Light dependent resistors (LDRs), also called photoresistors, change there resistance based on the amount of light falling on them. When measured with my LCR meter, I found that my LDRs gets down to close to 0Ω of resistance under extremely bright light, and over 200 million ohms of resistance if it gets dark enough.
The ad above is where I got my LCR meter. It is not a replacement for a multimeter, but is a nice additional meter for getting more info about inductor, capacitor and resistor components than you can get from a standard multimeter.
I assume you already know…
Min/max limit diagram
- Series resistance will make sure that the total resistance will never fall below the extra resistance no matter how bright it is on the LDR. The added resistance will add up with the LDR resistance.
- Parallel resistance will make sure that the total resistance never goes above the parallel resistance no matter how dark it is on the LDR.
- Combined. You can put resistance in series with the LDR while also putting resistance in parallel with the LDR.
How to set LDR min/max limits video
Demonstrations of the circuit fragments shown above.
Good topics to learn next
- Light Emitting Diodes – Indicator LEDs
- Capacitor component
- Capacitor RC time constant
- NPN Bipolar Junction Transistors (BJTs)
- 555 timer bistable mode – Flip flip basic circuit