Photoresistor (LDR) Symbol
Definition: The Photoresistor (LDR) symbol represents a two-terminal passive resistor whose resistance decreases as incident light intensity increases, drawn in schematics as the standard resistor body (IEC 60617: filled rectangle; ANSI Y32.2/IEEE 315: zigzag line) with two inward-pointing arrows to indicate incoming light, designated by the reference designator R or RV followed by a number.
Also known as: LDR, light dependent resistor, photoconductor, CdS cell, photocell, light-sensitive resistor.
What the Photoresistor (LDR) symbol means
The photoresistor (LDR) symbol denotes a passive semiconductor device whose electrical resistance is inversely proportional to the amount of light falling on its active surface. In darkness, a typical cadmium-sulphide (CdS) LDR exhibits resistance values from 1 MΩ upward; in bright sunlight, the resistance falls to 100 Ω or less. The symbol communicates this light-dependent behaviour to anyone reading the schematic, signalling that the component forms part of a light-sensing or light-activated sub-circuit.
In a circuit diagram the LDR symbol replaces a fixed resistor symbol wherever a variable, illumination-controlled resistance is required. Engineers use it in voltage-divider configurations alongside a fixed resistor so that the voltage at the midpoint varies with ambient light. The two light arrows on the symbol serve the same directional convention used for photodiodes and phototransistors, making it immediately clear that the component responds to optical input rather than electrical input.
How to identify the Photoresistor (LDR) symbol
The LDR symbol consists of the standard resistor body — a small rectangle in IEC-style schematics or a zigzag line in ANSI-style schematics — enclosed inside or accompanied by a circle representing the device package. Two short diagonal arrows point inward toward the resistor body from the upper-right direction, indicating incoming photons. These inward-pointing arrows distinguish the LDR from a plain variable resistor (which uses an arrow through or alongside the body) and from a photodiode (which uses the standard diode triangle-bar body with the same arrow convention).
Function in a circuit
An LDR (photoresistor) functions as a photoconductive device: absorbed photons excite electrons into the conduction band of the cadmium-sulphide semiconductor, lowering its resistivity. This variable resistance, placed in a voltage divider or Wheatstone-bridge circuit, produces an analogue voltage proportional to light intensity that can be read by an ADC, compared by a comparator, or used to directly control a transistor or relay. LDRs respond relatively slowly (10–200 ms) compared to photodiodes, making them suitable for ambient-light sensing rather than high-speed optical communication.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-04 (passive components) defines the photosensitive resistor symbol as a filled rectangle (the standard IEC resistor body) with two inward-pointing arrows; the IEC designator for a light-dependent resistor is R with the qualifier 'photo' or simply the LDR annotation on the schematic. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975 represents the LDR as the standard zigzag resistor body with two inward-pointing arrows and the same circular enclosure; the designator is R or RV (for variable resistor) followed by a reference number. |
| Key difference | The only visual difference is the resistor body shape: IEC uses a small filled rectangle whereas ANSI/IEEE uses a zigzag line. The two-arrow light indicator and circular package outline are identical in both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| a | A |
| b | B |
Typical values
Dark resistance: 1 MΩ–10 MΩ (typical); Bright resistance (1000 lux): 50 Ω–1 kΩ (typical); Maximum power dissipation: 50–200 mW; Peak spectral response: 540–620 nm (CdS, green-yellow region); Response time: 10–200 ms (rise), 20–500 ms (fall); Operating voltage: typically 5–150 V DC or AC.
