance to turn off at Icollector is R=VCC/(Icollector*3).

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