All component comparisons
Photoresistor (LDR) vs Photodiode
Photoresistor (LDR) and Photodiode / Phototransistor compared — what each is better at, and which one the job actually calls for.
An LDR is a resistor that changes value with light — cheap, easy, and entirely adequate for deciding whether it is dark yet. But it is slow (tens of milliseconds), varies wildly from part to part, and is not calibrated to anything. A photodiode produces a current proportional to incident light and responds in microseconds or faster, which is what you need for measurement, for data, or for any light that is modulated rather than merely present.
What each one is
Photoresistor (LDR)
A light-dependent resistor whose resistance decreases with increasing light intensity. Used to detect light levels or create light-sensitive circuits.
Photodiode / Phototransistor
Light in, current out. A photodiode produces a tiny current proportional to light — fast and linear. A phototransistor adds gain: more signal, slower and less linear. The receiving half of every IR link.
Key specs
Photoresistor (LDR)
- Dark resistance(kΩ (MΩ))
- Resistance in complete darkness (typically 1MΩ to 10MΩ).
- Light resistance(Ω (kΩ))
- Resistance under bright light (typically 500Ω to 5kΩ).
- Spectral response(nm)
- Wavelength range it's sensitive to (typically 400-700nm for CdS).
- Response time(ms)
- Time to change resistance when light level changes (typically 10-100ms).
- Power rating(mW)
- Max power it can handle (typically 50-100mW).
Photodiode / Phototransistor
- Spectral peak(nm)
- Wavelength of best sensitivity — match your source (940 nm IR pairs).
- Responsivity / current(A/W)
- Output per unit light; phototransistors quote collector current at a test lux.
- Response time(s)
- Photodiodes: ns–µs; phototransistors: tens of µs.
- Dark current(nA)
- Leakage with no light — the noise floor.
Types you will meet
Photoresistor (LDR)
- CdS (Cadmium Sulfide)
- CdSe (Cadmium Selenide)
- PbS (Lead Sulfide)
- InSb (Indium Antimonide)
- Ge (Germanium)
- Si (Silicon)
- Cadmium-free alternatives
Photodiode / Phototransistor
- PIN photodiode
- Phototransistor (2-lead)
- IR-filtered (940 nm)
- Ambient light sensor ICs
- Avalanche photodiode (specialist)
How to choose
Photoresistor (LDR)
1) Match spectral response to your light source (e.g., CdS for visible light). 2) Consider required light/dark resistance ratio. 3) Check response time for your application. 4) Ensure power rating is sufficient.
Photodiode / Phototransistor
Photodiode + transimpedance amp for speed/linearity (light meters, high-rate data); phototransistor when you just need a decent signal cheaply (object detection, remote receivers).
How each one fails
Photoresistor (LDR)
Becomes unresponsive (fails to change resistance) with age or exposure to high humidity. Cracking or discoloration of the epoxy coating.
Photodiode / Phototransistor
Saturation from ambient sunlight swamping the signal, dead from reverse-polarity abuse, or 'failed' units that are really pointing the wrong way — these are directional.
Full reference for each part
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