Phototransistor Symbol

Definition: The Phototransistor symbol represents a bipolar transistor whose base current is generated by incident light rather than an electrical connection, drawn per IEC 60617 as an NPN transistor (usually within an envelope circle) with two parallel arrows pointing toward it and — in most devices — no external base lead, leaving only Collector and Emitter terminals.

Also known as: phototransistor, photo transistor, NPN phototransistor, light-sensitive transistor, optical detector transistor, photodarlington, IR detector transistor.

What the Phototransistor symbol means

The phototransistor symbol denotes a light detector with built-in amplification: photons striking the exposed base-collector junction generate a photocurrent exactly as in a photodiode, but that photocurrent then acts as base drive and is multiplied by the transistor's current gain (hFE), so the collector current is typically 100–1,000 times larger than an equivalent photodiode would deliver. This makes phototransistors the workhorse for simple, cheap light sensing where sensitivity matters more than speed — line-following robots, optical limit switches, IR proximity sensors and the receiving side of most optocouplers.

The missing base lead on the symbol is meaningful: light is the base signal, so most two-lead phototransistors physically have no base pin. Three-lead variants do bring the base out so designers can bias the operating point or bleed off leakage for faster turn-off, and their symbol shows the base lead drawn conventionally alongside the light arrows.

How to identify the Phototransistor symbol

Look for the familiar NPN transistor symbol — vertical bar (base region), angled collector line, angled emitter line with its outward arrow — enclosed in a circle, with two small arrows pointing in toward the device, and typically no lead attached to the base bar. The inward arrows distinguish it from a plain transistor; the absent base lead distinguishes it from an ordinary NPN receiving those arrows by coincidence of layout. The emitter arrow still points outward (NPN); a PNP phototransistor (rare) would show the emitter arrow pointing in.

IEC and ANSI draw it essentially identically; the envelope circle is more consistently present in IEC-style drawings. Inside an optocoupler symbol you will see the same phototransistor facing an LED across the package outline, with the LED's emission arrows doubling as the coupling indication.

Function in a circuit

In a circuit the phototransistor conducts collector-emitter current proportional to illumination. The standard arrangement is common-emitter: emitter to ground, collector through a load resistor to the positive rail, output taken at the collector — bright light saturates the transistor and pulls the output low, darkness lets the resistor pull it high. Swapping the resistor to the emitter side gives a non-inverting (common-collector) response. The load resistor value trades sensitivity against speed: larger values give more output swing per lux but slow the response through the RC formed with the device's relatively large collector-base capacitance.

That capacitance, Miller-multiplied by the gain, is why phototransistors respond in microseconds rather than the nanoseconds of photodiodes, and why their linearity is poorer (hFE varies with collector current). Photodarlingtons stack a second transistor for another ~100× sensitivity at the cost of ~50× slower response and a higher saturation voltage.

Standards: IEC vs ANSI

Terminals / pins

Typical values

Typical NPN phototransistor ratings: VCEO 30–50 V (some to 70 V); maximum continuous collector current 20–100 mA; on-state collector current 0.5–10 mA at ~1 mW/cm² irradiance; dark current under 100 nA at 10–20 V; rise/fall times 2–15 µs with a 1 kΩ load (photodarlingtons 50–500 µs); collector-emitter saturation voltage ~0.2–0.4 V (darlington ~0.6–1 V). Spectral peaks near 850–940 nm for IR types (PT334, TEPT4400 class) and ~570 nm for ambient-light types like the TEPT5600. Optocoupler examples: 4N35 (CTR ≥ 100%), PC817 (CTR 50–600%).

