take a look at this circuit

[Fahime]

Hi, This is the schematic of a doorbell that is widely produced in my country. Please take a look at it and tell me your idea. As you see the supply is 220Vac and the piezo is directly connected to the mains. There is no trasnformer and the 220V is converting to a low voltage via just resistors and transistors. As far as the components get so hot, this would be so dangerous. But why the circuit is running well?

 

[Hero999]

What are the coponant values?

Providing there the insulation is good enough and there is protection agiant earth faults this circuit presents little safety risk.

The piezo might be a high voltage type but don't like the way it's connected normally a push pull output is required because it's capacitive unless it isn't a piezo of course.

Do you have a datasheet for the IC?

What about the transistors?

 

[Fahime]

values

What are the coponant values?
Do you have a datasheet for the IC?
What about the transistors?

Hi.
1. The values are :
R=2.7K
R1=R2=R5=39k
R4=100k
R7=2.2k
2. The transistors are all C945.
3. The datasheet Of Ic is available on www.alldatasheet.com I tried to attach it but it failed to attach.

 

[john1]

https://www.alldatasheet.com/datasheet-pdf/pdf/128672/INTEGRAL/BT8031-XX.html

 

[Nigel Goodwin]

As you see the supply is 220Vac and the piezo is directly connected to the mains. There is no trasnformer and the 220V is converting to a low voltage via just resistors and transistors. As far as the components get so hot, this would be so dangerous. But why the circuit is running well?

It's a crude and nasty design, using a simple resistive dropper to supply the low voltage power to the circuit and piezo - and the entire circuit is live, so must be double insulated for safety. There's no reason it can't work well, it's just a bit nasty!.