Have you measured the 'full dark black' resistance of your cells? You need to place them in a thick black box with the edges and leads foam sealed to eliminate absolutely every photon possible. Light sensitivity for our eyes and these CdS isn't a linear process. Is something like this possible with a simple analog circuit? Or am I better off using some other sensor instead i.e. In other words I'd like a circuit that converts a signal from a device whose resistance decreases exponentially with increasing illumination to a signal thats voltage decreases (or increases) linearly with increasing illumination. Anything brighter than that should be clamped at 5V. This value should appear as 5V on the circuit output. The sensor maximum should be about the brightness of a small in-door room lit by a single 800 lumen light bulb. In this case the circuit output should show 0V. The sensor minimum should be in a complete darkness like deep in an undergound cave without any light sources. by using some op-amp wizardry? The circuit output should span from 0 to 5 VDC. I was wondering, if there is a circuit to "linearize" this output i.e. I know I could create a "gamma correction" curve in software, but I would rather measure a linear signal to begin with. The problem with this approach is that the ADC steps aren't equal. Most circuits I googled show the LDR in series with another resistor to create a voltage divider between Vcc and GND, and the tap point in between them is connected to an ADC. I've already done some homework, and apparently the resistance of a light-dependent resistor is not linear, but rather falls exponentially as the illumination increases. ![]() It doesn't have to be ultra-accurate, but I'd like to at least have similar precision across the full measuring range. I'd like to measure ambient light by using a CdS photoresistor (got a bunch of them from ebay) on an Arduino.
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