LED driving BJT transistor

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Hi folks, i`m doing my own little HDD Clock project and using the "LED sensor" method for measuring RPM.

The output waveform actually looks pretty good about 1.25V, at least up to ~6000rpm with a 2mm slit. Have not tried any higher speed.

What i`m unsure of is if the current generated in the led would be sufficient to drive a small BJT, say a BC237 or similar, without any biasing? Load will be an AVR input. Or would it require a schmitt-trigger?

Thanks
Kim

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6000 RPM is 100 revolutions per second. Surely 100Hz would be easy to detect with phototransistor, but as they are slow so fast pulses may get lost.

So you are now using a LED as a photodiode? You should first try a real photodiode and good IR light source for it. Or at least a red LED and red light source for it. Most likely you need an amplifier (opamp, comparator, transistor), and since photovoltaic effect is quite slow and low in voltage, maybe reacting to the photocurrent would be better.

Driving Schmitt-trigger inputs takes more current than normal inputs, because you have to overdrive the current coming from the hysteresis.

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Here is a link to motion sensing and optical detectors..

http://www.avagotech.com/pages/e...

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Thanks for the replies, i think ill do some tests and see what the driving capability is, if it is to low i`ll start looking for real photodiodes.