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Recently, I have ordered smt-0440-t-r buzzer from Mouser.

Circuit is as shown in figure.

enter image description here

Microcontroller pin transmits square wave of 4kHz as mentioned in the datasheet. However, the sound isn't audible. When I am at a distance of 1-2cm from the buzzer, I am hearing very low intensity sound. Any fixes?

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  • \$\begingroup\$ Do you have a oscilloscope to check the waveform on the collector of Q4? Also, you could replace R109 with a 0 Ohm link, and remove the capacitor C103. \$\endgroup\$ – Steve G Apr 29 '16 at 12:00
  • \$\begingroup\$ I have got a square wave at the collector of Q4. Here's what I have done till now: a) I have replaced R109 with 0 ohm and also removed C103, the intensity of sound has slightly improved (2-4 cm range). Not close to audible range though. b) Removed R111 too, even this isn't working. \$\endgroup\$ – NareshR Apr 29 '16 at 12:30
  • \$\begingroup\$ Do you have D14 the correct way round? \$\endgroup\$ – Steve G Apr 29 '16 at 12:36
  • \$\begingroup\$ Now, I have removed the diode too. Still not audible. I have checked the current consumption of this circuit, it is 90 mA. I am attaching the datasheet of the buzzzer link . Can it be the buzzer parameters like wrong spl level etc? \$\endgroup\$ – NareshR Apr 29 '16 at 12:51
  • \$\begingroup\$ 1. You wouldn't normally run a transistor with no current limit into the base. Usually, a transistor switch is run with a beta of 10, so if your collector current is 90mA, your base current needs to be 9mA or so, which would make the base resistor about 240 ohms if your base drive is 3.3 volts. 2. Do you have the + side of the buzzer connected to the supply? 3. Do you have the transistor connected backwards? That is, with the collector and the emitter swapped? 4. How square is your square wave? \$\endgroup\$ – EM Fields Apr 29 '16 at 13:05
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  1. Look at your transducer. If it has a sticker over the sound hole remove it.
  2. The resonant frequency is 4K +/- 500Hz. Try changing the frequency through several other settings to see if resonance is the problem.
  3. Check the drive waveform from the MCU pin. Does it allow the NPN transistor to turn full on and off? Use an oscilloscope to check this.
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  • \$\begingroup\$ I have got a square wave at the collector of Q4. Here's what I have done till now: a) I have replaced R109 with 0 ohm and also removed C103, the intensity of sound has slightly improved (2-4 cm range). Not close to audible range though. b) Removed R111 too, even this isn't working. c) i have also played with frequency from 500 Hz-7 kHz \$\endgroup\$ – NareshR Apr 29 '16 at 12:33
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You have checked almost everything and nothing gave a definite improvement. I'm having a guess here that you don't have exactly the same buzzer as in the datasheet. With a specified coil resistance of 17 Ohm this should be a fixed coil / moving diaphragm buzzer, which will work OK with your original circuit.

However if you actually have a piezo buzzer you need a slightly different drive circuit to get a good sound level out of it. A piezo buzzer acts very similar to a capacitor and needs a push-pull driver circuit to charge and then discharge the capacitance of the piezo. However a piezo will work quite well with a small modification to your original circuit, with a 1k Ohm resistor placed across the piezo buzzer to discharge its capacitance.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I tried the above circuit, it isn't working though. I have contacted other manufacturer, CUI, waiting for the response. \$\endgroup\$ – NareshR May 10 '16 at 18:48
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I've had and solved this exact problem that you're having. What Steve said is the key, a piezo buzzer appears electrically as a capacitor.

Now with that in mind, look at your circuit. You're just going to charge the capacitor once, and then it's going to more or less stay charged and produce no more sound.

The loudest way to drive this circuit is using a full H bridge, this will provide a VPP of 2x your input voltage.

Another method that will work is to use a totem pole driver, or an op-amp so that you can drive one side of the piezo high and low. This is simpler, but it will produce half the amplitude.

What I ended up doing in my application is just driving both sides of the piezo directly using 2 microcontroller outputs. This will work fine with an appropriate current limiting resistor, and it may be loud enough for your application.

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  • \$\begingroup\$ can you give a more detailed illustration with a figure? \$\endgroup\$ – lukeluck Oct 12 '19 at 1:32
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How is your hearing and what is your age? If you are 45 years old then your normal hearing loss at 4kHz is -23dB so you will barely hear it. Hearing aids like mine (I am 73 soon) will fix the loss.enter image description here

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