i'm an eager electronics noob, so sorry if this is a dumb question...

I'm trying to build vactrols controlled by audio signals to control other circuits. So far this configuration with a 2n5457 enter image description here seems to work best on breadboard (i really dont trust breadboards).

How can that be? And is it due to the weakness of the signal coming out of my phone or would it apply to an amplified signal aswell?

Thanks and greetings, Jochen

Edit: refined image...

Edit2: After finding i will probably not be able to design a circuit as complex as that and the use (light vactrol in circuit bent device) isnt worth getting into electronic engineering for a month, if that would ever happen, I tried another way with an op-amp.

Back to "i hate breadboards"... at first it worked, not satisfiyngly, but it did. Then not anymore (Led lights up all the time). Unfortunately i always seem to fail with these simulation softwares aswell...

Design idea was this: enter image description here

Whenever there is an Input signal, there would be a difference between the Inputs and the Opamp would Amplify (ideally to ~5V, therefore the pot in the feedback loop), while the Schottky diodes for lower Voltage drop (input signal might not be larger than 1-2V p2p) might even filter out some noise (at least in my head)

  • 2
    \$\begingroup\$ A quick search tells me that the 2n5457 is an N channel jfet. Are you sure you want a Jfet in this application? Because you’ve drawn the part as a BJT. \$\endgroup\$
    – Bryan
    Jul 9 at 5:22
  • \$\begingroup\$ Sorry, my bad... yes its a N channel jfet. With a 2n3904 it didn't work, for me at least \$\endgroup\$
    – jochknoch
    Jul 9 at 21:56

1 Answer 1


You need to designate emitter and collector of the transistor and if it's PNP or (more likely) NPN. A link to its datasheet would be best. Also please characterize the input signal from your phone. It will need to have a peak voltage well above the typical 0.7 V B-E. And with a 700k resistor (if I read chicken-scratch correctly), to drive 1 mA into the LED with a transistor having hFe of 100, the LED will light only when base current is at least 10 uA. This will require 0.7 V plus 7 V.

Also, please use the provided CircuitLab software to draw a legible schematic (with legible values), like this:


simulate this circuit – Schematic created using CircuitLab

Voltage of input signal and voltage of LED

Current through LED and current of input signal

Your input signal has its own internal impedance and other characteristics which will affect performance. And simulators have their limitations, so real world results will differ.

(edit) Learning from the comment, it appears that the 2N5457 is actually a JFET, which changes everything.


simulate this circuit

Signal and LED voltages

LED current

As can be seen, the LED remains ON until the input signal goes below about -3 volts.


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