I am wanting to create an alarm system which would be triggered by someone or somthing passing through an infa-red beam, thus cutting off the signal to an LDR.
Nothing complicated, just wondered how I could go about it.
You can simply make a potential divider with a preset and the LDR and connect it to the base of a transistor which would function as a switch. Then Use a relay to switch on/off the alarm. U can tune the sensitivity by adjusting the preset. I hope this is helpful.
Here is the circuit. Note by the LDR configuration the transistor will switch on when light falls on the LDR. If u excange the LDR with the 100K Preset the operation will be reversed. U may need to choose a different value of the preset and tune it according to ur light intensity and LDR resistance for proper operation.
Sorry, I'm not too good at this, but how would you connect on an alarm with a buzzer to this circuit, I'd be using a 555 timer, unless anything else would be more appropriate.
Here is the circuit. Note by the LDR configuration the transistor will switch on when light falls on the LDR. If u excange the LDR with the 100K Preset the operation will be reversed. U may need to choose a different value of the preset and tune it according to ur light intensity and LDR resistance for proper operation.
Hmmm. this works but I do not think it is very practical for what the original poster intended. With this design, one would have to be sufficiently close to the LDR to cause the amount of light falling on its surface to change enough to turn on the transistor. I think that you will have to tune the preset to a point where it is sitting just at the border of ON and OFF so that the very small change in incident light on the LDR will be enough to trip your alarm. Of course you can try it but it looks like it will be very difficult to tune properly.