Pulse meter circuit

Posted on Feb 7, 2014

The input cap for The phototransistor at the bottom is what feeds the opamp. But the output from the phototransistor is always between GND and Vcc. So why the need for an input capacitor It`s most likely there because the photo transistor is not being switching fully on and off. It is likely giving a very small `wiggle` in response t

Pulse meter circuit
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o the small change in transmitted light during a heartbeat. The first op amp stage is configured for a lot of gain with that large feedback resistor. The AC coupling allows the feedback to establish a linear operation point for the first stage to amplify the small blips from the phototransistor. while I kinda agree that it`s probably needed to isolate it from the virtual ground, that`s not the only reason. For example, if you removed the virtual ground and powered the circuit with a bipolar supply, you would still need the AC coupling capacitor. for the reasons stated by w2aew. The ambient light is going to bias the transistor on in a small linear region. w2aew described it just about perfectly, it will be like little wiggles on the DC level, probably just a few microvolts, and the AC input capacitor will remove this BIAS caused by the LED and allow the signal to be amplified.

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