DD-T21 / DD-T31 / DD-D21 Compact Self-Contained Photoelectric Sensor

Name: DD-T21 / DD-T31 / DD-D21 Compact Self-Contained Photoelectric Sensor
Brand: DAIDISIKE
SKU: dd-t21-dd-t31-dd-d21-compact-self-contained-photoelectric-sensor

Compact self-contained NPN photoelectric sensor with built-in amplifier: wire to 12-24 VDC, get a 1 ms response.

DD-T21 / DD-T31 / DD-D21 Compact Self-Contained Photoelectric Sensor - product photo

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Technical Specifications

Models	DD-T21 / DD-T31 / DD-D21
Output type	NPN, Normally Open (NO)
Detection method	DD-T21 / DD-T31: through-beam; DD-D21: diffuse-reflective
Detection distance	DD-T21: 1.5 m; DD-T31: 2.5 m; DD-D21: 150 mm (adjustable sensitivity)
Light source	Infrared (confirmed for DD-T21 / DD-T31 through-beam models)
Supply voltage	DC 10.8-26.4V (nominal 12-24VDC)
Current consumption	20 mA max
Response time	1 ms
Protection rating	IP40
Minimum detectable object	6 mm

Overview

DD-T21, DD-T31, and DD-D21 are compact, self-contained photoelectric sensors with the amplifier circuit built into the sensor body, so no separate external amplifier unit is required, unlike a fiber-optic head plus external amplifier setup. DD-T21 and DD-T31 are through-beam (transmitter + receiver) versions with an infrared light source, detecting at 1.5 m and 2.5 m respectively, while DD-D21 is a diffuse-reflective version with a 150 mm sensing range and an on-site adjustable sensitivity control.

All three models output NPN, Normally Open (NO), and run on a 10.8-26.4 VDC (nominal 12-24 VDC) supply drawing 20 mA maximum, with a 1 ms response time and a minimum detectable object size of 6 mm. The series suits machine builders, panel and equipment integrators, and conveyor OEMs who need economical, single-point object or pass detection inside dry, indoor equipment enclosures (IP40).

Features & Benefits

Self-Contained, Amplifier-Built-In Design

The amplifier circuit is built directly into the sensor body, so you wire straight to a 12-24 VDC supply and the sensor is ready to detect. No separate amplifier unit or extra mounting rail is needed, unlike a fiber-optic-head-plus-amplifier setup.

Two Through-Beam Ranges Plus One Diffuse Option

DD-T21 (1.5 m) and DD-T31 (2.5 m) through-beam models suit cross-structure detection where both sides of the gap are accessible; DD-D21 (150 mm, adjustable sensitivity) suits single-side mounting where only one side can be reached.

1 ms Response Time

Fast enough to reliably catch objects passing at speed without missed counts, suiting small conveyor and in-machine detection points.

Low Current Consumption (20 mA Max.)

Multiple sensing points can be deployed on one supply line without straining the power budget.

On-Site Adjustable Sensitivity (DD-D21)

The diffuse model's sensitivity control lets integrators tune detection to the actual target and background, reducing false triggers caused by background reflectivity variation.

Detects Small Targets Down to 6 mm

The series' minimum detectable object size of 6 mm suits small parts, thin components, and small carriers in compact machine layouts.

Wide Supply Voltage Tolerance (10.8-26.4 VDC)

Tolerates typical industrial 24 VDC supply fluctuation without needing a dedicated voltage regulator.

Applications

In-Machine Workpiece Pass/Position Detection — The compact body and self-contained amplifier fit tight in-machine mounting spaces without needing a separate amplifier enclosure.

Small Conveyor Cross-Detection (Within 2.5 m Span) — The DD-T21 (1.5 m) and DD-T31 (2.5 m) through-beam models cover typical narrow-to-medium conveyor widths.

Tray and Carrier Detection on Electronics Assembly Lines — 1 ms response time and 6 mm minimum detectable object support small, fast-moving carriers common on assembly lines.

Multi-Point Low-Cost Detection Layout in Dry Indoor Rooms — 20 mA low current draw allows many sensing points on one supply circuit; IP40 rating suits dry indoor use only.

Single-Side Object Presence Detection Where Only One Side Is Accessible — DD-D21's diffuse-reflective design needs mounting on only one side, unlike through-beam types that require access to both emitter and receiver.

Fine-Tuned Detection Against Varying Target/Background Materials — DD-D21's adjustable sensitivity lets integrators tune the trigger threshold on-site to match the actual target and background.

Economical Replacement Point for Fiber-Optic-Head + Amplifier Setups — The built-in amplifier eliminates the extra amplifier unit and its mounting/wiring, reducing panel space and part count.

Models & Ordering Information

Through-Beam (Transmitter + Receiver)

DD-T21DD-T31

DD-T21 detects at 1.5 m; DD-T31 detects at 2.5 m. Both use an infrared light source.

Diffuse-Reflective

DD-D21

150 mm detection range with on-site adjustable sensitivity.

