Why does the voltage drop when I connect an 8 ohms 1 watt speaker, specifically the source to the circuit is GPIO pin output voltage?

Question

This circuit I am using to drive a speaker is acting strangely.

Case 1: When I connect the 3.3 volts source from the Nucleo board it works fine as expected (in terms of the sound.) When I check the voltage across source it is 3.1 to 3 volts. A drop of only .2 to .3 volts.

Case 2: When I connect the GPIO output pin 3.2 volts as a source to the circuit, the speaker sounds very low. When I check the voltage at the GPIO pin it is 2.1 to 2 volts. It drops nearly 1 volt.

Why is it dropping so much voltage? Due to this speaker does not sound as expected.

What is the issue? Can I resolve this?

Edit: The GPIO is from a BlueNRG-1 chip.

Answer 1

You need to do something like this:

^{simulate this circuit – Schematic created using CircuitLab}

That delivers power to the speaker as needed, but allows you to turn on the BT66 (which seems to be an FM radio receiver) with the signal from your GPIO pin.

As has already been noted, IO pins from microprocessors aren't intended to provide power. They are control or data signals, and cannot provide high current.

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The above sketch assumes that the BT66 is powered from something appropriate. You didn't show the power connections for it in your sketches, and I didn't make an effort to find a datasheet or other description of it.

Answer 2

The issue is that you assume that a GPIO pin is a strong output like a power supply, but it isn't.

The speaker is an 8 ohm load that would take more than 400mA when connected to 3.3V. That would consume about 1.36 watts.

A typical GPIO pin is much weaker, they typically can provide up to 20mA without getting permanent damage, and even that with reduced output voltage.

So expecting a GPIO to drive an 8 ohm load won't work. You need to drive a transistor or other amplifier with the GPIO pin, and use the transistor or amplifier to drive the speaker.

Answer 3

A GPIO pin isn't intended to power circuits. It's a logic output for signalling to other devices, or to turn on a transistor. To a GPIO pin, an 8 ohm speaker is pretty much a short circuit.

Use the GPIO to turn on a transistor, which then powers the rest of the circuit.

Answer 4

The main issue (even though the details of the circuit are unclear) is that the GPIO pin isn’t sourcing enough current to drive the BT66T. You may be able to resolve it by buffering the GOIO pin. A simple approach would be to use a general-purpose p-channel FET, connect the gate to the GPIO pin, drain to your 3.3V supply and source to the VI pin of the BT66T. Then drive the GPIO pin low to activate it. Be sure to choose a FET with a low threshold voltage of about 2V.