Electronics > Beginners
Adding a horn on a delay: where to begin?
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Zero999:

--- Quote from: vmallet on April 02, 2019, 06:46:49 am ---
* What is D1 used for, some sort reverse polarity protection?
* D1: I'm using a 1N4001; is that ok?
* What is R1 used for? Some form of current limiting? Why?
* What is D2 here for? Shunts the current in case V1 gets too high (> 15V) to protect the regulator?
* D2: I couldn't find BZX84C15L so I used BZX55C15 R0; it looked similar enough but I'm not sure what the differences really are, it's hard for me to tell at the moment what's important and what's not in a data sheet..
* C1/C2: do they need to be of a specific type? I used MLCCs for both but the two symbols on the schematic are different, not sure if I should read something through that
--- End quote ---

* Yes.
* Yes.
* When the zener conducts, the resistor will limit the current to a safe level.
* Yes, high voltage spikes are common in automotive systems, so over-voltage protection is necessary.
* That's a better choice: the higher the power rating, the better.
* They're decoupling capacitors. The symbol for C1 is polarised and C2 non-polarised. It's fine to used non-polarised for both. Be careful with MLCCs, they often have a much lower capacitance, when the voltage is near the rated value. This circuit will still work with lower capacitance values. Indeed, you'll probably find it works with no capacitors, but it just won't work very well and you may have problems with the power supply voltage dropping, causing the microcontroller to crash and reboot randomly.

--- Quote ---The "black" resistor resting (in peace) on top of the breadboard is the first 22R resistor I used; the cloud of smoke it produced when I ramped the input towards 12V helped me realized I had put the Zener backwards :) 1/4W is not going to deal very well with a few W of power, one more lesson learned!
--- End quote ---
I'm sure that sort of thing has happened to most people who build circuits at some stage.

I notice you appear to be using the LM7805, which is fine, but bear in mind it's rated to 1A and often doesn't current limit until 2.2A, so if there's a short circuit, R1 will get hot. Using a higher wattage resistor >0.5W or lower current regulator, such as the LM78L05, which will limit the current to  <220mA, will prevent this. I meant to write LM78L05 on the schematic: LM7805L was a typo.
David Hess:
The easiest way will be a 12 volt time-delay relay.  Building your own circuit includes building it for an automotive environment.

Zero999:

--- Quote from: David Hess on April 02, 2019, 09:58:49 am ---The easiest way will be a 12 volt time-delay relay.  Building your own circuit includes building it for an automotive environment.

--- End quote ---
Yes, that will work, but I thought this was just as much for educational purposes, as anything else.
https://www.littelfuse.com/~/media/protection-relays/catalogs/littelfuse_protectionrelays_time_delay_relays_catalog_section.pdf
Kasper:
To adjust the delay, you might find something you like on digikey in this category:
"Potentiometers, Variable Resistors"

Create a voltage divider with a resistor and a variable resistor and the variable output voltage can be used in discrete circuit or MCU circuit to set the delay time.
vmallet:
The goal is definitely to learn plenty along the way and so far it's been very helpful. The fancy relays from littlefuse.com are quite awesome though, it would make it ultra easy if the only thing I cared about was really just a delayed on/off. Epic!
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