Why is the geometric mean the best resistor value?

A voltage divider produces the largest output swing when the fixed resistor is the geometric mean of the LDR's two extremes, that is the square root of bright resistance times dark resistance. This centres the transition and spreads the voltage change evenly across the light range.

Should the LDR go on the top or bottom of the divider?

Either works. With the LDR on the bottom (between the analog pin and ground) more light gives a lower voltage. With the LDR on top (between supply and the pin) more light gives a higher voltage. The tool computes both output extremes for whichever you choose.

What output voltage does the calculator report?

It reports the divider output at the bright and dark resistance extremes. The output equals supply times R_lower divided by (R_upper plus R_lower), where the LDR and the fixed resistor occupy the upper and lower legs depending on the position you select.

What resistor should I actually buy?

Round the geometric-mean result to the nearest common E12 value such as 10 kΩ or 22 kΩ. The exact value is not critical because microcontroller code thresholds the reading; being within a factor of two of the ideal still gives good sensitivity.

Does this work for a 3.3 V board like an ESP32?

Yes. Set the supply to 3.3 V and the tool scales all output voltages accordingly. The recommended series resistor depends only on the LDR resistance range, not the supply, so it is the same for 3.3 V and 5 V boards.

LDR Voltage Divider Calculator for DIY Electronics

A light-dependent resistor (LDR, or photocell) changes resistance with light, but a microcontroller can only read voltage. A voltage divider converts that resistance change into a readable voltage. This tool picks the fixed resistor that gives you the widest, most sensitive voltage swing across your light range.

How it works

An LDR varies enormously — often from about 1 kΩ in bright light to 1 MΩ in darkness. Pair it with a fixed resistor R to form a divider. The output voltage depends on which leg each component occupies.

With the LDR on the bottom leg (pin to ground):

V_out = V_supply * R_ldr / (R_fixed + R_ldr)

The voltage swing is largest when the fixed resistor sits at the geometric mean of the LDR extremes:

R_fixed = sqrt(R_bright * R_dark)

For a GL5528 (1 kΩ to 1 MΩ) that is sqrt(1000 * 1000000) ≈ 31.6 kΩ, so a common 33 kΩ resistor is ideal.

Why the geometric mean

If you used a value near either extreme, the divider would saturate at one end of the range and the output would barely move across most light levels. The geometric mean centres the transition so the output voltage moves smoothly and the analog-to-digital converter sees the full usable range.

Tips

Round to the nearest E12 value (10 k, 12 k, 15 k, 18 k, 22 k, 33 k…). The exact value is not critical.
For an outdoor sensor that mostly distinguishes day from night, bias the resistor toward the dark resistance so the threshold sits near twilight.
Add a small capacitor (100 nF) across the analog pin to ground to filter flicker from mains-powered lighting.

All calculations run locally in your browser.