Where the Photoresistor (LDR) symbol is used
- Automatic street-light and garden-light controllers that switch on at dusk and off at dawn
- Analog light-meter circuits and exposure-control systems in photography equipment
- Alarm systems that detect beam interruption by a change in light level across the LDR
- Brightness-adaptive backlight dimming circuits in displays and instruments
- Flame-detection safety interlocks in gas appliances and industrial burner controllers
- Arduino and microcontroller light-sensing projects using voltage-divider readout on an ADC pin
- Solar-tracker circuits that steer a panel toward the brightest point of the sky
Example
In a simple night-light circuit, an LDR and a 10 kΩ fixed resistor form a voltage divider between +5 V and GND; the midpoint feeds the non-inverting input of an LM393 comparator, whose output drives the base of a transistor switching a 12 V LED strip relay. The schematic shows the LDR symbol (rectangle with two light arrows, designated R1) in series with the fixed resistor R2, clearly indicating that the circuit activates when ambient light falls below the threshold set by the comparator reference voltage.
Key facts
- An LDR (light-dependent resistor) is a two-terminal passive component whose resistance decreases with increasing light intensity, from several megohms in darkness to below 1 kΩ in bright light.
- The LDR symbol is the standard resistor body (IEC rectangle or ANSI zigzag) with two inward-pointing diagonal arrows indicating incoming photons; the arrows distinguish it from a plain variable resistor.
- Most commercially available LDRs use cadmium sulphide (CdS) as the photosensitive material, with peak spectral sensitivity in the 540–620 nm (green–yellow) range, close to human eye peak response.
- IEC 60617-04 governs the LDR symbol in European/international schematics; ANSI Y32.2-1975 / IEEE 315-1975 governs the equivalent symbol in North American schematics.
- The LDR has two terminals (pins A and B) and is non-polarised: it operates correctly regardless of which terminal connects to the higher voltage.
- Response time is 10–200 ms for illumination rise and up to 500 ms for the dark recovery fall, making LDRs unsuitable for high-frequency optical signals but adequate for ambient-light and slow-change applications.
- The reference designator for a photoresistor in schematics is R or RV (following IEC 60617 and IEEE 315 conventions), sometimes supplemented with a 'photo' or 'LDR' annotation.
Frequently asked questions
What does the LDR symbol look like in a schematic?
The LDR (photoresistor) symbol looks like a standard resistor body — a filled rectangle in IEC schematics or a zigzag line in ANSI schematics — with two short diagonal arrows pointing inward toward the body from the upper-right, indicating incoming light. The symbol is often enclosed in a circle representing the device package.
What does the photoresistor (LDR) symbol mean?
The LDR symbol means the component is a light-sensitive resistor whose resistance decreases as light intensity increases. In a schematic it signals that the node is part of a light-detection, ambient-sensing, or light-activated switching circuit.
What is the difference between IEC and ANSI symbols for an LDR?
In IEC 60617-04 schematics the LDR is drawn with a filled rectangle as the resistor body; in ANSI Y32.2 / IEEE 315 schematics it is drawn with a zigzag line. Both versions carry two inward-pointing arrows to indicate light sensitivity. The meaning and circuit function are identical.
Is an LDR polarised?
No. An LDR is a non-polarised, passive two-terminal component. It operates identically regardless of which terminal (A or B) connects to the higher potential, and it works on both DC and AC circuits within its rated voltage.
What is the designator letter for a photoresistor on a schematic?
The reference designator for a photoresistor follows the resistor convention: R or RV (variable resistor), followed by a sequential number such as R1, R2, or RV1. Some schematics add a 'photo' superscript or the abbreviation LDR to distinguish it from fixed resistors.
How fast does an LDR respond to changes in light?
A typical CdS LDR has a rise response time of 10–200 ms when light is applied and a fall response time of 20–500 ms when light is removed. This relatively slow speed is adequate for ambient-light sensing and dusk/dawn switching but rules out LDRs for high-speed optical communication, where photodiodes or phototransistors are used instead.
What is a typical resistance range for an LDR?
A typical CdS LDR has a dark resistance of 1–10 MΩ and a bright-light resistance (at 1000 lux) of 50 Ω to 1 kΩ. The exact values depend on the device size and material; manufacturers specify the resistance at defined illumination levels (e.g. 10 lux, 100 lux, 1000 lux) in the component datasheet.
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