I would like to detect motion to turn off my 3.8 V, 0.3 A DC light bulb using a PIR motion sensor.

Recently I tried to connect it to a 5 V relay, Arduino Uno, and two 3.7 V Li-ion batteries, and the bulb seemed to burn out.

Any ideas on what should I do to my circuit, and what values of battery I need? Or should I put in a resistor instead?

I tried to modify the schematic with battery and DC light bulb referring to the schematic from here:

enter image description here

  • 4
    \$\begingroup\$ Did you apply 7.4V across the bulb? \$\endgroup\$
    – vu2nan
    May 27, 2020 at 11:10
  • \$\begingroup\$ Apparently yes, is that why my bulb burnt? \$\endgroup\$ May 27, 2020 at 11:19
  • 1
    \$\begingroup\$ Most probably. Could you please furnish a schematic? \$\endgroup\$
    – vu2nan
    May 27, 2020 at 17:29
  • \$\begingroup\$ Do you have a 5V battery? Why do you need 2 x 3.7V batteries? Can you manually draw the schematic of what you wired and upload a photograph? \$\endgroup\$
    – vu2nan
    May 28, 2020 at 9:25
  • \$\begingroup\$ Thank you so much for the schematic. Sorry but does 5v battery exists? I can't seem to find any. \$\endgroup\$ May 30, 2020 at 0:59

1 Answer 1


Here's the schematic.

A 3.9Ω 1W voltage-dropping resistor is used in series with the 3.8V bulb.

enter image description here

  • \$\begingroup\$ For the relay, do we connect it to no or nc? Because I want to turn off the bulb when the pir detect the motion. \$\endgroup\$ Jun 13, 2020 at 9:14
  • \$\begingroup\$ Hi Aween, 'NO' contact is shown. To turn off the bulb with the signal, you would need to wire the 'NC' contact. \$\endgroup\$
    – vu2nan
    Jun 13, 2020 at 9:39
  • \$\begingroup\$ Hi Aween, Is my answer acceptable to you? \$\endgroup\$
    – vu2nan
    Jun 26, 2020 at 6:32

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