I have designed a circuit that will switch on a 12V siren through a IRF540N MOSFET. The siren positive is connected to ST-3 (12V), while the negative is connected to ST-4 (SIREN-12V). A flyback diode is used for protection.
The IRF540N has a gate threshold voltage of 2V to 4V. I am using an ESP32 to turn the siren on and off via the MOSFET. The output current of IO 13 of the ESP32 is 33mA and the voltage is around 3.2V, so this should be enough to turn the MOSFET on. However, this 12V siren when activated sounds very quiet.
When I apply the 3V3 voltage from the ESP32 to the gate of the MOSFET, the siren then sounds as expected (loud).
Why is there is a difference in the loudness of the siren? The MOSFET should turn on when GPIO 13 is high but it sounds like the is only turning on the gate slightly.