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Replacing the 2N3904 with an SCR Answered

so 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 saw 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 replace the npn transistor on my circuit, the tansistor on my cicuit is 2n3904, input voltage is 5v



6 months ago

SCRs are a type of transistor that does latch when you apply a current to the base (or gate?) However they are typically used/reserved for high power circuits. They may not latch properly at very low currents. Also you don't commonly find them in consumer appliances, or at least I wouldn't know where to find them. They are somewhat rare. If you find a suitable one, your circuit would be very simple and elegant indeed!

But if you can't get your hands on one of those, you can build any type of latching circuit. There are some based on 555 timers, others based on some combination of transistors, etc. Hell you can even do it with relays! Simply wire up the 'NO' contacts in series with the coil and apply a voltage to it. Add voltage trigger source in parallel to the relay to initially trigger it. The NO contacts will provide power and keep the NC contacts open until interrupted. I'll let you work out the details.

As you don't show your circuit you will have to work out how to incorporate the following circuit elements. Shouldn't be too hard.


what thyristor is the ''D1'', i mean is there a standard for them or something or are they all the same?

For this, anything you have will probably work fine. Bear in mind that the only way to turn it off will be to either turn off the power, or put a button that can short circuit the Anode and Cathode connections

or the real question, what transistors should i use to make the ''d1'' thyristor, or should i just use any npn and pnp transistor

Take care if it is a mech buzzer then it will turn off the SCR.

basically, they were suggesting that you use a latching circuit. once it trips, it stays on until the user forces it back off. do an internet search for "latching circuit" It's a handy tool to have in ones' subcircuit toolbox.


6 months ago

You trigger the SCR = Thyristor with a milliamp pulse.
Then the SCR will go into conduction mode for as long as the water flows and the rain falls.
The only way to turn the Thyristor Off is to interrupt the current.
If you are curies ask about dv/dt and holding current.

Click the pic to see the whole image !


6 months ago

it was a low quality image but i gave it a touchup, is that a NOT gate in the circuit you gave (sw1)? and what's D1? sorry i'm a beginner with electronics. can i just change the transistor on my circuit,