ledtransistors
As you can see on the illustration below, I have a voltage supply (a keyboard made of two piezo elements) with two terminals. Each one drives one LED, but since I've added a buzzer in parallel with both LED, my circuit drives the LEDs at the same time
Do you have any idea on how I could make flash one LED at a time using just one buzzer?
Thanks in advance 😉
The Atmega328 chip used in many Arduinos has a limit on the amount of current it can provide directly. You can find this in the datasheet section 28.1 page 313.
simulate this circuit – Schematic created using CircuitLab
If you are using the Atmega IO pins to turn on transistors (FETs or BJTs) and use the 5V or 3.3V supply to provide current to the LEDs through these transistors then there is no problem with the Atmega328 current limits.
FET circuits are simpler. For larger numbers of LEDs it may be better to use a suitable LED-driver IC instead of using large numbers of transistors.
The voltage regulator on the Arduino board that is providing regulated 5V or 3.3V power will itself have a limit which may also prevent you using it to drive a large number of LEDs (or a lower number of high-power LEDs).
Neither is very good for your intended task, but the coin cells are better than the A23. In the datasheet for each, you have the Typical Capacity spec (Capacity in milliamp hours at a given load) as well as some information on Load vs Capacity or Voltage Drain vs Capacity.
In short, the A23 is typically 55 mAh, when drained at 2mA (0.002A) (Maybe 0.2mA, considering the 20kΩ load, faulty datasheet dropping the leading decimal point?). Yet has only 25 mAh at a 15mA load. The CR2025 typically has 163 mAh at nearly the same load as the A23. While the discharge characteristics might not be the same in theory, in practice if everything else is equal, you will get 3 times the capacity.
Not knowing how you will actually power the lights, two 83mA lights is 166mA, so if you power them straight on 100% of the time, you will get considerably less than 1 hour life out of the CR2025s.
Best Answer
Add a diode in each lead from the buzzer to the collector of the transistor (in that direction).
Current can flow through a diode in one dircetion only (the 'arrrow' direction) so it can flow from Vcc through the buzzer and a diode to one of the transistors and to ground, but not from one LED to the other transistor, because one of the diodes would block it.
An 1n4148 will do up to ~ 100 mA, otherwise pick an 1N4004. (or any other 1n400*)
PS your opamps feed the base of the transistors directly. Unless it is a current-output opamp (in which case the circuit would not work as expected) this is not a good idea. Put resistors in series with each base, 10k will probably do.