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# Simple on off IR photoswitch(LM555CH)

2015-01-10 11:17
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This article describes the Simple on off IR photoswitch (LM555CH). The content is very simple, very practical. The components of the article can help you understand better grasp this principle. For example, in this part of the principle, you can go to find and purchase the components: LM555CH.

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 betw1een 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).

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