How to shorten the signal of a car horn?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. What you've got.







simulate this circuit

Figure 2. A capacitor pulse method solution.







simulate this circuit

Figure 3. With the addition of a relay a very short delay timer can be created.

How it works:


At the instant of ALARM turning on the horn will sound.
A very short time later RLY1 will energise, the contact open and the horn disconnect (and turn off).
When ALARM turns off the relay drops out and the circuit resets.
D1 prevents relay chatter when the horn button is pressed. It needs to be rated for the full horn current. It could be omitted if the chatter is not a concern.


The time delay will be a function of the response time of the relay.