Trigger a vision light on for two seconds at the inspection moment

The robot sets a part down under the quality camera and holds it there. You want the light to switch on the moment the part is in place, hold for exactly two seconds, then go dark until the next part. The clean way to build that is a light controller whose trigger input follows a digital output: the robot or PLC owns the two-second window, the controller switches the load, and the camera fires inside the window. You get the wiring order, the decision between a continuous window and a strobe pulse, and the timing arithmetic. The guide also shows why a dwell of this length is thermally a non-event at rated power, and where the real limits sit.

What you need

• A machine-vision light matched to the field of view. For a single part on a fixture, spot lights in the compact classes cover most quality checks.
• A light controller with a trigger input and enough power per channel for the light you hang on it.
• A source for the window signal: a spare robot digital output, a PLC output, or the strobe output of a smart camera.
• Shielded I/O cable, with the trigger logic matched at both ends; a 24 V sourcing (PNP) output into a 24 V trigger input is the common industrial pairing.

Both lights below run on a 24 V rail; pick by lit area and wavelength:

Continuous window or strobe: pick the drive mode first

Two builds satisfy a two-second light-on requirement, and the choice decides the wiring.

For a quality check whose verdict is recorded, take Build B. The two-second output becomes a light-ready window and the actual illumination is a flash timed to the shutter, so brightness repeats part after part no matter how long the window stays open. A light with an integrated strobe driver can even take the trigger directly and skip the external controller; the trade-offs are compared in integrated strobe drivers in machine vision lights: what they change.

A 4-channel digital control box with a strobe mode, an RS-232 port and 200 W total across its channels switches 24 V lights for either build:

Where brightness stability across a long shift matters most, drive the channel with constant current; this 2-channel high-power unit supplies up to 240 W per channel, 320 W total, to current-driven lights:

Wire and time it, step by step

1. Fix the exposure and switch off the autos. Set exposure time manually and turn off auto exposure and auto gain everywhere; an auto mode inside the window will chase your own light. The failure pattern is described in camera image brightness drifts over time: turn every auto mode off.
2. Run the dwell signal to the controller trigger input. Wire a spare robot digital output to the trigger input, matching the logic at both ends (24 V PNP out, 24 V trigger in). The robot-side technique is laid out in trigger strobe lighting from robot IO digital outputs. If the smart camera leads the cell, its strobe output replaces the robot output; the wiring is in smart camera strobe output: enable it and wire the light.
3. Set the controller mode. Continuous mode for Build A. External-trigger strobe mode for Build B, with the pulse width set equal to the exposure time: shorter loses brightness, longer only collects ambient. The pulse-width mechanics, including motion freeze, are in how strobe pulse width freezes motion blur in machine vision, and the driver's turn-on delay, which runs in microseconds, is quantified in LED strobe rise time: why microseconds of light delay blur your image.
4. Fire the flash inside the shutter. In Build B the chain runs: part-present output arms the window, the camera trigger opens the exposure, and the camera strobe output drives the light while the shutter is open. The full sequence with the reject gate is in machine vision trigger timing: sequence camera, light and reject gate. A black frame while the light visibly fires means the flash missed the exposure; how the strobe pulse lines up with the shutter is worked through in camera strobe output timing: exposure pulse and image timestamp.
5. Set the dwell where the part is known present. On the robot, set the output TRUE from the in-position signal, start a 2,000 ms timer, and drop it when the timer expires. Open the window 100-200 ms after in-position so vibration damps before the first shot. With Build B the dwell can overrun the inspection without cost, because nothing lights up between shots.
6. Prove repeatability over 50 parts. Record the mean gray level of a fixed region of interest across 50 cycles and hold it within ±5%. A brightness step or flicker band inside the run points to exposure and light overlap drifting; the diagnosis is in black bars in some camera images: fix lighting flicker vs exposure.

If doubt remains about overlap, an oscilloscope across the camera trigger and the light input settles it in minutes: oscilloscope check: lighting pulse timing and camera exposure overlap.

