Good evening, I would like to use a DB681 transistor through arduino, which, connected in NPN, must activate a 24V buzzer.
I would give the arduino command on the base, my difficulty is to calculate the resistor to put in the circuit and the possible diode.
That transistor is a Darlington type.
It is a high gain device HFE 750
The Vce saturation voltage is quite high so depending on the collector current, you may need a heat sink on the transistor.
We assume you are using this transistor as your buzzer requires high current.
What are the electrical properties of your buzzer ?
If the buzzer is the magnetic kind, place a snubbing diode across the device.
In the mean time, with the following assumptions, if you have an UNO and need 1A for the buzzer to operate:
1A /750 ≈ 1.3mA base current.
We will use 10mA to ensure the transistor is saturated.
(5Vcc - 1.6Vcb ) / 10mA = Rb ≈ 330 Ω (base resistor).
If the buzzer was 14v and Vce(sat) was 2v this means the transistor dissipation would be 2V * 1A = 2Watts HOT.
Is the 24V AC or DC?
Is this a real mechanical buzzer or a solid state device? I ask because a real buzzer creates a huge amount of electrical noise and will confuse your Arduino. There are alternate methods to drive such a load.
I know this is not your question however a reall
Thank you for your quick answer.
It is a two-tone buzzer used to indicate the reverse of forklifts.
Definitely solid state.
How would you have done to make the buzzer go on with 24 volts?
First off, is the buzzer rated for 24V ?
How much current does it need to operate ?
It's a Darlington! 
What does HFE have to do with it.
Please ignore this post, I just had to do it.
I would have suggested an opto isolator (which still may be a good idea) and careful wiring + capacitors to keep the voltage spikes away from the Arduino.
Back to the situation. Is your Arduino being powered from the same source as the 24V buzzer? i.e. like were you planning on using a DC to DC converter for 24V =-> 5V?
If not I suggest an opto isolator so you don't have to make the grounds common.
I answer all, the buzzer needs almost an Ampere. there arduino is powered separately at 5 volts.
JohnRob do you have any example of an opto isolator?
However I made the test circuit with the aforementioned transistor, and I used a 330 ohm resistor then I put a diode between the positive and the ground of the buzzer.
I don't know if I will use it maybe it would be better to use the relays for arduino.
As you may have understood, I am more of a programmer than an electronic one.
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