I have a circuit where my microcontroller is turning off and on a heavy load using a power mosfet (I've also tried it with a mosfet and a relay which yielded the same results). When I use a power resister at 1.5ohms everything works as expected. However, when I replace the power resister with a car horn (measured at around 1.5 ohms using my multimeter), then when the circuit triggers the horn, I hear a horn sound for about a quarter of a second and then it stops, like the horn is somehow resetting my microcontroller.

Does anyone have any idea why a horn might act differently than a regular resister?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ There's a big difference. No doubt that it is EMI from the rapid dI/dt of the discontinuous astable inductive resonator or basically a moving coil radiator with a cutout switch or as we call it " a noise generator" Use twisted pair for EVERYTHING or Shielded TP (STP) cables and EMI filters, Big single wire loops make great antenna for noise radiation \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 15 '18 at 21:55
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    \$\begingroup\$ And, as there is a coil in the horn, start with adding a fly-back diode. \$\endgroup\$ – Oldfart Dec 15 '18 at 22:04
  • \$\begingroup\$ I just ordered some diodes that should be able to do the job. Sorry, I'm not the most experienced EE, when it comes to an EMI filter, is that something I can just buy as a discrete IC or something an put in parallel with the horn? \$\endgroup\$ – Chase Roberts Dec 16 '18 at 1:46

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