Need a light sensor that plugs into a Pluto Trigger to create a laser cross-beam trigger...

[Warren Savary]

I recently purchased a Pluto Trigger device for my photographic pursuits (you can see the unit by searching the web for "Pluto Trigger") Among its features is the ability to trigger a camera (or flash) when a laser beam is interrupted, which is one of the things that prompted my purchase. The unit even comes with a small laser emitter for that purpose. The instruction manual suggests an even more elegant modification that would be particularly helpful to me in photographing small pollinators:

"Laser Cross Beam Sensor: Build a DIY light sensor and plug it into Aux port of Pluto Trigger. In Aux mode, set trigger mode to "Low Trigger" and adjust the sensitivity when laser beam aligned. Go to Laser mode, adjust sensitivity when laser beam is aligned. Finally switch to Fusion mode, "AND" Aux mode with Laser mode to set up a laser cross beam sensor" There is a YouTube video that shows such a setup in action.

I'm told by the manufacturer that "You just need a led, a resistor and a cable [I presume a 2.5mm audio cable] to make the light sensor.", with no further guidance. Alas, I am a biologist, not an electrician. HELP!!! Can anyone offer some guidance on how to build the requisite sensor??

 

[Doug3004]

I found this in the instructions:

5.7 AUX
Aux mode is for DIYers. You can add other sensors that are not included in Pluto Trigger, such as ultrasonic sensor, smoke sensor or temperature sensor.
You will need a 2.5mm audio cable to connect your DIY sensor to the Aux port of Pluto Trigger. The tip of the cable connector is 3.3V and the base sleeve is Ground. Pluto Trigger just samples the signal that is fed into the middle sleeve of the connector. Please make sure the voltage of this input signal is in the range of 0-3.3V. ...

The instructions for using the laser sensor (section 5.1) state that the lower threshold of the sensor is user-adjustable in the app, so apparently the middle sleeve is connected to an analog 3.3v input on the Pluto Trigger. That helps here because the DIY sensors don't need to put out a specific level of volts; they just need a definite [low] and [high] state.

You need a 1/8" TRS cable (tip-ring-sleeve), like for mini stereo headphones. You can cut the cord off a cheap pair of headphones and use that, but the wire can be difficult to solder well.

The manual also refers to a youTube cross-beam video, which is here--

I would guess that the resistor is just used to pull the LED back down? Presumably you connect the LED to create a positive voltage between the [ground] and [middle] sleeve, and the resistor is to bleed the LED's voltage (from the middle sleeve) back off to ground. The 3.3v (+) isn't connected to anything for this.

When a LED is used as a light sensor, it gives off voltage in the reverse direction that you would apply electricity to it to make it light up. There is web pages about "using a LED as a light sensor" online.

I would suggest a trim pot of 20K ohms and try that adjusted high at first, then lower its value if the trigger needs to turn off faster. Being able to turn off fast enough is important here because in cross-beam mode the trigger activates when the beam sensor goes low--not high.

I would get LEDs that were the same color as the laser being used. Is the laser an IR (invisible) one? The youtube video shows them using red lasers, but that would seem to spoil the photo...??? Usually these kinds of (photographic) sensors are IR--so they don't spoil the photo--but they will still work with red light good too. They may not pick up blue light at all, and you may need an IR-cutoff filter to block the beam light.

LEDs work as light sensors but they're not really optimized for it.

A phototransistor will switch faster and would be more sensitive, but it is oriented towards the red/IR spectrum. The PT334-6C is one example.

You also might try getting a real photodiode and trying with that too; the BPW34 is a common one. It is more sensitive and switches faster than a LED would, but it is more sensitive over the whole visible-light spectrum than the phototransistor above is. That could help or cause problems, depending on the lighting you have to shoot in.

 

[Warren Savary]

" Is the laser an IR (invisible) one? The youtube video shows them using red lasers, but that would seem to spoil the photo...??? Usually these kinds of (photographic) sensors are IR--so they don't spoil the photo--"

You bring up a good point, Doug. I'm likely stuck with one of the beams being a red laser, given that the manufacturer provides one for the purpose, but I imagine that the second beam (the one who's receptor plugs into the AUX channel) could be IR. The FUSION mode allows the user to set the trigger to respond when either one or both of two sensors are tripped (for cross beam, it would be both beams). It doesn't appear, however, that BOTH sensors have to utilize red laser beams. One, I suppose, could be an infrared beam. For that matter, it may be worthwhile to see if the built in sensor on the main Pluto Trigger can be used with an infrared beam. That would be better than having a visible red dot on the photo subject and would also be less likely to annoy a potential photo subject. Another alternative, should both sensors employ red lasers, is to place both laser sources on the far side of the photo subject (opposite side from the camera's view) and place both sensors on the side nearest the camera. If the photo subject is large enough, that would hide the red dot(s), but those dots might still be a distraction to the photo subject itself. You've definitely stimulated some thought. Now, if I can just climb the learning curve....

 

[Doug3004]

Usually, photosensors are more-sensitive towards the red and near-IR end of the light spectrum. If it picks up red laser light, it will probably pick up near-IR light too unless there is an IR filter on it.

Product manufacturers--at least in the USA--are loathe to put IR lasers on any normal consumer products, due to the risk (or claims) of eye injuries. It doesn't take much to block a 5mW laser beam, you can buy yellow-tinted wrap-around safety glasses that will do it. 5mW is low-power but goggles still aren't a bad idea here, and they don't cost much.

Aligning IR lasers can be difficult due to their being invisible. There are optical detection cards that are nice to use, but they're $80+ each. It is a plastic card coated with liquid crystal plastic that temporarily changes color where it is struck with a specific range of light, and you'd have to buy the IR one.

If the Pluto Trigger won't work with an IR laser and you really wanted to switch to using IR beams for a cross-beam trigger, it might be easier to just build that yourself rather than cannibalize the Pluto Trigger. There are guides for building circuits or using arduinos for triggering a DSLR camera online. You'd just use two photodetector inputs, and both of them would need to be [low] to trigger the camera.