i made a laser alarm system some time back, it worked good but had one fault, more like a limitation. it made the buzzer ''buzz'' only when the laser was interrupted and the buzzer turned off when the laser was allowed to hit the photo sensor again. i wanted it to stay on until turned off by a switch. someone suggested something called a monostable flip flop but i had no idea what it even meant, then i read about something called silicon controlled rectifier or a thyristor, but i also learned that they have to be made using a pnp and npn transistor, what i cant understand is what transistors should i use to make a thyristor to replace the npn transistor on my circuit, the tansistor on my cicuit is 2n3904, input voltage is 5v.
thanks for any help

 

A flip flop would probably be more convenient.

When discussing circuits, schematics and/or block diagrams are appreciated. Preferably of the well drawn variety.

Most would also appreciate it if you used paragraphs to organize your thoughts and proper grammar.

 

Whoa! Hold on there. You have way too many threads there in one post.
Forget the SCR, 2N3904, monostable flip-flop.

Rewind and go back to the initial question.

You have a buzzer that is turned on by some trigger.
You want to turn the buzzer off with a switch or push button.

Can you post a circuit schematic of what you have so far?

 

Although you can make an SCR equivalent, it would be easy just to buy one. They're readily available and cheap.
Post your schematic and we can suggest how to modify it.

 

just madness what is OP talking about where schematic

 

BASIC OPERATION AND PRACTICAL APPLICATION CIRCUITS FOR SCRS.

The above link will afford you some good and basic information as to SCR operation and will show you a few transistor circuits which can be used the same way, as a latching switch. Based on your limited description it would seem you want a simple SCR used as a latching switch. Give the link a read and come back with hopefully more information and a drawing of what you want to do exactly.

Ron

 

i made a laser alarm system some time back, it worked good but had one fault, more like a limitation. it made the buzzer ''buzz'' only when the laser was interrupted and the buzzer turned off when the laser was allowed to hit the photo sensor again. i wanted it to stay on until turned off by a switch. someone suggested something called a monostable flip flop but i had no idea what it even meant, then i read about something called silicon controlled rectifier or a thyristor, but i also learned that they have to be made using a pnp and npn transistor, what i cant understand is what transistors should i use to make a thyristor to replace the npn transistor on my circuit, the tansistor on my cicuit is 2n3904, input voltage is 5v.
thanks for any help

Although, yes, your post could be better organized, I want to thank you for telling us the underlying problem you are trying to solve. So often someone trying to do what you are and is at the position you are at would post a thread that only asked how to make a thyristor out of transistors.

Look at the link given by Reloadron -- I think you will find one or more circuits that do just what you want.

 

i made a laser alarm system some time back, it worked good but had one fault, more like a limitation. it made the buzzer ''buzz'' only when the laser was interrupted and the buzzer turned off when the laser was allowed to hit the photo sensor again. i wanted it to stay on until turned off by a switch. someone suggested something called a monostable flip flop but i had no idea what it even meant, then i read about something called silicon controlled rectifier or a thyristor, but i also learned that they have to be made using a pnp and npn transistor, what i cant understand is what transistors should i use to make a thyristor to replace the npn transistor on my circuit, the tansistor on my cicuit is 2n3904, input voltage is 5v.
thanks for any help

A simpler approach will be to use a small electromechanical relay in a latching circuit. IT will be easier to understand what is happening and a lot more forgiving of connection errors. The typical relay latch circuit has been a standard in control systems for almost 100 years and so there are a lot of examples available to learn from. IN addition, the circuit would only use two parts, the relay and the reset button. You can use that same transistor to operate the relay, and then the latching contact circuit will keep the relay latched and the buzzer buzzing until the reset button is pressed to stop it.

 

Assuming the problem is that the buzzer only sounds for a short time, all you need is a timer/delay circuit, given that these are available for next to nothing on eBay from China, it's crazy to do anything else. I used one recently myself where I needed a relay output to switch on for 3 hours, I bought a ready made solution from my Chinese supplier and modded the circuit to get the time I needed.

 

honestly speaking, i wasn't hoping for so much help in such a short time. its my first thread actually, didn't have much idea how to ask the question correctly. thanks for all the help

 

Although, yes, your post could be better organized, I want to thank you for telling us the underlying problem you are trying to solve. So often someone trying to do what you are and is at the position you are at would post a thread that only asked how to make a thyristor out of transistors.

Look at the link given by Reloadron -- I think you will find one or more circuits that do just what you want.

thanks, i'm actually a complete beginner and i've lost the original circuit diagram, which i didn't make, so i couldn't attach the diagram at that time, still thanks for the support

 

BASIC OPERATION AND PRACTICAL APPLICATION CIRCUITS FOR SCRS.

The above link will afford you some good and basic information as to SCR operation and will show you a few transistor circuits which can be used the same way, as a latching switch. Based on your limited description it would seem you want a simple SCR used as a latching switch. Give the link a read and come back with hopefully more information and a drawing of what you want to do exactly.

Ron

yeah the article you shared really helped, thanks for that, it also has a circuit diagram of exactly the thing i was trying to make, a burglar alarm, so that solves my problem, thanks a lot,

 

thanks, i'm actually a complete beginner and i've lost the original circuit diagram, which i didn't make, so i couldn't attach the diagram at that time, still thanks for the support

You should be able to trace out the circuit if you still have the assembly that still works. A photosensor device plus a transistor arrangement is not very complicated, and is actually a good way to start learning to trace out circuits. You already know that the 2N3904 is an NPN transistor, and you therefore know that the emitter goes to the negative power connection and that the collector goes to the buzzer, with the other side of the buzzer going to the positive power connection. That leaves the base of the transistor to trace out the connections of, which I can guess at but I might not get it right. So when you can get that done then I will describe the relay and pushbutton connections.