Questions about driving a car horn. - Page 1
Forum section: electronics / beginners

author: legrady
author_rank: Contributor
timestamp: June 06, 2022, 12:09:08 am
content: To perform sailing drills, such as tacking once a minute, I want to develop a device to provide a horn blast at defined intervals.I found a car horn that uses 12V, 1.5A ... so it's 8 ohms.Q 1 If I have a boost converter generating a voltage that I store in a cap and then feed into the horn via a transistor, How long a blast can I expect.Q 2 Can I use a lower voltage to obtain a quieter horn blast? Or would I have to increase the resistance, and thus reduce the current. I suppose relays switching inline resistors would provide the desired result.Thanks, guys & galsTom

author: Whales
author_rank: Super Contributor
timestamp: June 06, 2022, 12:39:06 am
content: QuoteYou can get boost converters that can supply 12V 1.5A continuously, they cost several dollars pre-assembled.Otherwise if I assume you are powering the horn with nothing but the charged cap: the cap will immediately start dropping in voltage as soon as you hook it up to the horn. The horn will change tone and become quieter and stop making sound at some voltage.I suspect that you will need crazy amounts of capacitance (hundreds of millfarads to whole farads)for 1.5A to last more than a few milliseconds. Using a boost converter to supply the full 1.5A continously would be much cheaper and smaller.QuoteMaybe. Some horns change pitch and timbre too. Your horn may have an adjustment screw on it too.QuoteAdding a resistor in series reduces the voltage across the horn. You may as well reduce the voltage across the horn in the first place.

author: David_AVD
author_rank: Super Contributor
timestamp: June 06, 2022, 12:42:59 am
content: Running an electro-mechanical car horn at a much lower voltage than normal will likely result in no output. It needs a certain voltage to operate the contact that causes the oscillation.

author: Whales
author_rank: Super Contributor
timestamp: June 06, 2022, 12:43:37 am
content: Why do you need a boost converter? What is your supply voltage?Would it be easier just to carry 12V worth of batteries (a string of 8 AA batteriesmightdo for short bursts, but 8x C cells would probably be more appropriate) and have a button you hold down to make the horn sound?

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2022, 02:15:29 am
content: 1.5A sounds a bit low for a car horn, maybe the modern ones are that efficient though. Forget about adding resistors, the horn volume is unlikely to vary much based on voltage, it will likely change in tone a bit and then stop working completely as the voltage is reduced.

author: rstofer
author_rank: Super Contributor
timestamp: June 06, 2022, 03:44:15 am
content: If you want to use some form of electronics to control the horn, you should look into using a single MOSFET on the low side of the horn (between one side of the horn and ground). Depending on the electrical noise generatedby the horn, you might want to have the electronics drive a relay that controls the horn.Amazon has battery holders and batteries to place 3 or 4 18650 batteries in series. 3 batteries seem about right, 4 is likely too many but it may work fine. Car batteries, with the engine running will be around 14V and that's not a lot less than 4 batteries in series.A suitable MOSFET:https://www.sparkfun.com/products/10213

author: vu2nan
author_rank: Contributor
timestamp: June 06, 2022, 05:14:39 am
content: Hi Tom,You may use an automotive time delay relay, adjustable from 0 - 60 s with the inbuilt trimpot.Nandu.

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2022, 12:38:49 pm
content: How about an electromechanical buzzer? The type in a metal can, not the plastic ones. I have a couple sitting around I ordered awhile ago for a project which didn't happen. They sound similar to a car horn, but not quite as loud. If I remember rightly they're specified to work at 8V, but work down to 6V and aren't harmed by 10V, making them suitable for use off a 9V battery. I can't remember what the current consumption is.Here are a couple of sites selling similar parts.https://www.shokaifareast.com/Products/Buzzers/Electro_mechanical/electro_mechanical_buzzers.htmhttps://www.componentonline.com/product/906A-9/DBZ-1Note that this kind of buzzer needs a back-EMF diode if switched using a transistor.

author: eugene
author_rank: Frequent Contributor
timestamp: June 06, 2022, 01:28:20 pm
content: Quote from: legrady on June 06, 2022, 12:09:08 amThe voltage across a discharging capacitor is one of the first things that students study in an electronics class:V(t) = V0e-t/RCSo if you have C=1F of super capacitor charged to V0=12V and you let it discharge through R=8 ohms you'll get the voltage shown in the graph. Note that 1F is a huge amount of capacitance. I have seen 1F super capacitors that could handle 5V. If you put 3 of them in series they'll be safe to 15V, but the net capacitance will be only 1/3 as much (0.33F) and the cap will discharge in 1/3 the time.Looking at the graph, I'd say that a full 1F of capacitance should get you more than one full second of horn blast before the voltage drops below 10V. With 0.33F you might be able to count on 0.5 second.These are just rough estimates. You might start out with more than 12V or your horn might work well below 10V. Also the voltage drop across the switching device is not considered. But, I would say your idea has a chance of working if you can get that much capacitance at a high enough voltage.I would also consider using rechargeable batteries instead of super capacitors.