Laser beam detector

It's connected between +5V and the CDSIn connector.

The opamp should be part of an LM3900 IC - but all radio shack had was an LM324.

I do have a 741 as well.

Steve

--
Not if it doesn\'t work, it\'s not! ;)

How is the LDR connected in the circuit and what\'s the part number
of the active device?

Wow!

Thanks for your time - I really appreciate the detail of your answer.

I'll let you know how I get on...

Steve

--- Redrawing your schematic for convenience:

+5>---+-------------+-E1 | |R1 | [LDR] | | | E2-+----|+\\ |R3 R4 | | >-- [100K]--+-------------+

With a +5V supply, the LM324 has a common mode range of from 0V to (V+) -2v, which means that you've got to keep both inputs between

0V and +3V for the thing to work right. With an LDR low resistance of 3k and R3 equal to 1k, the voltage on the non inverting input of the opamp will be:

E1R2 5V * 1kR E2 = --------- = ----------- = 1.25V R1 + R2 3kR + 1kR

with the LDR fully illuminated and its resistance at 3k ohms.

With the LDR less than fully illuminated its resistance will always be less than 3000 ohms, and the voltage on the + input of the opamp will always be less than the common mode limit, so you'll be unconditionally safe there.

Now, in order to trip the circuit, all you have to do is to adjust the pot to provide an output somewhere between 0V and less than

1.25V, 1.25V being the point where the LED will never turn ON and 0V being the point where it will never turn OFF.

In order to get a greater range of control and to keep the output of the pot below the common mode limit, you might want to increase the value of R2 to where E2 will be just under 3V with the LDR at 3k. For 3V that value would be:

E2R1 3V * 3kR R2 = ------- = ---------- = 4500 ohms E1-E2 5V - 3V

so a good choice would be something less than that, and I'd use a

3.9kohm +/- 5% carbon film just to make sure I stayed well below the common mode limit.

Also, to limit the range of the pot while increasing its sensitivity, (angle of rotation) you could do this:

+5V |E1 | [R1] | [100k]

That works!

Thanks again.

Steve

you need to connect the other terminal of your CdS cell to the +5V (or connect the 1K resistor between +5 and the input)

also ensure you have an op-amp that's suitable for 5v (or +/- 2.5v) operation (many aren't including the popular LM741) fortunately the LM324 is and they're common as dirt. so you shouldn't have much trouble getting one.

No. at certain light levels the op-amps output can be an undecided voltage between the the two desired states (and that can be bad for some digital circuits), to cure this connect a 220K resistor from output the to the op-amps non-inverting input, this will introduce hysterisis making the output snap from "off" to "on" and vice versa instead of meandering with that modification it should be fine to attach to a digtital input.

you don't need a special IO board, you can hook this up to one of the the joystick port's fire button terminals (if it's vacant) there's even a handy +5v supply there... just don't short circuit the 5V supply it tends to ruin the ribbon cable behind the socket :)

Bye. Jasen