What do you want your circuit to do? Please be as specific and detailed as possible. Why not buy a digital multimeter for $10 - $20? The circuit you show will exhibit high current gain, but it might even drive the LED and buzzer when there is nothing that you would consider to be detectable.
What do you want your circuit to do? Please be as specific and detailed as possible. Why not buy a digital multimeter for $10 - $20? The circuit you show will exhibit high current gain, but it might even drive the LED and buzzer when there is nothing that you would consider to be detectable.
Yes OK. I think my post went up about the same time as the one above it which explains the intended application. I guess it might work. I do not like having BJTs with base connected directly "out to the world" and no base-emitter resistor. As the circuit is shown, I think that any collector-base leakage current (or any strong AM band radio stations) might cause it to turn "on."
If I were going to try this I would consider a couple of other ideas. If we want to stay with 3 transistors, one approach would be to use a 2N3904 NPN driving a 2N3906 PNP driving another 2N3904 NPN. I would use some high value base-emitter resistor on each, and current limiting resistors in series between each.
Another idea would be a single comparator similar to LM339 but this would probably need to be built with surface mount (SOT-23-5) so a real pcb is probably needed unless you are really good hand wiring small stuff. The LM339 does not put out much current so driving a 2N3906 or 2N4403 with it would allow more current output. The comparator would allow triggering on lower voltages such as maybe 50 - 100 mV p-p coming into the "antenna". Another thing which could be included would be a potentiometer at the comparator input to adjust input voltage sensitivity.
But I guess what I am trying to do is to like taking a design for a foot propelled scooter and turn it into a bona fide motorbike.