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Simple optical switch
author:
Jonathan
Filippi

Introduction
The 555 is proved to be the most versatile and ubiquitous IC all
over the world.This is a possible use: simple inverting schmitt
trigger.
Circuit explanation
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 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).
Have fun!
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