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I have a project where I am driving a car horn that is rated for 12V@8A. I have a pack of NiHM batteries rated at 9.6V (usually get's up to about 11.3V) total and their spec'ed out to 10A for 8 minutes. I also have an Arduino mega and a Beefcake Relay kit.

There is a point in my program where the car horn sounds off 3 times (ON 500ms, OFF 500ms). However, when the car horn starts, it usually gets through 1 whole sound, then resets the Arduino. Both the Arduino and the car horn are being powered off the same 9.6V batteries.

My theory is, the Arduino is being starved of voltage when the horn goes off, then resets.

I want to know if it's possible to make a small (in physical size) capacitor bank that I can use to keep the Arduino alive while the car horn is being used in short 0.5sec bursts.

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    \$\begingroup\$ It might be useful to measure the voltage while the horn is sounding. It could be dropping out like you suspect, or it could be noisy. Those are slightly different problems. \$\endgroup\$ – AaronD Sep 24 '15 at 21:26
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    \$\begingroup\$ To reduce the hassle, I'd strongly suggest getting two battery packs, one for the electronics and one for the horn (via the relay) and isolating them. That might not be a possibility in your setup, but seriously consider it. It will make things so much easier with electrical noise. An alternative is to buy a good USB/power connector and run the Arduino off that, connected to the same battery (if you must). Most switching regulators usually do amazingly well in ways it will be impossible for an amateur circuit to emulate. \$\endgroup\$ – Dan Sheppard Sep 24 '15 at 21:52
  • \$\begingroup\$ AaronD, Yes, I have thought of that. I'm planning to do that this weekend. Dan Sheppard, Ideally, I only use this one battery. When you say a good "USB/power connector, you mean the actual plug (USB type-B I believe)? \$\endgroup\$ – jojoguy10 Sep 25 '15 at 17:30
  • \$\begingroup\$ I just tried getting a lower amperage horn (12V@4A=48W), and it didn't help. \$\endgroup\$ – jojoguy10 Oct 6 '15 at 20:49
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The real problem seems to be that your battery can't run everything you are trying to make it run. The best fix is to address that directly. Get a bigger battery or less stuff to run.

Does it really need to be a 96 W car horn!? How much noise are you really trying to make? Chances are that you don't need that much noise, and/or that there are more electrically efficient ways of making such noise. For example, there are some self-oscillating "buzzers" that are still quite loud but take a lot less power than that. Then there are also things like fire and burglar alarm horns that are meant to be run from backup battery, so probably don't need 96 W to make a lot of noise. 96 W is no big deal in a car, so efficiency was likely not much of a design criterion for the car horn.

In any case, it is probably a good idea to put a Schottky diode in series with the power feed to the computer, followed by a cap to ground. How big the cap needs to be depends on how much current the computer draws, which you haven't told us.

Another strategy is to charge up a cap slowly to be used only by the horn. When the energy in the cap is spent, the horn goes silent. That will likely give you a shorter horn burst (with this pig of a horn), but it won't be able to load the battery to the point it can't power the computer anymore. It will also take some dead time between horn blasts for the cap to recharge before the next horn blast is possible. This is very much like the concept of a camera flash having to charge up before use, then having to wait for it to recharge before it can be used again.

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  • \$\begingroup\$ I got it from Sparkfun. sparkfun.com/products/retired/10966 Plus, this is for a paintball field. The louder, the better :-) By "Computer" I'm assuming you mean the Arduino? And, should is be the entire circuit that is being controlled by the Arduino that I base my cap size on? Or just the Arduino draw itself? I thought about your last suggestion. Ideally, I get my 3 times of ON 500ms OFF 500ms out of the horn. Not sure if a capacitor can pull that off? \$\endgroup\$ – jojoguy10 Sep 25 '15 at 17:35
  • \$\begingroup\$ Hello Olin, I just bought a horn that says is 4A (equating to 48W). It seemed to work even worse. It would reset the Arduino in a shorter amount of time than the 96W horn did. Thanks for the help, but I'm not sure if it works. \$\endgroup\$ – jojoguy10 Oct 6 '15 at 20:43
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Are you switching the horn with a relay? If so, and the relay coil isn't clamped, the problem may be inductive spikes, which can have considerable energy.

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