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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.
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). |
