Photo switch / transistor - any suggestions?

[TheRealStark]

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.

Basically, I want the circuit to simulate a laser "charging up" the LEDs to a point of self-sustaining power, lol.

Thank you!

 

[Diver300]

I suggest that you separate out the functions. I see them as:-

Light detector
Latch to keep the circuit on when the laser has turned off (plus some way to reset the circuit)
Fade circuit
LED driver

A phototransistor will detect the light. A couple of transistors could latch the signal. A switch could be used to reset the circuit, but a capacitor could also be used to make sure that the circuit started in the "off" state so that you could be sure that turning the power off and on again would leave the LED off.

As you suggested a resistor-capacitor circuit will make the LEDs fade on but won't supply much current. You should make the output driver produce a current that is proportional to the input voltage so that the LEDs fade on nicely.