Laser-activated switch... phototransistor?

[bennyd]

Hi all, great forum here!

I want to replace a simple momentary switch (low DC voltage, no real load) with a unit that will trip each time the beam is broken. The unit needs to react quickly (<1ms) to brief breaks in the beam.

So...

• Quick reaction time
• Work with a basic laser pointer
• Trip when beam is broken

I'm guessing the right phototransistor could fill in the blank nicely, but that's just a guess What do I need to get this running?

 

[ericgibbs]

Hi all, great forum here!

I want to replace a simple momentary switch (low DC voltage, no real load) with a unit that will trip each time the beam is broken. The unit needs to react quickly (<1ms) to brief breaks in the beam.

So...

• Quick reaction time
• Work with a basic laser pointer
• Trip when beam is broken

I'm guessing the right phototransistor could fill in the blank nicely, but that's just a guess What do I need to get this running?

hi,
Something like this would do the job.

Use photodiode or phototransistor as the detector.

 

[bennyd]

Thanks for the diagram

This is a whole lot more that this small project call for, tho... I just need to replace a momentary contact switch that is just used to trigger a controller, for practical purposes there's no load at all and I don't need any latching or reset functionality.

I can get some better numbers once I can tear into it, but I do know it's less than 3v across the switch. I'm thinking a transistor alone might be up for the task, simple is better for what I'm doing Am I way off with that idea?

 

[bennyd]

Here's a highly-simplified diagram of what I'm after... I know the final circuit will probably require more components, just don't want to muddy the water with my guesses

 

[ericgibbs]

Thanks for the diagram

This is a whole lot more that this small project call for, tho... I just need to replace a momentary contact switch that is just used to trigger a controller, for practical purposes there's no load at all and I don't need any latching or reset functionality.

I can get some better numbers once I can tear into it, but I do know it's less than 3v across the switch. I'm thinking a transistor alone might be up for the task, simple is better for what I'm doing Am I way off with that idea?

hi,
The reason the latch is included is because you stated * Trip when beam is broken

If you are trying to replace a mechanical momentary switch, a single phototransistor will most likely
not give the contact closure action you require.
Especially as I do know it's less than 3v across the switch .

You also said react quickly (<1ms) to brief breaks in the beam. I doubt an existing mechanical switch
would be able to give a clean momentary 1mSec closure.

Perhaps it would be best if you told us what you are trying to do.

 

[MorskiBG]

Hi Eric.
Could You specify what is correct drawing for continuous beam. Thanks

 

[bennyd]

Oops, I thought I replied to this last week...

Thanks for the suggestions Eric, and I'm sorry my wording was confusing. You are correct on all counts - by the '1ms' spec, I only mean that it shouldn't have a delayed reaction time to beam breaks, such as a photocell might.

So, an updated requirement list:

• emulated a closed momentary switch WHILE beam is broken
• reaction time less than a few milliseconds
• ideally, should be operable using the available 5v DC power supply

Again, this doesn't have a load behind it, so a minimum of components would be ideal. Need to keep this as compact & efficient as possible.

Thanks again...

 

[q5101997]

Watch out for ambient light

Hi bennyd
,if you tie the photo transistors emitter to ground (assume NPN), and its collector to 5V through a 10kΩ resistor, then it will only take 5V/10kΩ = 500µA of ambient light current to saturate the CE junction, and no amount of laser light will change this. It dosn't take much ambient light to achieve this, so keep this resistance low, say 1kΩ. Then the ambient light current would have to be 5V/1kΩ = 5mA for saturation; very difficult to achieve. Your laser might produce 1mA on a good day, giving a voltage drop across the resistor of 1mA x 1kΩ = 1.0V. This would have to be amplified to TTL levels, say Vih = 3V, using an opamp with a gain of 3, but this also amplifies the ambient voltage, say 1mA x 1kΩ x (gain=3) = 3V. Not much good. I've been messing around with photo detectors for a while, and unless the triggering environment is kept very dark, you have to start using AC methods of detection and DC blocking.
I dont know if this helps, but all things aside, under the right conditions, you could hook your photo diode straight to an opamp and onto to your MCU.

 

[bennyd]

Thanks Q, awesome info!

I'm learning as I go, so please forgive my ignorant diagrams (attached). Just trying to make my bad assumptions as clear as possible

Ambient light: the beam will only have to travel a few inches in a tightly controlled environment. By placing a 3-4" long carefully-aligned tube over the eye, I'm pretty sure I can control any ambient light well enough to get a usable result from the phototransistor. I'm unclear how to proceed from there, though...

• I need some suggestions on which op-amp should I be looking at, and a good place to buy small quantities of these components from.
• How should the op-amp be connected to the existing switch leads to emulate a closed switch while the beam is broken?

 

[Inaweofitall]

Infraredlight- activated switch.

Hi, all. I just discovered the world of LED,s and resistors. Thought I might be able to learn enough in here, with loads of help to make a few simple yet useful devices. I am trying to make a infrared
LED activated switch thar runs on 9v and in daylight or night. I am interested in something similiar to the laser-activated switch...phototransister that bennyd was talking about. I must admit that even his highly-simplified diagram left me in awe! My device will be similar to the switch that stops a garage door from closing when the beam of light is broken. The distance between the light and the (phototransister?) will be about 2 to 3 feet. Any ideas?

 

[MikeMl]

Post number 10 in this thread should do it. Unfortunately, the schematic diagram is missing due to a recent electro-tech hard disk crash. I will restore the diagram tomorrow, when I am at my home computer.

H'mm, it didn't work here, either.