Help in understanding the math behind the schematic.

Ok. little background on myself. I am a high school graduate with no formal electronic education. I read books and tutorials etc online. I think I am above the curve when it comes to understanding electronics, but where I struggle at is the design of circuits. I think a lot of this comes from a lack of knowledge of the math behind the design.

So what I want to accomplish is to teach myself effectively how to design circuits.

I was asked if I could make a simple trip laser security alarm that a friend had seen online, here http://www.wonderhowto.com/how-to-home-laser-beam-security-system-251050/

Now, afaik, The way this circuit works is that the laser upon hitting the photocell causes the resistance to decrease enough so that the flow of current is higher threw the photocell to ground. this effectively keeps the 2n3904 in an 'off' state. As soon as the laser is interrupted the resistance threw the photocell now is very high causing the flow of current to the transistor to increase, effectively putting the 2n3904 into a 'on' state. This completes the siren circuit and causes current to flow threw the load across the collector-emitter and to ground.

Now if all I said above is correct and I understand the principle of the circuit, how do I go about figuring out the math? What is important to know about this circuit that I would need to know from the datasheets of the components?

I am under the assumption the transistor works on current, if enough current is applied to the base, the transistor will turn 'on' But, I can not for the life of me figure out the values needed for the resistors. This is where I have major problems. I though I read somewhere that I need to break down the circuit into it's sub-circuits to do the math. I see two sub-circuits in this design, the alarm circuit and the photocell circuit. Does the variable resistor work as a voltage divider in this instance?

So I'll end there for now. I hope someone can help me with some insight, even if it's to post a link to a website where I can read and learn the exact procedure for doing this sort of stuff. I WANT to learn.

Also, isn't there some easy electronic simulation software I can use to design this circuit and tinker with it in simulation? I tried 'solve elec' , but that program seems to be severely lacking or I am just completely incompetent.

 

Laplace, well I appreciate your answer. But, you haven't really left me with anything to understand or even attempt to learn.

Is there another way that would be better? Can you explain how the transistor would blow? That is all part of what I am trying to figure out. I am really glad that you see how this won't work, But I am still at a lost...

I tried to complete this circuit in multiple simulation programs now and I haven't been able to accomplish it at all. I tried the yenka program, it pretty much showed what you stated, the transistor would blow because of too much current across the C-E. What I didn't understand about the yenka program was the C-E current was 2 kA. which doesn't make any sense to me since I only have a 9v battery as the source.

I did actually make a working model of this schematic, the alarm surely was working. So the circuit does work or at least part of it does. What I was not capable of was getting the alarm to shut off once the laser was pointed at the photocell.

 

Ok, well i took your advice and re-arranged the circuit or so I think I did. What I got now is a voltage divider across the base of the transistor.

I tested my photocell out and I get fluctuating numbers, but the most consistent numbers I got was 500 Ohms when the PC was illuminated and 33k Ohms when the cell was just sitting on my desk in a dimly lit section of my room. So I used these numbers in the simulation. As you can see below in the images. Please tell me if I am on the right track now.

 

You're exactly right, I was interpreting the simulation wrong. When the alarm is 'on' the voltage should drop across the load, as such, there should be less then 3 volts potential on the collector of the transistor since the load works from 6 - 14 volts.

How does this look now?

Couple more questions if you don't mind. Why am I seeing current even when the transistor is in the 'off' mode? Is this standard leakage or am I seeing the current being drawn by the voltage dividing circuit? I guess this is where the math would help explain to me exactly what is going on a little better.

No, but I WANT to learn. So I will find out what I can. please feel free to expand upon whatever knowledge you think that will help me at my current stage.

Just FYI, I got bigger plans for this project. What I would really like to do is expand the alarm circuit to instead of just sounding the alarm, I want the alarm to sound continuously and at the same time short a button on a cell phone keypad for a set number of seconds to make a phone call to myself. Maybe you see where I am going with this??? I really haven't thought about this much, but I am guessing I would need some sort of switch to turn on a sub-circuit and then have a way to keep it on. I had also plans on using some sort of timing circuit for the speed dial on the phone.

 

with my handy calculator and this webpage http://www.opamp-electronics.com/tutorials/thevenins_theorem_1_10_07.htm

I came up with .43 volts, that is with the 10k Ohm resistor and the 500 ohm photocell/resistor. Does that make sense?

 

I really appreciate the time you took to help me Laplace.

I have a working model sitting on my kitchen table now. Works exactly as it should!

 

Well this was my original thread. This was one of the first projects I ever took on. The link below is what I ended up with. It uses a flip flop and will sound the alarm for a set time period based on the 4060. Check it out, it is fully functional and easily up-gradable. You could even use magnets and reed relays for windows and such.

http://gurutechy.wordpress.com/2011/09/14/laser-alarm-circuit/