LED Laser Target

Hey everyone,

I would like to build a box to act as a target for a handheld laser pointer. The box I have in mind would have several individual LEDs on the front of it to act as the targets. There will be plenty of space between LED's. What I want to do is have each LED light when the laser beam hits it. After the beam hits it, it should stay lit.

I haven't been able to figure out a circuit to do this. I have seen plenty of ways of using an LED as a light sensor, but haven't seen how to make it light up when this happens. I'd prefer a way to do this without using a microcontroller, if possible.

Any ideas?

A flip flop will help...

Without actually trying it one way to go about this might be using dual LEDs. I mean the three wire flavor that share a common cathode. I am guessing the target area needs to be a single LED at a specific location.

I dig out a generic single red LED. At ambient room light the photovoltaic action produces about 25 to 52 mV. Striking it dead on with a small LASER it produces about .5 Volt. That voltage could likely be amplified using an operational amplifier and passed on to a comparator. If the op amp had a gain of maybe 10 that would give us about 5 volts when the LASER was directly on target. Using a simple inexpensive op amp like a TL084 would work for this and afford 4 op amps in a single package. If the LEDs were slightly recessed in the target that would help eliminate ambient light.

The TL084 output could be passed on to a comparator circuit using a chip like the LM339. The LM339 would afford 4 comparators in a single chip. When the op amp output exceeded a set level (reference) the comparator output could be configured to swing high. Even if only momentarily when the LASER was striking the target, the comparator output would be a high.

(Possibly the comparator could be eliminated, I am not sure.)

That transition could possibly be used to trigger a F/F (Flip Flop) as was mentioned. The F/F would toggle states on the clock input and the F/F output would trigger the LED on. THat LED being the other LED in the dual LED package. The F/F could be a chip like the 7474 which would provide two flip flops per package. Once turned on the LED would remain on till the F/F was reset.

Something along those lines might work. I have no clue as I have not tried it. I can tell you this, hitting and holding the LASER steady on a 3 mm LED was not easy.

Ron