Detecting red laser with an IR receiver/photodiode?

I'm currently working on a project where I need to start a stopwatch when the first person runs past the starting line, so the detection must be really fast and must detect quick movements.

I have a 5mw red laser and I need something to detect when the laser beam is interrupted.
The detection must be fast. A photoresistor isn't fast enough. What can I use?

I've been thinking about using an IR receiver or a photodiode.

The distance between the laser and the receiver to detect the beam is 5m.

The receiver must detect really quick movements (as in) people sprinting through the beam.

I basically need the 0/1 logic.

If the laser beam is present = 1
If the laser beam is interrupted = 0

What exactly can I use to detect this?

Thank you!

Photodiode or phototransistor.

Red LED as a photodiode might also work.

Some IR receivers have a filter (they look black) so won't be sensitive to visible red.

I assume that this is for athletics timing so you need something like 1 mS resolution - not really fast. Yes a photodiode (or LED) or phototransistor should be fine. You might also want to consider the effect of sunlight if outdoors or fluorescent lights if indoors either of which could cause a problem. One improvement could be to modulate the laser with say 10 kHz pulses and look for the presence of those in the photodiode output.


A photodiode/phototransistor will work fine but you will need to tweak it depending on how sunny the day is. Do not use a photoresistor.

An IR receiver (TSSP4038, specifically) operates with a granularity of 1/38000th of a second and will give you a digital output when the beam is broken. As long as you don't have direct sunlight on it you won't get any interference from ambient light. It will work fine with a red laser.

How do you plan to align the laser? Slightest movement on transmitting side will cause significant deflection on receiving side.
On a very similar project (that is still in development) after several iteration and modification I discarded the laser approach as being too fiddly and too sensitive to bumps and too prone to misalignment. Now I'm using high power, narrow beam IR LEDs - TSHF5210. For the receiving end I use TSOP31238.

I'm covering the distance of 7m indoors and have three parallel lines. I'm driving my LEDs at 75mA and I am experiencing some issues that I believe I will fix when I add another LED in parallel to increase the irradiance.
Currently, maximal range where I still get reliable and robust "connection" between transmitter and receiver is about 4 to 5m.

I've pondered the same thing in the past but haven't sacrificed an IR module yet. I wonder if the small bubble filter can be removed or sanded down to expose the detector underneath for better sensitivity to visible light?

Wouldn't that just let in additional light frequencies that aren't desired?

Wouldn't that just let in additional light frequencies that aren't desired?

If the laser is being modulated at 38khz and the IR receiver is a 38khz IR receiving unit, then maybe the ambient light might not be a significant factor. There are other projects like below that might be of interest. I made a parallel port IR receiver a long time ago for changing the tv channel that was interesting.

The modulated approach is a good idea, to avoid sensitivity to ambient light. Actually a focused IR LED would probably make a decent source for the beam. Indeed, the AFC would be correctly set by the continuous beam. But the system should be calibrated to avoid oversensitivity. The signal might bounce around in the environment and leak into the receiver even when there is an obstacle in the path. At least there should be some housings around the emitter and detector to avoid this.

It does assume that nobody around is using an IR remote.

However, the laser beam is easier for officials to verify. They can stick their hand in it and see where the line is.

A few years ago I designed and build devices to measure speed/travelling time of curling stones over ice.
I used standard IR remote sensors with integrated round lens. The PNA 6402.
And I designed/build battery powered IR transmitters with the ICM7555.
The transmitter powers the IR LED with a continious 38Khz square wave.
LED used was the SFH400. Narrow beam with integrated glass lens.
I did not push the IR transmitter LED (~30mA), because it had to work on battery power (2.5volt).
With that, five meters is not a problem in normal daylight, and seven meters is the absolute max.
Beam is about 5mm thick, about the size of the receiver lens. Tested that with a pencil.
Reaction time with a 38Khz signal is not as expected 1/38,000 sec but, because of the receiver's inbuild error correction delay, ~1/1000 sec.
The fastest runner in history would have travelled ~10mm in that time.
When I flick a pencil through the beam as fast as I can, it still detects it.
Detection works equally well if the gate is 10cm or 5meters apart.
The receiver enclosure has a normal LED for testing and easy alignment of the transmitter.
If you use a separate start gate and a stop gate on the track with the same parts, start/stop delay errors will cancel each other out.
Can share circuit diagrams if you are interested.

I've done something similar involving a reflected ray. I don't see why the red laser shouldn't be picked up by a photodiode. Even though the IR photodiode is meant to receive IR, it should still have a response in the red spectrum as well. My advice would be to test the setup and verify it trips at a distance of 5m. I wrote a blog article on reflected IR, so your project wouldn't be much different. If interested, check it out here:

I see you have added this link to several dead/old posts.

I don't see why the red laser shouldn't be picked up by a photodiode.

  1. A beambreak 'gate' for runners/bikes/cars can be several meters (or more) wide.
    Not easy to align (and keep aligned) a laser + photo diode over that distance.
  2. A photo diode can be swamped by ambient (sun)light.
    It needs a special amplifier to keep it in it's linear region.

Much easier to use modulated IR and 3-pin IR receivers (TSSP4038) for beambreak.

When I was working for a photonics company, they used long tubes (like more than a few inches) to keep out ambient light so all the say when a laser was pointed at the photodiode. If the last was off, all you had was dark current. But Wawa makes a good point, aligning it and staying aligned will be a pain to do.