Screenshot from 2025-01-09 16-01-49.png (13.98 KiB) Viewed 588 times
If the transistor is a jfet, is the led normally on and will turn off when there is a sensed signal ?
What if is a mosfet instead ? Will turn on the LED only when the gate senses a signal ?
Why would this circuit be drawn without a current limiting resistor ?
Would this work in proximity of a DC charge or an AC (radio) signal or both ?
"If the transistor is a jfet, is the led normally on and will turn off when there is a sensed signal ?"
Normally "ON" - yes, reverse bias to pinch off. I don't see any bias.
Gate-Source Cutoff Voltage is -8 V. (I do not want to stand in a field that strong holding a sniffer.)
"What if is a mosfet instead ? Will turn on the LED only when the gate senses a signal?"
Which MOSFET? Is it depletion mode (gate pinches), or enhancement mode (voltage to gate opens)?
"Why would this circuit be drawn without a current limiting resistor?"
Zero-Gate Voltage Drain Current @15 V 2.0 mA to 20 mA (internal resistance). So no LED resistor needed with MPF-102.
"Would this work in proximity of a DC charge or an AC (radio) signal or both?"
AC. An antenna would only produce a voltage while moving in a DC field.
The one at: https://www.eham.net/article/38074 has a meter instead of an LED. There are a number of comments explaining why it does not work. I think the original designer ever actually build and tried it!
Putting an LED in place of the meter would be worse.
It may be intended as non-contact, but it is too easy to contact a live part or push the pickup into an outlet.
The battery is not shown in the image.
So AFAIC, except a bit of music, youtube is the land of perpetual motion machines and flat earthers, and should be avoided.
If circuit in post 1 is meant for AC mains detection, then it is also dangerous.
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something has been irritating a long time
example from this discussion -
"Is this what your trying to make?"
should be -
"Is this what you're trying to make?"
also consistently swapping their, there and they're (not by occasional typo)
{or are wrong homonyms on purpose to look like a friendly ignoramus? which obviously you're not}