How to Select

Choose through-beam (DD-T21 or DD-T31) when both sides of the gap are accessible for mounting an emitter and receiver; choose diffuse-reflective (DD-D21) when only one side is accessible or the target itself must reflect the beam back.
For a gap or span up to 1.5 m, select DD-T21 (through-beam); up to 2.5 m, select DD-T31 (through-beam).
For a target within 150 mm of the sensor face on a single-side mount, select DD-D21 (diffuse, adjustable sensitivity).
All three models in this series are fixed at NPN, Normally Open (NO) output; no PNP or Normally Closed (NC) variant is confirmed in current product data.
Confirm the smallest object or feature to be detected is 6 mm or larger, the series' minimum detectable object size across all three models.
Confirm line voltage falls within 10.8-26.4 VDC (nominal 12-24 VDC) and that the circuit can support 20 mA maximum per sensor times the number of points deployed.
These sensors are rated IP40 (protected against solid objects larger than 1 mm; no water protection); use only in dry, indoor equipment environments, not for washdown, outdoor, or wet locations.

📘 Need the full method? NPN vs PNP Sensor Outputs Explained

When NOT to specify the DD-T21 — and what to use instead

Outdoor use, washdown, or any wet/splash-exposed mounting location

Why not: This series is rated IP40 - protected against solid objects larger than 1 mm, with no water protection stated.

Use instead: an IP65/IP67-rated sensor such as the strip-beam DK-D461/D561 or the BGS-CX61

A PNP output or Normally-Closed (NC) logic is required

Why not: All three models (DD-T21, DD-T31, DD-D21) are documented as NPN, Normally Open (NO) only, with no confirmed PNP/NC variant.

Use instead: the BGS-CX441/CX442 (NPN/PNP with field-selectable N.O./N.C.) or the E3JK series (M1-M4 transistor logic)

The smallest feature to detect is under 6 mm

Why not: The published minimum detectable object size for this series is 6 mm.

Use instead: the DX-15/30 ultra-thin self-contained series, rated down to a 1 mm minimum detectable object

Through-beam span needs to exceed 2.5 m

Why not: The longest through-beam model in this series, DD-T31, is rated to 2.5 m.

Use instead: the BX-H4000 long-range background-suppression sensor (up to 4 m, single-sided) or a longer-range through-beam pair

Personnel guarding or any safety-related presence detection

Why not: No safety rating or safety certification is published for this series.

Use instead: a certified safety light curtain (e.g. DQC/DQA series) for personnel protection

Specifications we do not publish yet

We only publish a figure once we are confident it is true. The items below are still being validated through further testing and real-world customer feedback, so for now we would rather leave them open than guess. If one of them matters to your design, ask us for the latest validated status.

Hysteresis
MTBF
Switching frequency (only response time is published)
Operating and storage temperature range
Cable/connector type
Exact infrared light source wavelength (nm)
Certification status (documents said to be available on request but not specified)

Frequently Asked Questions

What does "self-contained" or "built-in amplifier" mean for this photoelectric sensor?

The amplifier circuit that processes the light signal into an on/off output is built directly into the sensor body. You wire the sensor straight to a 12-24 VDC supply and it outputs a switching signal, no separate external amplifier unit is needed, unlike a fiber-optic-head sensing setup.

What is the difference between DD-T21, DD-T31, and DD-D21?

DD-T21 and DD-T31 are through-beam (transmitter + receiver) sensors using infrared light, detecting at 1.5 m and 2.5 m respectively. DD-D21 is a diffuse-reflective sensor with a 150 mm detection range and adjustable sensitivity, requiring mounting on only one side.

What output type does this series use?

All three models (DD-T21, DD-T31, DD-D21) output NPN, Normally Open (NO).

What is the response time?

1 ms.

What is the power supply requirement?

10.8-26.4 VDC, nominal 12-24 VDC, with a current consumption of 20 mA maximum.

What is the smallest object this sensor can detect?

6 mm minimum detectable object size, applicable across the series.

Is this sensor waterproof or suitable for outdoor/washdown use?

No. It is rated IP40, which protects against solid foreign objects larger than 1 mm but provides no water ingress protection. It should be used in dry, indoor environments only.

Does DD-D21's sensitivity adjustment help reduce false triggers?

Yes. DD-D21 has an on-site adjustable sensitivity control that lets you tune detection sensitivity to match the actual target and background, which helps reduce false triggers caused by background reflectivity variation.

Is a PNP or Normally Closed (NC) version available?

Not confirmed in current product data, this series is documented as NPN, NO only. Please contact us to check availability of PNP/NC variants for your application.

Can you provide certification documents for this series?

Certification documents are available on request.

Related Categories and Example Products

Explore other functions that may be needed in a machine or line. The products below are category examples, not pre-validated electrical or safety combinations. Confirm each interface and application separately: Proximity Sensors · Laser Switch Sensors · Relay & Timer Modules