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I have seen circuits that are used to detect the presence of AC voltage without any contact, like the one below.

Non-Contact AC Line Detector using BC547

Non-Contact AC Line Detector using BC547

I have tested this circuit and it works fine.

I understand that when the probe is closer to the AC line, a small amount of electrical signal is transferred to the circuit due to capacitive coupling, and the transistors amplify the weak signal in each stage so that it can make the LED glow and the buzzer emit sound.

Can I do the same AC line detection using single Darlington transistor with a high current gain, for example BC517? The datasheet mentions that it has a minimum current gain of 30000.

Can I do the same AC line detection system with single BC517 instead of using three BC547? Like can I connect the probe directly to the base of the transistor and the buzzer and LED to the collector of the transistor? Will it work? If it doesn't, what are the reasons?

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    \$\begingroup\$ We don't know if it will work or not for your purposes. These are sort of gimmick toy circuits anyway, which you should not trust your life on. If the BC517 is a Darlington transistor with gain of min 30000, that equals two transistors with gain of min 173. The BC547 has min gain of 110, so three of them should have a gain of 1331000, which is 44 times larger than single BC517. You can try building it and then answer if it could be done or not. \$\endgroup\$ Commented Jul 9, 2024 at 11:10
  • \$\begingroup\$ Why not test your idea using a simulation tool. \$\endgroup\$ Commented Jul 9, 2024 at 12:25
  • \$\begingroup\$ Really you got to try it. In a real circuit. Probably will take less time than asking a question. \$\endgroup\$ Commented Jul 9, 2024 at 17:02

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