I am a student and my Applied Physics teacher gave an assignment to create a schematic diagram of a security alarm. The main Components are,

  1. Switch (Simple on/off)
  2. SCR C106B (Silicon Controlled Rectifier)
  3. 10 KΩ Resistor
  4. Buzzer
  5. 9 V Battery or DC power supply

(Note: A combination of npn and pnp transistors can be used in place of SCR as Follows)

npn and pnp Conversion from SCR

I created a circuit diagram but my simulator does not seem to work, so i cannot verify it. Please Confirm it that is it correct. The schematic is as Follows,

Circuit Diagram Schematic that i Created

If There is any error in this Diagram or there is a better and easy way of doing it, please tell me.

(Note: Our teacher just asked us to create a Diagram, So type of SCR or any component does not Matter)

Thank You.


1 Answer 1


"Doesn't work" isn't a good description of a fault so you should really describe what does happen. As you have drawn it there are a couple of potential problems.

  1. With the switch open the SCR will be triggered as soon as your simulation starts.
  2. You have no way to reset the circuit.

Try replacing your switch with a normally-closed version. This will hold the gate at ground during power-up and prevent the buzzer from sounding. Add another normally closed pushbutton on the battery positive line. This will be your reset button. (The SCR latches on until the current through it is reduced below the hold-on value.)

Number your components R1, SW1, SW2, etc., and show the buzzer resistance or current so that we can calculate if it will pass enough current for the SCR to hold-on.

  • \$\begingroup\$ ,I did The changes You Asked. Schematic Diagram The Word "Doesn't work" means That my simulator (Circuit Lab) that i use online gives the error that SCR is non-simulatable. So i cannot simulate this circuit to check it's working or its correctness. \$\endgroup\$
    – Usman
    Jun 9, 2020 at 4:19
  • \$\begingroup\$ I also made some changes to the circuit. I used npn and pnp instead of the SCR in the circuit, and the circuit simulation started, But it stopped again Schematics of my circuit is in this Link. Kindly review it once more and explain the error. Moreover The error in simulation here is basically on The Buzzer an Push to Break Switch. Everything else checked out. \$\endgroup\$
    – Usman
    Jun 9, 2020 at 5:57
  • 1
    \$\begingroup\$ Please edit your question to add in the CircuitLab schematic. You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers and run the simulator. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$
    – Transistor
    Jun 9, 2020 at 7:25
  • \$\begingroup\$ I will keep that in mind next time. As for now i gave CDDX file of my circuit to a friend and he made some changes and simulated it. It is correct Now. Really Thanks for your Time and Effort. \$\endgroup\$
    – Usman
    Jun 9, 2020 at 9:04
  • \$\begingroup\$ Thank you for accepting my answer. I'm glad I was able to help. Are you clear what the problem was? \$\endgroup\$
    – Transistor
    Jun 9, 2020 at 9:41

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