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I'm making AC detector using bjt amplifier below is my circuit.

enter image description here

When it detects AC signal it will show result in LED and I want to add buzzer(3V) for show result in LED and buzzer ( 3V) so I connected buzzer in series with LED but result is buzzer not work. How can I add buzzer to this circuit to make it works?

Thank you ( sorry my English is not good )

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  • \$\begingroup\$ Please link the data sheet for the buzzer. Your circuit shows a loop sensor and, for a loop to work correctly it needs two connections. Maybe the guy who wrote loop meant something else like "plate"? \$\endgroup\$
    – Andy aka
    Oct 23, 2022 at 15:04
  • \$\begingroup\$ @Andy aka... Here my buzzer electropeak.com/3v-active-buzzer-tmb12a03. And you're right loop sensor he meant plate. \$\endgroup\$
    – Heroz
    Oct 23, 2022 at 15:28

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The buzzer has no reviews and no details on current required for the device.

so I connected buzzer in series with LED

However, it does say it can hang off an IO pin but, an IO pin might be able to source 10 mA. So, if it needs 10 mA (who knows?) then putting it in series with the LED isn't going to work because you'd drop 10 volts across R7 but the supply is only 9 volts and you need at least 2 volts for the LED to work. This is the most unlikeliest of scenarios to work.

Putting it parallel to the LED is an improvement but, if it needs 10 mA to make it work, you still have the same problem as mentioned earlier and, the LED (assumed to be a standard type) will limit the voltage across the buzzer to about 2 to 2.5 volts. I still don't think this will work.

And, you can't really put it directly from TR3 collector to positive supply either because, the buzzer maximum supply is 4 volts so, maybe, with a little bit of luck you can wire it from collector of TR3 to 9 volt supply with a series resistor of 470 ohms.

If this doesn't work then, unfortunately, with things bought from hobbyist sites with no reviews and no data sheet then this might prove to be a good lesson learnt i.e. you should buy from a reputable source that also provides a data sheet and has some good reviews.

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  • \$\begingroup\$ thank you. Fortunately it's work I reduced R7 and found that buzzer started to lound but the sound is low. \$\endgroup\$
    – Heroz
    Oct 24, 2022 at 12:38
  • \$\begingroup\$ OK that's good, you are getting there so, try and do what I said in my 2nd to last paragraph i.e. use a series resistor of 470 ohm for the buzzer but keep the 1 k series resistor for the LED @Heroz \$\endgroup\$
    – Andy aka
    Oct 24, 2022 at 12:53
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A 3V buzzer can work on 9V through a series resistor. Connect between TR3 collector and 9V. A better solution is to get a 9V buzzer. The C458 will handle up to 100mA. There is lots of choice.

Here is an example. You should research depending on what sources you have access to.

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  • \$\begingroup\$ Unfortunately the link doesn't work for me. \$\endgroup\$
    – ErikR
    Oct 24, 2022 at 0:49
  • \$\begingroup\$ Try the link now. There are several examples. \$\endgroup\$
    – RussellH
    Oct 24, 2022 at 0:55
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Here's an extract from the buzzer data sheet.

enter image description here

Considering a mean operating voltage of 3.25 V and an operating current of 15 mA, the value of the voltage dropping resistor would be 220 Ω.

enter image description here

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