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Beam alarm system

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:idea: Make sure you don't have a PNP in place of Q1 with the emitter tied to the 9v (10v). This would also cause the buzzer to stay on. Check and double check your circuit. Also check that Q1, if not a PNP by mistake is not faulty. I often use a simple trick (after voltage measurements to identify the culprit and before desoldering ) to check transistor switching in low power circuits by briefly shorting the b-e junction of a transistor to force it to turn off, when on.

Make sure that you have not created a "mirror image" of the circuit on the pcb. I have seen this a lot with 1st pcb attempts by students! :) If this is the case the components needs to be soldered to the track side of the pcb to correct the error. (which gets real messy when IC's are involved, as the legs have to be folded back in the opposite direction so the device is actually upside down)
 
i cannot stand it wen it works on the breadboard n not in the final project!!

okay, ive replaced Q1, still dont work, and yes it definetly is an NPN. i also shorted the be junction of Q1 but the buzzer still stays on.i am beginning to think that the problem is in Q2 Q3 OR Q4. i will short the be junction of each of those transistors to find the problem.

now that ive got that figured out, what is reeeealy driving me insanse is that i am getting 10v between the LDR, Q1's base and R1. i am measuring relative to ground. WHY!?!? is this not very strange??
 
figured it out: Q2 was bad. simple as that :lol:

WHY WHY WHY did it make that junction (base of Q1, R1 and LDR) be 10v!?!?!
 
Remember what I said in a previous post

If Q1 ever manage to switch on hard enough (accidents happen), Q2 will be fried because of high base current into Q2. Good practice will be to limit base current to Q2 in the collector of Q1 with a resistor. Maybe move R3 to Q1 collector

Just to prove accidents do happen..... :D
 
naw, thats not the reason. hate to disprove ur proving :lol:
i was trying to use up some old crap transistors that had been used in other things (bak wen i had less of a clue wut i was doing) and well, it mustve been fried. besides, it never fried on the breadboard (i was using different transistors) and it works now. if i moved the resistor to the collector of Q1 then Q2 wouldnt be able to turn on tho right?

and somebody plz satisfy my curiousity: why did it make that junction (base of Q1, R1 and LDR) be 10v!?!?!
 
zachtheterrible said:
figured it out: Q2 was bad. simple as that :lol:

WHY WHY WHY did it make that junction (base of Q1, R1 and LDR) be 10v!?!?!

Although it is impossible to know what happened, here is one possible sequence of events:

R1 or Q1 b-c was shorted accidently. This caused a high current into the base of Q1, causing an even higher current through Q1 into the base of Q2. Q1 fails with a short b-c (explains the 10v on the base), open b-e and Q2 shorted c-e (causing the buzzer to stay on)
 
oh, i c wut ur sayin now, i thot u wer talkin about not shorting or anything, just the circuit turning on really hard for some reason or another. im not worried about shorting nething because even in a perfect circuit, if something gets shorted accidently, things can get ruined. its very much mor likely tho that i ruined the transistor previously. in fact i no it for a fact because i cheked the circuit over very good b4 turning it on, to make sure i hadnt shorted anything :lol:

thanx theone, and everyone who helped me, you've all been very kind to me :lol:
 
dude can you send me on my e mail a copy of you invention so i can try it by myself i am a computer Engineer trying to be a electronics someday can you send the copy of you projrct also the schematic diagram and the values of the electronics parts thankssss by the way here is my e mail adress Fermae14@yahoo.com please send it to me i will be waiting on it thank you veery much that there are a person like you thanks.... LOL....
 
can i ask what is the meaning of 4k7 of R2 for the given circuit.
could it really work perfectly,because i'm having problem working out with our school project... we are running out of time to finish our laser burglar alarm project...anyone or someone who has experienced this and had been given help please send me your circuit diagram... thanks a lot & God bless... :confused::(:(
 
please help me huhuhuhu

can i ask what is the meaning of 4k7 of R2 for the given circuit.
could it really work perfectly,because i'm having problem working out with our school project... we are running out of time to finish our laser burglar alarm project...anyone or someone who has experienced this and had been given help please send me your circuit diagram... thanks a lot & God bless... :eek:
 
can i ask what is the meaning of 4k7 of R2 for the given circuit.
could it really work perfectly,because i'm having problem working out with our school project... we are running out of time to finish our laser burglar alarm project...anyone or someone who has experienced this and had been given help please send me your circuit diagram... thanks a lot & God bless... :confused::(:(

Hi Aeran,

Welcome to the site! Please post your question in a new thread, including the schematic you are actually using and what problems you are having. This will get you the best chance of a helpful reply.

Even though I gather that English is not your first language, that is not so important. Try to give as much information as you can as to *exactly* how you are doing things, and *exactly* what is going wrong for you. If you are writing your post and you start to wonder whether you are giving too much information, remember that *all* information is potentially important. Too much information is far better than too little information.


Good luck,

Torben
 
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