Photoresistors and LEDs

[arduinodoer3000]

How would one wire more than one photoresistor to detect light for one LED without using more than one analog pin? I heard it’s possible to put them in parallel with one another and one 10k resistor, but it hasn’t been working properly. I want to avoid using a multiplexer. I’m a beginner so any advice is welcome:)

[jim-p]

Do you just want to detect the presence of light or are you trying to actually measure the light level?
Also what is the part number of the LDR you are using?

[arduinodoer3000]

I just want to detect the presence of light, and I’m using mini photocells from SparkFun. I believe the part number is SEN-09088.

[jim-p]

If you put two in SERIES with a 10K resistor then the arduino will read about 512 when light and go to almost zero if EITHER or BOTH LDRs are in the dark.

[anon57585045]

Application details please. Also, if the parallel circuit is not working, please post a diagram as recommended in the forum guidelines:

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and fully describe what "not working" means in this case.

[anon57585045]

@jim-p

If you put two in SERIES with a 10K resistor then the arduino will read about 512 when light and go to almost zero if EITHER or BOTH LDRs are in the dark.

This is an "and" function. The OP didn't say, but the usual application is an "or" function... if there is light here OR there, then...

The problem with paralleling them is isolation, you could do that with a diode and resistor for each LDR.

If you use phototransistors, you can parallel them with a single load resistor.

[dlloyd]

Depending on your application, you might want to use an inexpensive LDR module. Here, I've attached an oscilloscope to one and you can see the relationship of its A0 and D0 outputs and its builtin LED. Just click the module and vary the slider ... the last 2 channels on the scope will show the graph.

README_Scope_Chip.md - Wokwi Arduino and ESP32 Simulator

[TomGeorge]

Hi, @arduinodoer3000
Welcome to the forum.

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See also FAQ - Arduino Forum for general rules on forum behaviour and etiquette. Hello, Welcome to the Arduino Forum. This guide explains how to get the best out of this forum. Please read and follow the instructions below. Being new here you might think this is having rules for the sake of rules, but that is not the case. If you don’t follow the guidelines all that happens is there is a long exchange of posts while we try to get you to tell us what we need in order to help you, which is fru…

Can you please tell us the application?
Do you just want to detect the presence of the LED light,
AND/OR
detect its direction?

Thanks.. Tom..

[ec2021]

@arduinodoer3000

How would one wire more than one photoresistor to detect light for one LED

Hi @arduinodoer3000,

it might be worth to specify your application more precise ...

• More than one photoresistor to detect the light of ONE (the same) LED?
• More than one photoresistor to detect the light of different LEDs?

If more than one LED:

• Do you want to distinguish which LED is on or
• Is it sufficient to know that at least one is on?

If you do not have enough analog inputs It might be much easier to add a 16-Channel ADC Analog Digital Multiplexer Breakout Board

https://www.aliexpress.us/item/2255800257647286.html

Good luck!
ec2021