Read LDR with A/D converter with adjustable senstivity

Question

I'm working on a small project, where I read up-to 12 brightness values from light dependent resistors (my schematic below only shows three). The basic layout is a voltage divider per sensor, which is feed into one of the ADC channels of a teensy microcontroller.

^{simulate this circuit – Schematic created using CircuitLab}

From the ADC I get a pretty good reading from 20-1000. I actually tried different resistor values for the voltage divider (R5) and I ended up with 47kOhms.

In some environments, I have lower light and I would like to make the voltage divider adjustable. With low light, my readings only go to 200-300.

I've tried to add a potentiometer, but this does not work. I'm not 100% sure why to be honest. Is there another way to do this? I want to have ONE adjustable potentiometer to change the voltage dividers on all channels.

Answer 1

You want high dynamic range... but you don't have enough bits. Here is a hack.

Let's suppose you want to measure your LDR's resistance from 100 ohms to 1 MOhms with decent precision...

Remove the resistors, and replace them with 10nF C0G or film caps. Now...

• Set ADC pin to digital output, and output 0V.
• Wait 100µs
• Set ADC pin to high-Z analog input
• Wait for a time delay T
• Acquire analog value

Now, depending on time delay T, the capacitor will have more or less time to charge through the LDR. In order to measure a much higher LDR value, simply wait longer for it to charge! This gives you easy range switching without any extra parts or pins.

Other solutions include :

• ADCs with more bits
• Analog MUX followed by variable gain amplifier
• dannyf's proposal above

Answer 2

Use GPIO pins to short the dividers through different resistors to obtain different "sensitivity".

Answer 3

If you are not careful you will end up with a LOT of I/O lines. What I would do is to use a 16-channel analog multiplexer to switch all the LDRs to a common digitally-controlled pot, and read the voltage from that. This way you need only 7 lines - 4 for the 16-way address, 2 for the SPI lines to the pot, and the ADC input.

OK it takes more ICs, but you lose 12 resistors and 5 I/O lines.