Circuit diagram
Introduction
The 555 is proved to be the most versatile and ubiquitous IC all over the world.This is a possible use: simple inverting sch mitt trigger.
Circuit explanation
When the photo transistor is stroked by IR light it conducts and the voltage between the 1Mohm resistor(arbitrary) and the photo trans 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 photo trans necessary to bring his collector to VCC/3 is determined by the resistor (V drop = I collector * R , so , if Vdrop= 2*VCC/3, the resistance needed to set the threshold on current is R=2*VCC/(I collector*3)). High sensibility photo trans would need a smaller resistor, and weaker photo transistors higher value resistor, you can also use a trimmer to set the on threshold level with precision. The time of photo transistor isn't critical. The 555 has high current capability and can drive various devices, such as Bi polar, relays, bi polar+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 coil gun) 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 photo transistor with the resistor, so the voltage will drop under VCC/3 when blocked: The formula to determine the resistance to turn off at Ic ollector is R=VCC/(I collector*3).
author: Jonathan Filippi, jonathan.filippi@virgilio.it
circuit from http://www.electronics-lab.com/
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