Hallo Aman,

the free-wheeling diode is added sometimes. Although rather small, inductivity of mowing coil loudspeaker is not negligible. Most Hifi-amplifiers do contain fast switching free-wheeling diodes.
It depends a bit on how fast the driver is turned-off: The higher the change rate of current, the higher the induced voltage. May be, free-wheeling diode can be omitted in here discussed applications, because often used small signal transistors are only able of slow switching. But I would always add a free-wheeling diode in combination with loads showing relevant inductivity.

The capacitor stuff works only in combination with push-pull output. Also, output impedances when providing low state and high state should be rather similar, otherwise the cap is only charged and not discharged, or vice versa.

I would recommend you this very simple circuit:



If you need rather high currents through loudspeaker a NPN transistor is not recommended. Standard '51 micro cannot deliver high output currents, when producing high state. A PNP is much more useful, because much bigger currents can be sinked by the micro. Also, '51 works with 'negative logic', means when micro is resetted all outputs are producing high state. If reset time then is lasting longer, then all the time a rather high current would flow through loudspeaker. Anyway what happens to the loudspeaker, there's at least a rather high supply current which flows. Not a good idea...

How to design such a circuit?
First, current through loudpeaker must be decided. At current less than 100mA I would use something like BC556 or so. These transistors need a base current of about 1/20 of collector current. So, if collector current is choosen to be about 100mA, then base current must be in the range of 5mA. This results in base resistor of about 1kOhm. If collector current is choosen much smaller, then also base resistor can be increased.

To guarantee that collector current is limited to less than 100mA, collector load impedance must be higher than 50Ohm! If highest sound pressure is wished, it would be the best idea to choose a loudspeaker with a DC impedance of 50Ohm. If types are used, which provide much smaller DC resistance, then a protection resistor Rp must be inserted. Assume your loudspeaker is 8Ohm type, then Rp = 47Ohm should be used. But then only a fraction of total power consumption is used to make sound.

If, on the other hand, maximum sound pressure is not the key parameter, then loudspeakers can be used providing much higher impedances than 50Ohm.

Keep in mind, that this extra portion of current, which draws the loudspeaker, should be adequately decoupled! In the schematic a 10µ/10V Tantal makes the job. It should be connected directly to emitter of transistor and ground pin of loudspeaker, otherwise ground noise can develop. Think about how the current is flowing through this loop, when the transistor is turned-on: If capacitor is connected correctly, the loop current does NOT flow over PCB ground routing.

A 1N4148 is used as free-wheeling diode. This is a good solution as long as the current through loudspeaker is not increased over 100mA. If higher currents are needed, a 'stronger' diode should be used.

Keep in mind, that this circuit does not provide a DC protection for the case, that the port pin is not put to high state after toggling (making sound)! If port pin is permantly emitting low state, all the time current is flowing through loudspeaker. In most cases this only causes an increase of current consumption, if a loudspeaker is choosen, which is powerful enough to withstand this condition. Keep an eye on this issue, when writing your code: After making sound, according port pin must always return to high state. This can only be a problem, when you forget this when dealing with interrupts! So, don't forget it...
To prevent damage of loudspeaker in this condition (think of testing phases...), power rating of loudspeaker should be high enough: Im = 5V / (Rp +RLS) gives maximum current 'Im', Pm = (Im x Im x RLS) gives maximum power 'Pm', if permanently current is flowing through loudspeaker.
By the way, Im x Im x Rp is power consumption of resistor 'Rp' in this case.
When making sound only the half of this power must be withstood, assuming a duty cycle of 50%, of course!


Kai