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IR diode+Photo diode sensor for Tachometer

abuhafss

Aug 3, 2010
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I am considering to use an IR diode + Photo diode as revolution sensor, with a simple 555-based tachometer.

Kindly check and confirm if this the correct way to use those diodes. And is it okay to use 12V supply or do I need to incorporate a 78L05 for 5V?

Thanks
Screenshot 2014-10-25 23.27.13.png
 

Arouse1973

Adam
Dec 18, 2013
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Hello
You dont need the cap and diodes for D.C and also photo diodes have low gain so you might be better with a photo transistor like the SFH309FA.
Adam
 

KrisBlueNZ

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Nov 28, 2011
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The capacitor (C1) is needed to make a short trigger pulse for the 555. D1 is a good idea, to avoid feeding a +24V pulse into the trigger input. D2 isn't needed.

If the LED and photodiode are part of a photointerrupter, you probably don't need high sensitivity. You can increase the trimpot if necessary. The data sheet for the photointerrupter will probably have a suggested circuit.

You're running the LED at around 110 mA... is it designed to run continuously at such a high current?
 

abuhafss

Aug 3, 2010
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If the LED and photodiode are part of a photointerrupter, you probably don't need high sensitivity. You can increase the trimpot if necessary. The data sheet for the photointerrupter will probably have a suggested circuit.
Yes, D10 and D11 make up a photo interrupter but, they are separate devices.
Increasing the trimpot means, increasing the value....right?

You're running the LED at around 110 mA... is it designed to run continuously at such a high current?
Sorry, it was supposed to be 470-560Ω.
 

KrisBlueNZ

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Yes, increasing the resistance. Move the LED and the photodiode to, say, twice their normal spacing and make sure the photodiode still conducts enough that T1's collector goes high.
 

Arouse1973

Adam
Dec 18, 2013
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The capacitor (C1) is needed to make a short trigger pulse for the 555. D1 is a good idea, to avoid feeding a +24V pulse into the trigger input. D2 isn't needed.

If the LED and photodiode are part of a photointerrupter, you probably don't need high sensitivity. You can increase the trimpot if necessary. The data sheet for the photointerrupter will probably have a suggested circuit.

You're running the LED at around 110 mA... is it designed to run continuously at such a high current?
Good point Kris.
Adam
 
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