This is a simple RC circuit using thyristor. You might have done similar projects using transistors. Let me provide you some of my previous RC projects:
b. Light operated security alarm– Make a simple light operated musical bell and security alarm using RC circuit.
What about this project?
When you press sensor switch SW2, the LED D3 and the alarm switches ON for a while. The timing of the circuit is dependent upon R3 and C1. You can get all the details of the RC circuit from the links given above.
You can see on the schematic that the circuit is based on C106M Thyristor. Download data sheet of C106M Thyristor