Hi all. I'm wiring the aftermarket car alarm system siren output to the car horn. i wired it directly but it did not work. So i added a horn relay to it and it worked fine. But the issue is that when the car is armed, the horn is triggered for too long for like 1 sec and this really blows the ears up. Is there any that the pulse can be lowered to around 0.5sec. Would really appreciate any help because i have been figuring out this for quite long. Thanks.

 

Hi K_Dinnoo, I guess the simplest thing would be to use a timer relay such as this:
https://www.aliexpress.com/item/GEY...lgo_pvid=fbd520b5-ae9a-4a65-acdb-4472bb8eae3a

 

Below is a simple one-shot circuit using a MOSFET to pulse the relay.
The simulation shows the relay being on for a about a half second (red trace) after receiving a 12V signal (yellow trace) from the alarm siren (V1).
U2 is a 100kΩ potentiometer to adjust the pulse time.

 

Hi K_Dinnoo, I guess the simplest thing would be to use a timer relay such as this:
https://www.aliexpress.com/item/GEY...lgo_pvid=fbd520b5-ae9a-4a65-acdb-4472bb8eae3a

I am installing this in a car. i would like something more compact so i can look for a place to hide it inside. thanks anyway.

 

Below is a simple one-shot circuit using a MOSFET to pulse the relay.
The simulation shows the relay being on for a about a half second (red trace) after receiving a 12V signal (yellow trace) from the alarm siren (V1).
U2 is a 100kΩ potentiometer to adjust the pulse time.

View attachment 172062

Hi. I believe this will work but the siren sends voltage lower than 12v i guess probably around 9v.
I have also heard of something called high or low pass filter. Do you think this will work. I think its a capacitor and resistor circuit. Thanks

 

the siren sends voltage lower than 12v i guess probably around 9v.

9V should work also.

I have also heard of something called high or low pass filter.

Those are normally for suppressing undesired frequencies in analog signals.
This is a digital (2-state) signal.
Note that my circuit has an R and a C, configured in a high-pass filter configuration.

 

I may be confused, but is this the same signal as the one that will actually sound the horn for the alarm? If it is, will this approach interfere with the normal operation?

 

I may be confused, but is this the same signal as the one that will actually sound the horn for the alarm? If it is, will this approach interfere with the normal operation?

Yes. The car is equipped with a siren as attached. I have already wired tried wiring the siren wire directly to the car horn but it did not sound. I added a horn relay to it and it worked. But the relay trigger time is what I want to reduce because i guess this is what make it sound loud.

 

9V should work also.
Those are normally for suppressing undesired frequencies in analog signals.
This is a digital (2-state) signal.
Note that my circuit has an R and a C, configured in a high-pass filter configuration.

So do you mean I can use the high pass filter method and play around with the resistor value and capacitor ratings to get the desired pulse?

 

Yes. The car is equipped with a siren as attached. I have already wired tried wiring the siren wire directly to the car horn but it did not sound. I added a horn relay to it and it worked. But the relay trigger time is what I want to reduce because i guess this is what make it sound loud.

I am sorry if I am being dense but what I am confused about is your modification of the alarm output. Is this a separate “armed” signal LINE or an armed sent on the same line that will be used to turn on the siren in an alarm state?

If it is the same as the line being used to power the siren in the case on an alarm, won’t changes potentially interfere with normal operation?

 

I am sorry if I am being dense but what I am confused about is your modification of the alarm output. Is this a separate “armed” signal LINE or an armed sent on the same line that will be used to turn on the siren in an alarm state?

If it is the same as the line being used to power the siren in the case on an alarm, won’t changes potentially interfere with normal operation?

I'm getting you now. Yes, it is the same line. Any solution for this issue?

 

I'm getting you now. Yes, it is the same line. Any solution for this issue?

Nothing comes to mind, but maybe if you explain all the moving parts once more, it would help. A drawing of every part hooked up, and a little narrative about what you want to happen could help. I get a little lost in the thread.

 

So do you mean I can use the high pass filter method and play around with the resistor value and capacitor ratings to get the desired pulse?

Yes.
The potentiometer in the circuit is for adjusting the resistance and thus the time.
You do know what a potentiometer is (?).

 

Yes.
The potentiometer in the circuit is for adjusting the resistance and thus the time.
You do know what a potentiometer is (?).

Yes. I do know. I will try wiring the high pass filter one. Do you have any approximate values of the components for me to start with.

 

Yes. I do know. I will try wiring the high pass filter one. Do you have any approximate values of the components for me to start with.

Did you not see post #3?
Use those values.

 

You could use an astable instead of a one shot, that way you could set the “on” time at .5 seconds and the off time a little longer, at least then you would have a pulsing alarm when triggered.

A delay “on” timer could also be used…but then you would lose the “beep” at arming.

You could add a small piezo buzzer to get the “armed” beep back, just place it before the delay timer.

 

Below is a simple one-shot circuit using a MOSFET to pulse the relay.
The simulation shows the relay being on for a about a half second (red trace) after receiving a 12V signal (yellow trace) from the alarm siren (V1).
U2 is a 100kΩ potentiometer to adjust the pulse time.

View attachment 172062

I went to my local store, they do not have the 2N7000. Any replacement for this?

 

Below is a simple one-shot circuit using a MOSFET to pulse the relay.
The simulation shows the relay being on for a about a half second (red trace) after receiving a 12V signal (yellow trace) from the alarm siren (V1).
U2 is a 100kΩ potentiometer to adjust the pulse time.

View attachment 172062

Can I also ask you if the potentiometer has a connection between the wiper and the +ve terminal?
See attachment.

 

I went to my local store, they do not have the 2N7000. Any replacement for this?

Just about any N-MOSFET.
What is your store and what kind does it have?

Can I also ask you if the potentiometer has a connection between the wiper and the +ve terminal?

Yes, that is a connection between the wiper and one end of the pot as you circled.
But why do you call it "the +ve terminal"?
There's no +ve on the schematic.

 

Just about any N-MOSFET.
What is your store and what kind does it have?
Yes, that is a connection between the wiper and one end of the pot as you circled.
But why do you call it "the +ve terminal"?
There's no +ve on the schematic.

I shall see with the store and let you know by tomorrow.

The other pin in the potentiometer and the mosfet needs to be grounded if i am not wrong?