Where the Phototransistor symbol is used

• Optocouplers/optoisolators (4N35, PC817) where a phototransistor receives light from an LED across an isolation barrier
• IR proximity, reflectance and line-following sensors on hobby robots (paired IR LED + phototransistor)
• Slotted optical switches and encoder pickups detecting beam interruption for position and RPM sensing
• Ambient-light detection for automatic brightness, night lights and dusk-to-dawn controls
• Object and end-stop detection in printers, vending machines and 3D printers
• Flame and pulse sensors, and legacy punched-tape/card readers built on phototransistor arrays

Example

In an IR break-beam sensor, the Phototransistor symbol's Collector pin connects through a 10 kΩ resistor to +5 V and its Emitter pin to ground, with the output taken at the collector node into an Arduino digital input; while the opposing IR LED beam reaches the lens, the phototransistor saturates and the output reads LOW (~0.2 V), and the instant an object blocks the beam the collector rises to +5 V — no base connection exists or is needed, because the light beam itself is the base signal.

Key facts

• The symbol is an NPN transistor with two arrows pointing toward it; most phototransistors have only Collector and Emitter leads because light replaces the base connection.
• Light generates base photocurrent that is amplified by the transistor's hFE, giving 100–1,000× the output current of a bare photodiode under the same illumination.
• The amplification costs speed: response is in microseconds (photodiodes: nanoseconds) due to Miller-multiplied collector-base capacitance.
• Standard hookup is common-emitter with a collector load resistor: light pulls the output low, dark lets it float high — an inverted logic signal.
• The load resistor sets the sensitivity/speed trade-off; bigger resistance means more sensitivity but slower response.
• Most hobby phototransistors are tuned to near-infrared (850–940 nm) to pair with IR LEDs and reject visible ambient light behind dark epoxy lenses.
• A photodarlington adds a second gain stage for ~100× more sensitivity but ~50× slower switching and higher saturation voltage.
• Inside almost every optocoupler, the output device is a phototransistor; the coupling ratio is specified as CTR (current transfer ratio) in percent.

Frequently asked questions

Why does the phototransistor symbol have no base lead?

Because the base signal is light. Photons striking the exposed base-collector junction generate the base current internally, so a physical base terminal is unnecessary and most two-lead devices simply omit it — the symbol honestly reflects the package. Three-lead phototransistors do exist and their symbol shows the base drawn normally; the extra lead lets designers add a base-emitter resistor to speed turn-off or set a bias point.

How do you tell the phototransistor symbol from a normal NPN transistor symbol?

Two differences: the pair of small arrows pointing inward at the device (light arriving), and — on most devices — the missing base lead. A plain NPN has no arrows aimed at it and always shows its base connection. The emitter arrow on the transistor itself still points outward exactly as on any NPN; don't confuse that emitter arrow with the light arrows, which sit outside the symbol body.

Phototransistor or photodiode — which should I use?

Use a phototransistor when you want a big, easy signal for on/off light detection — beam-break sensors, line followers, opto-isolation — since its internal gain delivers milliamps without an amplifier. Use a photodiode when you need speed (data links, encoders at high RPM, LiDAR) or measurement-grade linearity (lux meters, oximetry), accepting that you'll add a transimpedance amplifier to boost its microamp output.

How do you wire a phototransistor to an Arduino or microcontroller?

Common-emitter is simplest: emitter to GND, collector to a 10 kΩ pull-up resistor to 5 V (or 3.3 V), and take the signal from the collector into a digital or analogue input. Light = LOW, dark = HIGH. For a non-inverted signal, put the resistor between emitter and ground instead and read the emitter. Adjust the resistor between roughly 1 kΩ (fast, less sensitive) and 100 kΩ (sensitive, slower) to suit your light levels.

What does CTR mean on an optocoupler's phototransistor?

CTR — current transfer ratio — is the phototransistor's output collector current divided by the LED's input forward current, expressed as a percentage. A PC817 with CTR of 100% delivers 10 mA of collector current for 10 mA of LED current. CTR spreads widely between parts (50–600% bins for the PC817), falls as the LED ages, and varies with temperature — so designs should work at the minimum guaranteed CTR, not the typical value.

Related symbols

• IR Receiver symbol
• Photoresistor (LDR) symbol
• NPN Transistor (BJT) symbol
• Optocoupler symbol
• Photocell / Dusk Sensor symbol
• Photoelectric Sensor symbol

Place the Phototransistor symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.