How much current does the buzzer draw?

Conceptually, when the port pin is a logic '0' then voltage should be 0v, but reality sets in. The 8051 port pin has an effective resistance, so depending on the current passing through the port pin to 0V determines the actual logic zero voltage as seen on the port pin. The datasheet for the actual device usually gives you the voltage at a certain current value and will normally specify the maximum current per port pin.

Futher to this, the port pin when read, can actually register a logic '1' even when written to logic '0' if the current causes the port pin voltage to rise above a specified value.


In short, you could expect to sink a few mA though a 'standard' 80C51 port pin whereas for something like a Atmel 89C2051 you could sink around 20mA per port pin. Always read the actual datasheet for the device you are using for its values.

If this current value is not enough, you must add circuitry to amplify the current capacity. Obviously, this circuitry differs depending on the actual currents and voltages involved as well as the type of load. So it is always necessary to qualify and quantify these in order to come to a solution.