Photodiode / Phototransistor
Optoelectronic
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.
Schematic symbol
Photodiode
How photodiode / phototransistor appears in a circuit diagram.
Types & variants
Key specs
Spectral peak
nmWavelength of best sensitivity — match your source (940 nm IR pairs).
Responsivity / current
A/WOutput per unit light; phototransistors quote collector current at a test lux.
Response time
sPhotodiodes: ns–µs; phototransistors: tens of µs.
Dark current
nALeakage with no light — the noise floor.
Laws that govern it
A photodiode is a diode with a photocurrent term added: light shifts the whole I–V curve down, which is what you measure.
- Inverse square law of illuminationE = I / d²
The illuminance reaching the die falls as 1/d², so doubling the distance to the source quarters your signal.
- Ohm's lawV = I × R
In photoconductive mode the photocurrent is turned into a voltage across a load or feedback resistor — V = I × R does the conversion.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
How it compares
- Photoresistor (LDR) vs Photodiode
Side by side with Photoresistor (LDR) — specs, variants and which one the job calls for.
Markings
Look like clear or black-tinted LEDs; black package = IR filter. Shorter lead = cathode (photodiode) or collector (check datasheet).
Standard values
BPW34 (photodiode), PT334/TEPT5600 (phototransistors), 940 nm IR pairs.
How to choose
Photodiode + transimpedance amp for speed/linearity (light meters, high-rate data); phototransistor when you just need a decent signal cheaply (object detection, remote receivers).
Pinout & package
2 leads. Photodiode is used reverse-biased; phototransistor's leads are collector/emitter — base is the light.
Example circuits
- IR remote receiver (with 38 kHz demodulator module)
- Break-beam object counter
- Heart-rate pulse sensor
Common failures
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.
How to test
Multimeter diode mode across a photodiode while waving a phone flashlight or IR remote at it — the reading changes with light. Phototransistor: resistance C–E drops from ~open to a few kΩ under light.
Substitutes
Photodiode ↔ phototransistor with circuit tweaks; LDR for slow ambient sensing; packaged ambient-light ICs (BH1750) when you want calibrated lux over I2C.