Hi,
I am do a Non-Contact Voltage Detector by:
https://envirementalb.com/non-contact-voltage-detector/
every thing same except the BC547 (no handy) replaced by BC517.
The LED on as long as the antenna near to any cable with 6VDC, doesn't on with 3 VDC, whats wrong?
Thanks
Adam

 

BC517 is a darlington: you should be able to replace two of the BC547 s with it.
The circuit isn't properly biassed, so I'm surprised that it works at all.

 

BC517 is a darlington: the bias values will need to be adjusted to give twice the base voltage.

Thanks.
It's why the 3v no work and 6v does. then why it so sensitivity?

 

Without any proper bias setting, it's pot luck whether it works or it doesn't. It depends on its operation on leakage current, which is highly temperature dependent, and will vary wildly from one transistor to another.
Try cooling it down if it's on all the time, or warming it up if it's off all the time!

 

Without any proper bias setting, it's pot luck whether it works or it doesn't. It depends on its operation on leakage current, which is highly temperature dependent, and will vary wildly from one transistor to another.
Try cooling it down if it's on all the time, or warming it up if it's off all the time!

Thanks.
Replaced the BC517 by PN2222a, SEEMS ok NOW.
BTW. what's the easy way to set bia of here?

 

I am very surprised to read that it can sense DC. If it only sees DC, what causes base current to flow?

 

I am very surprised to read that it can sense DC. If it only sees DC, what causes base current to flow?

It's easy to see what causes current to flow - a gain of about a million at a conservative estimate, probably 10 million, would only need 1nA leakage C to B in the first transistor to cause 10mA in the LED. But I don't see how it can sense the presence of DC in a nearby wire - probably it works like divining rods.

 

I am very surprised to read that it can sense DC. If it only sees DC, what causes base current to flow?

Thanks.
Not even DC, any cable means even a piece of long wire without connect to power.

 

It's easy to see what causes current to flow - a gain of about a million at a conservative estimate, probably 10 million, would only need 1nA leakage C to B in the first transistor to cause 10mA in the LED. But I don't see how it can sense the presence of DC in a nearby wire - probably it works like divining rods.

Thanks.