DIY laser schematic

[[email protected]]

A friend of mine dug up this schematic he wants to build with his son.

http://www.diyforums.net/laser-alarm-schematic-noob-questions-459027.html

If you scroll down a bit, you'll see an annotated schematic. It shows
the +9V battery and on the right side it has Positive side of B1 and
on the left side it has Negative.

I'm not much of an electronics whiz, but that only makes sense to me
if the - side of the battery is connected to chassis ground. If you
connect the - side of the battery to the voltage divider consisting of
the pot and the photocell Q would never trigger.

Am I totally out in left field?

 

[[email protected]]

The buzzer will sound when the photocell is in the dark, if everything
works right. But 9 volts through an emitter follower, with a weak base
pullup, is pretty wimpy to drive a 12 volt buzzer.

Ugly.

Do you have a better one? This project was devised by a dad so his
enterprising son wouldn't take apart the family dvd player, build a
handheld laser, and attack who knows what. ;-)

Seriously, this is a fun project for a father and son who know little
of electronics. I got pulled in because I know a few basic
principles, but certainly not enough to design a circuit. The
requirements are that it has to be simple, be triggered by a "laser" -
LED in a wand - and make noise.

 

[[email protected]]

Well, then it's backwards. If the photocell is a photoresisor as
drawn, it beeps in the dark, not when hit by light.

DUH! That makes sense. I kept trying to figure out how that
worked... The circuit is cribbed from a 'burglar alarm' - break the
beam and it goes off. What we want is "make the beam and it goes off"
- i.e. reverse the pot and photocell.

Why not buy a cheap laser pointer? An LED will practically have to
touch the photocell, in a dark room, to be detected.

I think that's the intent. In my sloppy thinking, that is.

 

[Rich Grise]

No, I think you're in the ballpark. Unless I have my head up my donkey*,
as wired the circuit can deliver no more current than will flow through
the 5K pot and the photocell.

I _suspect_ that the 9V battery is supposed to go from ground to the
transistor collector, but if you do that then your siren will go off until
the photocell starts conducting (presumably it conducts when it gets lit
up).

The circle labeled "+9V" isn't the battery, it's only the positive
terminal of the battery - it's "assumed" that the negative terminal
goes to ground.

It's still a pretty cruddy circuit - John Fields's is better.

Cheers!
Rich

 

[palvi]

Hi,
Could you please forward the link to a better laser trip circuit ,I have been trying to use the circuit above as an external trigger to an oscilloscope but in vain