I am pairing an LDR with a capacitor to to make a light sensing project by returning the value of the variable COUNT when the capacitor is charged. But when i do this with 2 or more LDRs the values of COUNT are very inconsistent. The values work fine if I connect the LDRs at pins 3 and 5(GPIO1 and GPIO2) but if I connect them anywhere else the values don't make any sense even if I use the exact same LDR and capacitor in identical conditions (you can see it in the output). Why is this happening ? Here is the code and output.

import RPi.GPIO as GPIO
import time


pr1 = 3
pr2 = 5

def rc_time1(pr1):
    count = 0
    GPIO.setup(pr1, GPIO.OUT)
    GPIO.output(pr1, GPIO.LOW)
    GPIO.setup(pr1, GPIO.IN)
    while(GPIO.input(pr) == LOW):
        count += 1
    return count

def rc_time2(pr2):
    count = 0
    GPIO.setup(pr2, GPIO.OUT)
    GPIO.output(pr2, GPIO.LOW)
    GPIO.setup(pr2, GPIO.IN)
    while(GPIO.input(pr2) == GPIO.LOW):
        count += 1
    return count

    while True:
        print("LDR_one: ---- ", rc_time1(pr1))
        print("LDR_two: ---- ", rc_time2(pr2))

except KeyboardInterrupt:


output at pins 3 and 5

LDR_one ---- 242 LDR_two ---- 246 LDR_one ---- 241 LDR_two ---- 203 LDR_one ---- 209

output at pins 3 and 8

LDR_one ---- 482 LDR_two ---- 141400 LDR_one ---- 482 LDR_two ---- 142865 LDR_one ---- 480


  • Frankly this code makes little sense and I doubt the LDR is having ANY impact on the result. Post circuitry.
    – Milliways
    Feb 21 at 6:26
  • You're right, i changed the code, thanks for the advice. I ditched the logic of counting.
    – Coder
    Feb 21 at 19:05
  • You are right about the LDR as well.
    – Coder
    Feb 21 at 19:18

2 Answers 2


Both of pins 3 and 5 have hard wired 1k8 resistors to 3V3.

Either use different pins or ensure the others have the same pulls added to the circuit.

  • Thank you for the answer, really appreciate it. About the answer though, I currently am using the 3v3 as voltage line. So when I connect the LDR to pins other than 3 and 5 I just have to make sure the resistance is the same. Have I understood correctly?
    – Coder
    Feb 21 at 5:57
  • Yes. You have to be as sure as possible the resistance-capacitance circuit is electrically the same. The current flowing from the 1k8 resistor to the GPIO on pins 2 and 3 change the circuit. Either add the same value resistor to 3V3 to all other GPIO you want to use or don't use pins 2 and 3.
    – joan
    Feb 21 at 8:29
  • Ok got it :). Thanks a ton !! I will try it.
    – Coder
    Feb 21 at 9:12

Using a capacitor is an established technique for making a rough and ready ADC but relying on loop count is inaccurate and will not give reproducible results.

You should instead time how long it takes to change.

  • I should check how long it takes for the COUNT to change by using time() method ? Thanks for the response, appreciate it.
    – Coder
    Feb 21 at 9:16
  • I tried using time() method as well. The times were inconsistent too depending on the pins and the number LDRs I connected.
    – Coder
    Feb 21 at 9:24
  • @Coder If using pins 3,5 you are measuring the resistance of the 1.8kΩ pullup - these will totally swamp LDR as they are 2 orders of magnitude less. If you disconnected the LDR you would see the same result.
    – Milliways
    Feb 21 at 10:00
  • After numerous attempts I've gotten it to work. I'm using the time.time() function in a way figure out how long it's taking for the capacitor to charge. But the returned values became consistent once i added a 1k resistor to each LDR. Thanks a lot, all your advice has helped a ton.
    – Coder
    Feb 21 at 19:16

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