Electrical – Arduino buzzer resistor requirements

Your NPN power transistor seems to be connected up wrongly.

I'd expect something like this ...

^{simulate this circuit – Schematic created using CircuitLab}

where

• R1 limits base current and protects P1. \$R_1 = 5V - V_{BE(sat)} / I_B(20mA?)\$
• R2 limits LED current. \$R_2 = (18V - V_{LED} )/ I_{LED}\$
• Q1's collector is connected to the LED's cathode.

Perhaps your LED is actually an 18V LED module that has current-limiting built in?

Note: I believe piezo buzzers don't draw much current, you probably don't need a TIP31 to drive it. Check the specs and choose a more suitable transistor. To be connected as shown, the piezo buzzer should be rated for more than 18V.

One issue is that you do not have a fixed grounding resistor on the transistor. This may not always be needed. However the Uno ports are not programmed at reset or when there is no program running. In those cases the port may be set to a high impedance input with or with out a light pull up resistance. In this case you might be unknowingly turning the transistor on intermittently with noise or pull up leakage current.

To insure stable operation be sure to set the controlling port pin quickly to a digital output and set the initial logic value. Adding a resistor to ground, (before or after the existing 10k) will help prevent an open port or a light pull up from activating the transistor. With a 5v Uno a matching 10k should be OK. Lower resistor values reduce the chance of unknowingly activating the transistor, but at some point the high signal will not be able to turn it on.

If immediate switching time is not critical adding a small cap on the transistor base to ground can prevent fast multiple ON-OFF cycles that could reduce the life of the bulb. (Maybe 1uf).