Electronic – arduino – LDR not detecting light from LED when finger placed between the two

arduinoldrled

I'm currently working on a project which uses an LED and a photoresistor to work out my BPM.

I have a schematic which I've built into a prototype which all works fine and when I run the serial monitor in Arduino I'm getting output values.

The problem is, when I place a finger in between the LED and photoresistor, for some reason the readings are going to 0 and I'm unsure why?

I've encased with led and photoresistor to avoid any other light but for some reason I'm not getting any values.

Here is the schematic that i'm following:
Here is the schematic I'm using

You'll have to excuse my choice of encasement:
enter image description here

Unsure of what reasons which could be preventing the LDR of outputting.

Best Answer

The situation is simple. If you make it impossible for light to enter the LDR then the resistance of that device is increasing as you can see from the added picture.

enter image description here Removing the light with your finger makes the resistance go up. The result is that the base of Q1 does not receive enough current anymore to make it conduct. The collector of Q1 goes up and Q2 starts to conduct in full. So the led D! starts to glow.

The next step is to understand what happens when the led is putting it's light on the LDR.

Looking at the schematic again you can see that if Q1 starts to conduct complete Q2 will be cut of and the led goes off. However that is not possible because then there is no light falling on the LDR anymore.

The result will be a balance whereby the resistance of the LDR lowers enough with the light from the LED to keep Q1 and Q2 conducting so much that the light from the led is sufficiant to keep the LDR at the resistance needed.

Now if you start playing with this combination then you are able to change the balance and send the resulting signal to the analog input of arduino.