PNP Switch self triggering

Thread Starter

Willen

Joined Nov 13, 2015
313
Hi all,
I got a very basic doorbell. It has a cheap chip directly driving a 2 inch speaker. The input has a PNP switch. Problem is it triggers itself and ringing too. I think because of the 5 meter long switch wire "twisted together with 220V high volts wires". Maybe also the leakage around PNP. There is a 10k pullup resistor exist and a 10uF too, but I guess this is not sufficient. What can I do?

Later I found that to save wire, an electrician used neutral wire of mains as a one wire of the switch. As I know the neutral wire has great amount of shocking voltage compared to earth.
 

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Thread Starter

Willen

Joined Nov 13, 2015
313
The neutral wire should be at essentially earth potential.
Once I turned on a small TV and a VCD player with neutral supply and earth supply. That was free electricity to me because meter does not count neutral electricity. Later, distributer added two big earth poles around main transformer of mains, then my free electricity stopped working. It was more than a decade. I cannot figured out what was that!
How long have you had this problem?
Is it a new installation?
I am not sure, but I think problem was from starting. When I turned ON the mains of my house, it starts ringing.
 

MisterBill2

Joined Jan 23, 2018
8,760
Most probably it is the capacitively coupled voltage that is triggering the device. Even on our 120 volt systems there is a voltage present on wires not connected but running near mains powered wires. And using the neutral as a return assures that there will be enough voltage present to trigger that circuit. Probably adding a capacitor across the lines to the button will work the best. I suggest a value of 0.47 Mfd, or possibly 0.22 Mfd. That will act as a capacitive voltage divider and reduce the voltage appearing at the input by quite a bit, since the capacitance coupling. The problem with the resistor shunt is that it will result in a constant current drawn from the battery, and I presume that the supply is 2 batteries.
 

MisterBill2

Joined Jan 23, 2018
8,760
If it is, then it's illegal under most wiring codes.
Probably it is in violation of the NEC , but probably it would be quite a challenge to change. Of course, if the low voltage wire had the higher voltage rated insulation then it would be OK and legal. In most construction the doorbell wiring is not serviceable at most of it's locations.
 

Thread Starter

Willen

Joined Nov 13, 2015
313
Probably adding a capacitor across the lines to the button will work the best. I suggest a value of 0.47 Mfd, or possibly 0.22 Mfd.
Hi, your suggestion worked amazingly. Thank you! I added 220nF film 100V capacitor across lines (switch wires). I know the leakage voltage across the lines are shorted through the 220nf capacitor and the circuit started to worked without false trigger. But placing the capacitor there, take little bit current at first as a charging current. when this capacitor takes charging current, it need to trigger the PNP there, why it is not triggering while charging the cap? Because of pullup resistor of 10k or because tiny amount of charging current?
 

MisterBill2

Joined Jan 23, 2018
8,760
Hi, your suggestion worked amazingly. Thank you! I added 220nF film 100V capacitor across lines (switch wires). I know the leakage voltage across the lines are shorted through the 220nf capacitor and the circuit started to worked without false trigger. But placing the capacitor there, take little bit current at first as a charging current. when this capacitor takes charging current, it need to trigger the PNP there, why it is not triggering while charging the cap? Because of pullup resistor of 10k or because tiny amount of charging current?
The combination of a small charging current and the 10mFD capacitor, as well as that 22K ohm resistor, limit the charging current of the 220 nF capacitor.
 
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