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Old 23rd April 2012, 03:57 PM   #1
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Default light beam switch needed

Hi,

I need a circuit to allow a relay to be activated and power a small geared down motor when a photo transistor sees the light from a IR LED and to deactivate the relay when the beam isn't seen. I am passing an object of roughly .450 in diameter inside a 1/2 ID pipe. When the pipe is full I need the motor to stop.

Any ideas?
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Old 23rd April 2012, 04:10 PM   #2
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The problem with such a design is ambient light and false triggering.

Simplest way to ensure no false triggering is to use an NE555 (for example) to generate a known frequency and then just use its PLL counterpart (I can't remember it off the top of my head by I seem to recall the NE564) to detect it.

http://noel.feld.cvut.cz/hw/philips/acrobat/1057.pdf

Last edited by KatieandDad; 23rd April 2012 at 04:13 PM.
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Old 23rd April 2012, 04:14 PM   #3
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It would be a black tube so light shouldn't be a problem.
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Old 23rd April 2012, 05:11 PM   #4
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In which case its pretty simple.

You simply need to wire up the IR LED exactly as you would any other LED with a series resistor.

The sensor is an IR transistor (assuming its NPN) with its Base OPEN CIRCUIT (Normally - they do differ), Emitter to 0V and its Collector to the -ve of the motor. The +ve of the motor is connected to VCc.

Might be better to switch the motor with a relay to reduce the current through the IR transistor.
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Old 23rd April 2012, 07:54 PM   #5
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will this work?
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File Type: jpg IR control.jpg (91.2 KB, 17 views)
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Old 24th April 2012, 07:17 AM   #6
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Use 555 as schmitt trigger.

Gajanan Phadte
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Old 24th April 2012, 12:25 PM   #7
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I don't want a schmitt trigger.
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