PNP transistor turns on at 3.3v

Question

I have connected a 3906 PNP transistor base to a GPIO pin of ESP32 board. Emitter is connected to 5v and collector is connected to a buzzer. When I program the buzzer (send a HIGH from ESP32), it is permanently on. When I check it against a multimeter, the transistor turns on even when base current is 3.4v (when I though it turns on only if voltage is less than 0.7v).

To cross check the transistor and buzzer, I directly connect base to 5v and the transistor is turned off. Connecting to ground turns on. So, it works correctly when connected directly.

For those who would like to suggest a NPN transistor, I have tried using 3904 NPN transistor and everything works as expected. However the issue is that when programming, all the pins of ESP32 are high by default and the buzzer is ON until programming done.

Can someone tell me how to solve this issue? I need to use PNP transistor only. Thanks in advance.

Answer 1

I suspect you are trying to drive the PNP base with a 3.3V logic signal while controlling 5V to the buzzer. That just isn't going to work. To turn the transistor off (non-conducting) you need to raise the voltage on the base to be close to the emitter voltage, or about 5V. You can't do that with a 3.3V logic signal. You make a PNP transistor conduct by lowering the base voltage about 0.7V below the emitter voltage, not by setting the base voltage to 0.7V above ground.

You might be able to add a pullup resistor from the base itself to 5V to turn the transistor off when the logic output is 3.3V. Select resistor values so that when the 3.3V signal is low the voltage at the base is less than 4.3V. You would also have to select resistor values that limit the current that will flow back into the 3.3V logic output when that output signal is high. This is all just speculation, you haven't provided a datasheet for the device that is driving the base.

Answer 2

try this

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

The classic way to do this is with a circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}

Here, any input signal above ~0.7V switches the NPN transistor Q2 on, which pulls current through Q1's base to ground. When Q2 off, R1 pulls the base high, to keep the PNP transistor off. R2 and R3 are here to limit current through the bases of the transistors.

Answer 4

This will work:

^{simulate this circuit – Schematic created using CircuitLab}

The emitter of Q1 is at 3.3V so it is off when the input is floating or at 3.3V. When the input goes low, Q1 turns on, providing almost 3.3V at the top end of R2, providing about 0.5mA to the base of Q2. R3 drains away any leakage Q1 has when it is off.

If the buzzer takes more than about 5-10mA then decrease R2.

Answer 5

^{simulate this circuit – Schematic created using CircuitLab}

You may have to play with resistor values.