Question

This is a heart rate sensor hardware, the photo-transistor detects the light coming from the LED. I have a few questions

1. what is the purpose of C4 at the non-inverting terminal of amplifier?

2. how is C1 operating? (i know its for the gain but don't know how)

3. Can the 3 ouput terminals of the circuit itself be explained? they are going towards an arduino.

3 al 2 8 8 100 C6 R4 100 (mu) R3 T1 SV1 100(mu

Answer 1

1. C4 is a coupling capacitor which couples the voltage at collector terminal of phototransistor to the non-inverting terminal of op-amp. It is used to pass only AC components from phpto-transistor to op-amp. It is maintaining the stability in the circuit.

2. C1: Pin 1 and 8 of LM386 IC is used for gain control setting

If these pins are left open, LM386 will provide a gain of 20. If a a capacitor is place in between 1 and 8 pin, the gain can be increased from 20 to 200.C1 is used here to increase the gain of op-amp.

3. 3 output terminal of the circuit is the ground of this circuit. As you mentioned, this circuit will be connected to the Arduino, So the ground of Arduino and this circuit should be common for proper voltage reference.

This pin 3 of SV1 connects ground of this circuit to the ground of Arduino.

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