Electronic – arduino – Switching Piezo Buzzer with a Transistor

Do the math on the currents. That transistor only has a gain of 20 that you can count on. With 1 kΩ base resistors to 5V, the base current will be about 4.3 mA. That times 20 is only 86 mA. That will be shared by all the LEDs that the transistor is driving. Since you say there are 15 LEDs in parallel, that means each only gets 5.7 mA, which will be visible but dim.

What current do you want to run each LED at? That times 15 is the current the transistor needs to be able to sink. That divided by the transistor's gain is the minimum base current required.

There are several solutions that come to mind:

1. Use a transistor with more gain.

2. Give it more base current. The limit is what the arduino can supply. You'll have to check the arduino datasheet for that. Probably in the 10-20 mA range, but I haven't checked.

3. Use a FET instead. This is actually a good application for a FET. I like the IRLML2502 for low side swithing from 5V logic. In this case you drive the gate of the FET directly from the microcontroller digital output without a resistor in series.

It would help if you said what the max current per LED is intended to be.

You said you were a novice, so here is something simple:

The page you linked to turned out not to be a datasheet, so we don't know anything more about the piezo speaker than what you say. The above circuit should work well enough for most piezo speakers, although it can possibly be driven from a higher voltage, which would yield higher volume.

R1 limits the current thru the base of Q1 when the digital output is high. Figure the B-E junction drops 700 mV, so that leaves 4.3 V accross R1, which causes 4.3 mA base current and will draw that much from the digital output. Most digital outputs can source that much without issue. Check your datasheet. If not, make R1 higher. You most likely don't need as much base drive as this.

When the digital output is high, Q1 will have 4.3 mA of base current, which should easily give it enough current sink capability to pull the collector as low as it can go. Figure you can count on a gain of 50, so that means 215 mA of collector current, which should be way more than required.

D1 is there to give the kickback current of SP1 a place to go when Q1 turns off suddenly. Piezo elements have a significant inductive component as viewed from the driving circuit. If you don't give the kickback current a place to go, then the voltage can get high enough to fry Q1. R2 cause some back voltage to this current to make it die down more quickly, helping the high frequecy response. R2 can probably be a lot lower, but again, you didn't supply a datasheet so all the component values are just rough ideas or guesses at best.