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Simple Sensor

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I want to measure the frequancy of a IR beam interrupted by a spinning fan blade.

I am usieing an MSP430G2553 

 

Here is a link to the sensor circuit.      http://www.dickbrom.com/circuits/dark.jpg

 

The voltage across the Red Led varies from 1.0 volts to 2.0 volts depending on the beam being interrupted or not. 

 

Can I connect the emitter of the 2n2222 directly to an analog input pin and the battery minus to the launchpad ground pin? 

 

There are a few Energia programs that show hot to compute the frequency. 

 

Thanks 

 

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Hi,

 

You can do this, you won't damage your MSP, but it will not work I think because generally in digital ICs, logic 0 has a short range (something like GND <-> 0,8V) and it's a logic 1 above. So if your signal don't have a "clean" 0V state, you may miss some "shots".

 

Usually, open drain circuit are choosen in this case :

open-collector.jpg

 

The signal is captured on the collector of the transistor. Advantages : you can use the voltage of the

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