Best way to amplify the photoresistor signal

Question

I have the following schematic.

^{simulate this circuit – Schematic created using CircuitLab}

And it graphs to an almost stratight line. It is supposed to be a heartbeat sensor. From what I have read, I come to know that amplification is a must. But my knowledge of electronics is very limted and I need guidance.

I am emitting a red. My photoresistor responds to IR signal very poorly and I had to use the red one.

Am working with Arduino Uno board.

Answer 1

I suspect R2 is much too low; the 100 \$ \Omega \$ of the LDR is probably under high light condition, you probably have much less light.

I suggest empirically choose R2 to center the A0 point with nominal light. You may still need amplification, but this should give you the best performance without changing the topology.

Answer 2

Biological signals are generally very small (in the uV range). You should construct an amplifier of some sort. A instrumentation op-amp would do fine.

general info on instrumentation op-amps

Before you construct a circuit though you should have an idea about the signal. Get a good oscilloscope and you should be able to confirm you have a signal. It can be helpful to simulate a signal using a function generator rather than your heartbeat to test the circuit beforehand.

Answer 3

You can use a simple op-amp circuit (non-inverting configuration). No need for, or value in, an instrumentation amplifier.

Try this:

^{simulate this circuit – Schematic created using CircuitLab}

Adjust R3 to get a couple volts at the output. If there is not enough range, reduce R1 a bit. This circuit gives 10x the voltage of yours (due to the 1K R1) and has a gain of +11, so 110 times the output. You can increase R5 to increase the gain further.