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6. LDR: sensing light

Bench build · Fundamentals · Beginner · about 20 minutes

Step 6 of the automatic light: R2 47 k from the supply to SENSE, LDR1 from SENSE to ground. Bright: the LDR is low, SENSE is low (well under 1 V). Dark: the LDR is high, SENSE rises to several volts. Nothing uses SENSE yet: measure it. Component 6 of 14: the ldr (photoresistor). A light-dependent resistor is a resistor whose value falls with light: a few kΩ in daylight, about a megohm in the dark. On its own it does nothing; in a voltage divider it turns light into a voltage that the rest of the circuit can use.

6. LDR: sensing light: the finished build running on the simulator's bench

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Parts list

Test procedure

Each test has an expected value and a pass range, like a factory test sheet:

  1. SENSE in daylight (300 lux)
  2. SENSE in the dark (1 lux)

How it works

The ldr (photoresistor). A light-dependent resistor is a resistor whose value falls with light: a few kΩ in daylight, about a megohm in the dark. On its own it does nothing; in a voltage divider it turns light into a voltage that the rest of the circuit can use.

Reading it. GL55xx: the two digits after 55 set the range (GL5528: about 10–20 kΩ at 10 lux, 1 MΩ dark). It is slow (tens of ms) and not precise, which is fine for “is it dark?”.

Testing it. Ω across it: cover it with your thumb and the reading climbs; shine a light and it drops. If it does not move, it is dead or the lead is open.

In the build. R2 47 k from the supply to SENSE, LDR1 from SENSE to ground. Bright: the LDR is low, SENSE is low (well under 1 V). Dark: the LDR is high, SENSE rises to several volts. Nothing uses SENSE yet: measure it.

Builds on lesson 5: everything from before is still there.

Next: lesson 7, npn transistor.

Things to try

  1. Click the LDR and drag its light level from 1000 to 1 lux while metering SENSE.
  2. Swap R2 and the LDR: SENSE now rises with light instead of darkness.

Background

One circuit, built up one part at a time: by lesson 14 it is a solar-powered, dusk-sensing, motion-triggered timed lamp with an alarm.

More builds: fundamentals