Have you ever wanted to really blast someone to get their attention on the road, but don't want to blast yourself when you lock your car or just need to beep real quick? If so, this is for you!

The purpose of this is to add a second louder horn to my car that I can use sometimes, but not have to add an extra button somewhere or have it going off all the time. In order to accomplish this I will make a circuit that turns on the second horn with the first one, but slightly delayed in time.

Is this legal? I'm not an expert, so the burden is on you if you choose to do this project. I would guess that adding a second horn to a vehicle (as long as it's a reasonable one) is allowable in many places, but I'm sure there are others where making a modification like this wouldn't pass inspection if your state/country has one. Be cool!

Step 1: The Circuit

A good practice when adding something electrically demanding to a vehicle is to use a relay to switch the load. This is a good idea for a couple of reasons: it doesn't put extra load on the existing wiring (which wasn't designed for it, and could blow a fuse or start a fire), it lets you use a small switch for control (instead of switching the entire load), and it can minimize the amount of heavy gauge wire needed by making a shorter run between the battery and the load.

This circuit uses the 12v signal from the existing horn wiring to switch the relay which in turn switches the current from the battery directly to the second horn.

Components used:

R1 - 2.2k resistor
R2 - 0-10k potentiometer
R3 - 10k resistor
C1 - 100uf electrolytic capacitor
D1 - 10v zener diode
U1 - IRFZ44 N-Channel Mosfet

These may not be the optimal components to use, but they are what I have on hand. This circuit is almost certainly not the best design for the purpose, but I'm an amateur and like to keep expectations low!

The purpose of the potentiometer is to give some adjustability of the time between powering the circuit and the relay switching on.

I was aiming for a second or less of delay, but this can easily be increased by using a larger capacitor and/or fiddling around with the resistance. Experiment with it!

Step 2: How It Works

The first image above demonstrates the delay in time between 12v being applied and the relay switching. The blue trace is the "new horn" side of the circuit, and the yellow trace is the "signal" side from the car's existing horn. There is about a half of a second between the signal and the relay switching - exactly what we're hoping for.

Second image: Idle

This should show that while 12 volts is always present on the wire from the battery, it is not flowing through the relay

Third Image: Horn on, relay off

At this point the existing horn has been switched on, supplying 12 volts though the wire at the upper left.

Voltage is present at the coil of the relay, but no current flows and the relay does not switch because the Mosfet is not conducting to ground
Voltage is also present and rising at the zener diode. The voltage here rises slowly, the rate is determined by the resistance of the potentiometer and the capacity of the capacitor.
Current does not flow through the zener diode because the voltage is below it's threshold of 10 volts. Because no current is flowing into the gate of the Mosfet the relay remains open

Fourth image: Horn on, relay on

At this point the voltage at the zener diode has reached the threshold and it immediately begins conducting
Now the mosfet completes the circuit allowing current to flow through the coil of the relay
When the relay closes it switches power to the load (second horn)

No Image: Done

When the horn is released no more current can flow through the gate of the mosfet, this causes the relay to open and all the action is stopped

Step 3: Putting It All Together

I'm not going to explain exactly how I blobbed this together in too much detail, if you are reproducing this I recommend using the circuit diagram and doing it however suits you. I used a small plastic container to make this somewhat waterproof.

Use a good length of heavier gauge wire (depending on the load) for the battery, ground and load, and you can use a smaller gauge wire for the "signal" as it will carry very little current (probably 50-150 mA depending on your relay).

Step 4: Hooking It Up

Getting power from the battery: absolutely use a fuse, and
connect directly to the battery. The fuse tap pictured above is not for powering the second horn, it was to pick up the "signal" voltage from the existing horn and would carry a very small current.

Getting the horn "signal": the easy way to do this is to splice into your car's wiring. I didn't want to do that, so I chose to get it out of the fuse box. If car's horn fuse is normally hot you may need to figure out a way to get power from the existing relay instead. This was what happened in my case (the fuse tap was a good idea, but ended up not working out).

Step 5: Test It Out

This part is easy. Quick beeps of the horn are just like normal, but if you hold it down for more than a half-second or so the second one starts up and makes more noise. Now I've got more options to express myself while I'm driving!