Photo switch / transistor - any suggestions?

I’m trying to make a very compact circuit that will detect a laser beam’s presence and then allow 3.7V from a lipo battery to flow continuously through 3 LEDs in parallel, regardless of whether or not the laser is still present. Basically, I need a switch that is activated by intense light. Can anyone suggest a component or simple combination of components that will help me achieve this effect?

And, while I’m here asking questions, how could I set this up to have the LEDs fade on, instead of just turning on? I’m assuming using a capacitor-resistor circuit inline, but if anyone knows how to do this off-hand, I would really appreciate if you could post a schematic for me. This is an added bonus for my project, but would be really awesome if I can fit it in. :slight_smile:

Thank you! :smiley:

A lot of factors to consider.
A Cd-S photocell could be used as a light detector.
You’d stay in a loop reading the photocell [analogRead] till it crossed your intensity threshold where it would go to an LED routine.
The LED routine could ramp up your intensity with a series gradually increasing PWM [analogWrite] vs time/dwell.

[quote author=Runaway Pancake link=topic=128199.msg964498#msg964498 date=1350770545] A lot of factors to consider. A Cd-S photocell could be used as a light detector. You'd stay in a loop reading the photocell [analogRead] till it crossed your intensity threshold where it would go to an LED routine. The LED routine could ramp up your intensity with a series gradually increasing PWM [analogWrite] vs time/dwell. [/quote]

Thanks for the quick response. If I was incorporating an Arduino, this circuit would be a breeze. I'm trying to do it without including an Arduino. Any ideas for that circuit?

Too many parts then.

You could use an ATtiny85 (they're DIP-8).

TonyStark:
I’m trying to make a very compact circuit that will detect a laser beam’s presence and then allow 3.7V from a lipo battery to flow continuously through 3 LEDs in parallel, regardless of whether or not the laser is still present.

I did something similar for a magic show last June. I used an ordinary photocell connected to an analog input.

fungus:

TonyStark: I'm trying to make a very compact circuit that will detect a laser beam's presence and then allow 3.7V from a lipo battery to flow continuously through 3 LEDs in parallel, regardless of whether or not the laser is still present.

I did something similar for a magic show last June. I used an ordinary photocell connected to an analog input.

Yeah, if I was doing this with an arduino, it would be no problem. I'm trying to figure out a way to do it without an Arduino.

Then look up comparator circuits, that'll take care of the on/off. If you want it to go on and stay on till it's reset then you'll have to add a latching circuit (RS latch).

[quote author=Runaway Pancake link=topic=128199.msg964567#msg964567 date=1350774317] Then look up comparator circuits, that'll take care of the on/off. If you want it to go on and stay on till it's reset then you'll have to add a latching circuit (RS latch). [/quote]

Okay, I'll take a look for those. Thanks. Are any of the latching circuits triggered by light? Any suggestions on how I could include that functionality?

The comparator's output will go high/low based on the photocell. The output of the comparator will be the Set for the RS latch.

[quote author=Runaway Pancake link=topic=128199.msg964579#msg964579 date=1350774657] The comparator's output will go high/low based on the photocell. The output of the comparator will be the Set for the RS latch. [/quote]

Awesome. That sounds like the solution I was looking for. Any chance you'd want to do up a quick schematic for me Runaway Pancake? I would be forever in your debt. :D

google is still your friend

http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html

http://thalia.spec.gmu.edu/~pparis/classes/notes_101/node115.html

[quote author=Runaway Pancake link=topic=128199.msg964586#msg964586 date=1350775067] google is still your friend

http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html

http://thalia.spec.gmu.edu/~pparis/classes/notes_101/node115.html [/quote]

I'll take it! :) Thanks.

youtube works, too

https://www.youtube.com/watch?v=y2_CJ49C5UE&feature=related

using an LDR, when it sees bright light, the resistance drops to only a few Ohms, when it's dark it's between 100k - 1m or so.

As you increases the light, the resistance will drop across the LDR, so basically you will have a rising/falling voltage across your resistor divider.... now If you take that voltage and place it in series with a cap (not sure the value without getting out the calculator) you can make the Cap slowly rise and fall in voltage - still with me?

The Cap's + terminal goes to a base of a transistor (via a resistor to limit the current further) - then have the LED slowly turn on :) (and slowly fade out as the cap discharges)

I'm curious myself and might go and make it on a bradboard, i'm so bored right now.

and here's a video of it ... You're going to have to replace a couple of Resistor Values, to match what your setup requires, eg your voltage, how powerful the laser is to light it, how bright you want your LED, etc.

https://www.youtube.com/watch?v=obPiIJ6opsU

cjdelphi:

and here's a video of it ... You're going to have to replace a couple of Resistor Values, to match what your setup requires, eg your voltage, how powerful the laser is to light it, how bright you want your LED, etc.

https://www.youtube.com/watch?v=obPiIJ6opsU

Holy crap man. You whipped that up pretty quick! THANK YOU SO MUCH! :D That's almost exactly what I wanted! :D Is there any way to make the LED stay on if I have a 3.7V battery in the circuit somewhere?

ACK - I just noticed something wrong, the 10k resistor I used to quickly discharge the cap, but…

  1. Ground is not connected, does not matter what value that resistor is LOL - it’s going glow just as bright no matter what you use, after hooking up ground, 10k bled the cap dry, if you’re going to use it to discharge the cap, a lot higher value, try around 100k, it works for me…

Video uploading.

To make it work, connect ground, then place a 100k (or whatever suits you) between gnd and + on the cap, as the Light goes off, the voltage drops, the cap will drain via ground .

P.S how do you embed a video via this forum ? or is it disabled here…

cjdelphi: ACK - I just noticed something wrong, the 10k resistor I used to quickly discharge the cap, but...

  1. Ground is not connected, does not matter what value that resistor is LOL - it's going glow just as bright no matter what you use, after hooking up ground, 10k bled the cap dry, if you're going to use it to discharge the cap, a lot higher value, try around 100k, it works for me...

Video uploading.

To make it work, connect ground, then place a 100k (or whatever suits you) between gnd and + on the cap, as the Light goes off, the voltage drops, the cap will drain via ground .

P.S how do you embed a video via this forum ? or is it disabled here..

Gotcha. Thanks again - really appreciate you taking the time to figure this out. :)

And, no idea how to embed a video here - the link worked fine though - very helpful. :D

Ahh, you wish the LED's to stay on and not fade out?...

Good question, some kind of Latch then :) - probably require an IC of some kind to remember the state, someone else can probably explain how that works, all i can think of is a Latching IC, not sure how'd you do it with basic circuit components.

because as soon as the light level drops, the cap will slowly discharge making your LED's dimmer, you'd need a latch to stay delivering current until some kind of user input?

cjdelphi: Ahh, you wish the LED's to stay on and not fade out?...

Good question, some kind of Latch then :) - probably require an IC of some kind to remember the state, someone else can probably explain how that works, all i can think of is a Latching IC, not sure how'd you do it with basic circuit components.

because as soon as the light level drops, the cap will slowly discharge making your LED's dimmer, you'd need a latch to stay delivering current until some kind of user input?

Yeah, exactly. I was hoping to essentially have some sort of photo transistor. Like, as soon as enough energy is put into the photocell, it opens the "flood gates" and allows the 3.7V to flow continuously. And, if it could fade on, that would have been ideal too. Don't know if it's possible, or how many components I'd need to make it possible. Just wanted to see if anyone had a simple solution for it. :) Thanks for all of your tips though. :D