Suggestion for Magnetic Buzzer Sound Low

[sakibnaz]

Hello All.

In my project I designed Buzzer circuit like below:

Its a small size (7.5mm x 7.5mm) Magnetic Buzzer CMT-7525S-SMT-TR. Burt when I applying PWM then the Buzzer sound is too low. Its barely to hear.

I did applied various Freq range (Low to HIGH) and I can hear the sound changes but its too low in volume.

Can anyone advise on the schematic?

Thanks.

 

[wwfeldman]

you might want reduce R74, R72

reducing R74 will increase the current in the buzzer
since its rated at 100 mA, using 0.33 mA is a small
i suggest 1000 ohm, (3.3 mA) or less

your schematic doesn't show the voltage applied to the base of the transistor
reducing R72 will increase the voltage at the base and let more current into the base

what frequency are you driving the speaker?

 

[danadakk]

Their test circuit also shows a transient protection diode across the buzzer,
definitely put that in place to protect the PWM output from HV getting fried.

Note their test circuit does not show collector limiting R, do NOT omit that, limit
the current into the buzzer/transistor as you are now doing.


Regards, Dana.

 

[sakibnaz]

Hello.

- I replaced R72 with 1K
- I removed R73 totally
- I replaced R74 with 0Ohm (i.e. Buzzer +Ve terminal connected with +3.3V directly)

Now the sound level increases but very little bit. Still I need to place my ear close to Buzzer to hear the sound

Any more advise please.

Thanks.

 

[KlausST]

Hi,

Take a photo, so we can see details of wiring.

Klaus

 

[danadakk]

R74 = 0 ohms burn out buzzer and possibly transistor, if you haven't already.

Buzzer coil is min 13 ohms. Limit of 100 mA.

R74 ~= (3.3 - Vcesattransistor) /.1 amps = ~ 3.1 / .1 ~= 30 ohms. Power of R = I**2 x R ~=
30 x .01 or .3 W. Use 1/2W Resistor.

Base drive wants to be ~ Icollector /10, or 10 mA. This insures transistor is in saturation when on.

R72 ~= ( Vohpwm - Vbe) / .01 amps so look up Voh of processor and calc
that value.

If I use Voh ~= 2.5V, Vbe ~= .7V then R72 ~= ( 2.5 - .7 ) / .01 ~= 180 ohms.

Buzzer helps if mechanical cone placed over it. Small end on buzzer. Use a piece
of cardboard to try out. Normally in finished products this is a plastic piece. It wants
to be audio reflective, a hard surface like metal or plastic.


Regards, Dana.