Gentle.....?

I just connected up a small speaker in series with a <big>1.8 Kohm </big> resistor that was driven from a function generator with a square wave at 5 volts peak to peak amplitude. The speaker is one of those 300 ohm jobs that one finds in a airline first class headset (the electrical type as opposed to the acoustic type headset you get in the cattle car seats).

I found the "noise" emitted from the speaker to be anything but gentle over frequencies from 100 Hz to up over 3 KHz. And it was also quite audible. So if the application is one to have an 8051 emit an acknowledge tone, a key click, or a 'bad input' type thing this will be a quite good implementation.

On the other hand if your trying to wake up the engineer in the next lab then you may want to further buffer a speaker like in the simple circuit below and use a speaker more on the order of 4" -> 6" (100 mm -> 150 mm) size.



If you use this circuit then make sure to select a proper NPN transistor and make the wattage rating of the 22 ohm resistor big enough.

Michael Karas