# Can I switch a transistor “ON” when the base/gate voltage drops?

I am using a sound board from Adafruit (this one) to play a small sound file.

What I would like to do is to close a transistor while the sound is playing, which will then close the circuit on a few LEDs. This will cause the LEDs to light while the sound is playing, and then turn off.

After finally finding the pin-out explanation there, I see that it has an ACT[ivity] pin which signals when audio is playing. While the board is on, the pin normally sits at roughly 3.35V, and then when the sound plays, the voltage drops by around 2 - 2.5 V. I see no change in amperage across the pin while the sound is playing.

As I understand it though, transistors switch when a positive voltage is applied to the base/gate. So I'm not sure what to do here. Is there a way to switch the transistor in such a way that when the ACT pin is sitting at 3.35V the transistor is open, and then closes when the applied voltage drops?

I tried to google on this, but all the articles and pages coming up have to do with the voltage drop across the transistor, nothing about switching the transistor based upon a voltage drop.

Below is the segment of my circuit I am trying to figure out at this point...

simulate this circuit – Schematic created using CircuitLab

Originally, I was planning on using the second speaker output on the board to switch my transistor, but that seems to have a constant voltage and amperage whether it's playing or not. Then I found the board's pin-out and the ACT pin, so I'm trying to make this work off of that now.

• You'll want to look up the difference between NMOS and PMOS FETs and enchancement and depletion mode FETs. – jramsay42 Feb 28 '18 at 4:35
• @jramsay42 that would be a PNP transistor? So far the handful I've been experimenting with are all NPN. For this usage, I should switch to a PNP? PNP transistors also, need to go up-stream from my LEDs, correct? – eidylon Feb 28 '18 at 4:40
• I'm not sure exactly what you are doing without seeing a schematic but yes a PNP BJT will turn on if you bring the base below the emitter. I mentioned FETs rather than BJTs just because they are more common in switching applications. – jramsay42 Feb 28 '18 at 4:47
• Have a look at electronicshub.org/transistor-as-switch - there is a PNP used as a switch – Sredni Vashtar Feb 28 '18 at 4:47
• PNP would work in that circuit to turn on at low voltage. at 3.3V on the base the emitter would be high enough to keep the LEDs dark. – Jasen Feb 28 '18 at 10:49