IR diode+Photo diode sensor for Tachometer

[abuhafss]

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

 

[Arouse1973]

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]

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]

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]

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]

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