laser alarm circuit-[PROBLEM]

[zhongbie]

ok....this is the first time i had involve in the electronic component things due to assignment, my assignment title that i had choose is to construct a laser alarm circuit, and  eventually i had face a lot of problem,  so i seriously need some help from you guys to solve it.  (gonna pass up my report at this Friday T.T )

the component that i had used for my circuit is:
1x 9V battery
1x cds photocell
1x buzzer
1x 5k potentiometer (trimpot)
1x 2N3904 transistor
1x 1000microF 35V capacitor

so this is the circuit that i try to used.


and this is the circuit that i had construct it on the breadboard


this is the zoom in for the connections of the transistor and potentiometer.


so the problem is here, i had connect everything according to the circuit shown , but i'm not sure what is the problem here, because the buzzer just can't sound even in the dark area, sun light area or normal in the light room area.  since it is a laser alarm circuit, so when there is no laser shoot at the photocell, the buzzer should sound, but things just goes not well.

i had try connect everything just by using potentiometer , photocell and buzzer , it's works, but its sounds when there is light  shine at the photocell, everything been opposite.  so i assume this is the reason why we need transistor right?  but after we add transistor, the circuit got problem, so i think mostly the problem is at the capacitor part?  but why and how?
need some advise,  should be very appreciate. thx.

[Soeren]

Hi,

the component that i had used for my circuit is:
1x 9V battery
1x cds photocell
1x buzzer
1x 5k potentiometer (trimpot)
1x 2N3904 transistor
1x 1000microF 35V capacitor
[...]
so the problem is here, i had connect everything according to the circuit shown , but i'm not sure what is the problem here, because the buzzer just can't sound even in the dark area, sun light area or normal in the light room area.  since it is a laser alarm circuit, so when there is no laser shoot at the photocell, the buzzer should sound, but things just goes not well.

i had try connect everything just by using potentiometer , photocell and buzzer , it's works, but its sounds when there is light  shine at the photocell, everything been opposite.  so i assume this is the reason why we need transistor right?  but after we add transistor, the circuit got problem, so i think mostly the problem is at the capacitor part?  but why and how?
need some advise,  should be very appreciate. thx.

Assuming the mentioned components are the (only) ones you can use, you will get an alarm when the LDR receives the LASER beam.
I've attached a better way of doing that with the components mentioned (the transistor is named differently, but that is of no consequence).

If you can use other components, you could swap the 2N3904 (NPN) with a 2N3906 (PNP) (or any other small signal PNP) and "reverse" the circuit. Tell me if you can use other components (and which) and I'll draw you a schematics.

Also... Measure the resistance of the LDR (Light Dependant Resistor/Cds) with and without the LASER on it. LDR's can be very different and these measurements are needed to make sure it works on first try.

[infurl]

zhongbie in your schematic you have 9 volts connected directly to the base of the transistor. Surely that can't be right. It would keep the transistor saturated for as long as the battery (or transistor) lasts and variations in the light level hitting the light dependent resistor would make no difference at all.