Worked example: a 4 W spot light on for 2 s of a 10 s cycle

Inputs:

• Light power P = 4 W: the rated continuous power of the LS-SPT-0133, from its catalog entry.
• Window t_on = 2 s: the cell requirement, light on while the part sits under the camera.
• Cycle time t_cycle = 10 s: an assumed takt of one part every 10 s; scale the result linearly to your own.

The rule in words: energy per part is power times window; duty cycle is window over cycle; average power is rated power times duty cycle.

Arithmetic:

• E = 4 W × 2 s = 8 J per part
• D = 2 s ÷ 10 s = 0.20, i.e. 20%
• P_avg = 4 W × 0.20 = 0.8 W

The trigger side never becomes the limit: controller trigger stages gate in the microsecond range, and reference strobe driver designs demonstrate 200 ns gates, so the dwell command clears a 1 µs minimum by a factor of 2,000,000 (2 s ÷ 1 µs). Duration is not the limiting spec; power and duty are.

Decision: 8 J per event and 0.8 W average sit far inside a 4 W continuous rating, so a continuous-rated spot light holds a two-second dwell cold and Build A is legitimate at any sane takt. The threshold is simple: at rated continuous power, any such window passes; in overdrive mode, no window longer than the rated strobe pulse does.

Checks before you order

• Drive type match. A 24 V constant-voltage light needs a constant-voltage 24 V channel; a constant-current light needs a constant-current channel. Mixing them under-drives or kills LEDs, and the drift physics is compared in constant current vs voltage LED lighting: why voltage drive drifts.
• Channel power budget. Sum every light on the channel and stay under the channel rating: a 4 W spot fits anywhere, but check before hanging a 100 W bar on a small box.
• Trigger logic. PNP 24 V into a PNP 24 V input, or a dry contact into the input the data sheet assigns; mismatches show up as lights that never fire or never turn off.
• Channels for the extras. Keep one channel for the inspection light and give an indicator lamp a second one, so operators see the window too.
• Strobe availability for Build B. Most smart cameras and industrial cameras expose a strobe output; confirm yours before wiring the enable-and-flash scheme.

The controller configurator steps through voltage, channels and trigger type in order, and the lights finder filters spot lights by size, colour and wavelength.

When the usual answer is wrong

Three shortcuts fail:

• Switching the light straight from an I/O port. Camera and robot I/O source trigger-level currents in the tens of milliamps, while a 4 W light on 24 V draws 4 ÷ 24 ≈ 0.17 A. Put the controller between them; what direct driving can and cannot carry is worked through in direct drive LED lighting from a camera: current and wiring basics.
• Overdriving to make the two-second window brighter. Duty cycle, not duration, is the currency; the arithmetic of strobe duty budgets is in strobe machine vision light: overdrive, duty cycle and timing.
• Leaving ambient light uncontrolled. A short exposure taken somewhere inside the dwell collects whatever the hall lighting does, and AC-driven ceiling fixtures band the frame at twice the mains frequency. Enclose the station or keep Build B's short flashes; the beat and its cures are in camera flicker under artificial lighting: diagnose and fix.

Commissioning checklist

• Exposure fixed, every auto mode off: nothing in the loop may chase the light.
• Trigger wired into the controller input, not into the light leads: the controller does the load switching.
• Build B only: controller pulse width equals exposure time, so the flash lands fully inside the shutter.
• Dwell set to 2,000 ms on the robot or PLC side: one device owns the window length.
• 50-part run with mean gray inside ±5%: repeatability proven before sign-off.
• Output verified low between parts: a stuck-high channel ages the LEDs and shifts the next cycle's contrast.

Send us the cycle timing through the contact page if your takt runs under a second or the light is bigger than a spot class, and we will size the channel and duty budget with you. The rest of the illumination techniques, from geometry to strobe timing, sit on the lighting topic page.