Fig1. My buzzer driving

When I turn on/off buzzer via D17 using my Ardunio nano board, the two Leds RED and AMBER turn on/off too. I don't want this occur but don't know how to do this. Can anyone can help to show me what root cause make the Leds turn on?

Buzzer characters: Large Piezo Transducer

· Rated Frequency: 3,500Hz · Rated Voltage: 4 - 28 Vdc (DC input) · Current Consumption: 6mA @ 12Vdc · Sound Pressure Level(30cm): 87dB @ 12Vdc · Flying Leads Fitted · Kingstate Buzzer - KPEG253 · Dimensions: 28mm Diameter, 25.4mm High, 33.5mm between

  • \$\begingroup\$ You sure it's not your arduino program? \$\endgroup\$
    – Bradman175
    Commented Aug 6, 2016 at 6:39
  • \$\begingroup\$ In the circuit, you got one pin (D16) turning both "LAMP" and "ALARM" on and off at the same time. They can never be in different states to each other. \$\endgroup\$
    – Bradman175
    Commented Aug 6, 2016 at 6:41
  • 1
    \$\begingroup\$ Your code may be an issue, but I don't know why you have a flyback diode on a buzzer. That's normally reserved for relays or solenoids. \$\endgroup\$
    – Passerby
    Commented Aug 6, 2016 at 7:03
  • 1
    \$\begingroup\$ Have you accidentally not set pins 2 and 3 as outputs and driven them low? If they are left as inputs there's a slim chance that you're getting interference between the buzzer driving and the high impedance input pins, which might be enough to switch on the ULN2803 transistors. \$\endgroup\$
    – stefandz
    Commented Aug 6, 2016 at 7:24
  • 1
    \$\begingroup\$ +lhphuc Also why do you have an extra diode for the buzzer? The COM pin of the ULN2803 transistor array is for that exact purpose. So if you hook COM to VSUP, you can omit that flyback diode. \$\endgroup\$
    – Bradman175
    Commented Aug 6, 2016 at 8:26

1 Answer 1


Recheck all your wiring or board for shorts. If you can, start from scratch. If you manually pull the input high and the others low, does it still happen? Or only with your microcontroller? If only your microcontroller it's likely a code issue. A ULN2803 is a simple transistor pack and would not experience these issues unless there was a short or code problem.

Also, you should actively pull the other outputs low. Do not leave them floating or as inputs.


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