laser security

[harpo]

Hello i am trying to make a laser security system
so far i have...
photo resistors
led's
and a laser pointer
my problem starts with the photo resistor i made the circuit and it worked but there was 1 problem
i want the led to light up when the laser beam is broken but it this case its lighting up when its complete and turns off when its broken
i need it to turn on when its broken and off when its complet

thanks for reading
note (please no "how to make a laser security system tutorials")

[waltr]

You just need to invert the LED drive. This can be done with a transistor.

Post a schematic of your photo-detection circuit for specific suggestions.

[harpo]

ok thank you for your post
so (i just read the transiter tutorial) i take it that transiters can also be turned on with a lower than voltage
for example "5v above transiter is off, below 5v transister is on"
please tell me if i get it

[waltr]

Not sure which transistor tutorial you read (a link would be helpful so that I can read the exact same one).

If this is on Bi-polar transistor, NPN & PNP types, then these are turned on with current flow through the base and emitter. The voltage required is about 0.6V due to the emitter diode but it is the current that turns on a bi-polar transistor.

If the tutorial is on MOSFET transistor then a voltage applied to the gate turns on the MOSFET. For an N-channel the transistor turns on when the gate voltage is higher than the source. The threshold voltage is different on different MOSFETs therefore one needs to refer to the data sheet for the MOSFET being used. Many will turn on with the gate 5 Volts greater than the source but other may need as much as 10 Volts.

Either a Bi-polar or MOSFET transistor can be used to invert the LED drive. Which type did you read about?

[harpo]

i read the sor guide
http://www.societyofrobots.com/electronics_advanced_components_tutorial.shtml#mosfet

[harpo]

so what do i need to get to turn off a led when its say above 5v and turn on when its equel or below say 4 or 3 volts

[harpo]

any one?

[Soeren]

Hi,

A schematic of how you wired it up would help.
If you created a voltage divider with the LDR and a resistor, just swap the two.
If not... Post a schematic of what you have done - we're not mind readers.

[harpo]

...
let me try to make this clear forget what i said above

what would the name off a item be that would take for example , if 5 volts were givin to a input it would stay off
but if it was below 5 it would turn on. if its on then it allows a seperet power source thou to a output

[harpo]

i do not need the exact name of the product i just need to know what somthing like this in general would be called
would it be a mosfet?

[waltr]

The SoR transistor tutorial is a very basic introduction and really does not give enough information on properly using any of transistor. Do additional search for detailed tutorial on transistor usage.

In my post above I listed the possible transistor types that could be used. These would be called:
NPN Bi-polar transistor
PNP Bi-polar transistor
N-ch MOSFET transistor
P-ch MOSFET transistor
Electronics catalogs, google and the SoR tutorial page search function can use these names.

Do you have a schematic to post here?

[Soeren]

...
let me try to make this clear forget what i said above

Clear? 

what would the name off a item be that would take for example , if 5 volts were givin to a input it would stay off
but if it was below 5 it would turn on. if its on then it allows a seperet power source thou to a output

A circuit 

Like eg. this: