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Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 1122

Posted: 03:06am 02 Apr 2020

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We are adding a very loud truck air horn to our site alarm, this will hopefully help deter would be thieves; it will be driven off the other sonar alerts already fitted to the system.

The horns air compressor runs off 12 volts and draws approx 30 amps, running it for more that 30 seconds seriously over heats the motor. So to avoid this I need something to pulse the motor on\off....on\off in a sequence over a 5 minute period that prevents the motor burning out. A simple timer with a car horn relay may work, but as its continually switching on\off, its contacts would rapidly burn out.

So decided to make a simple intelligent solid state switcher, driven by the continuous input signal from the existing alarm and an optocoupler input. Power this off a separate small 12v sealed lead battery. Replace the relay with two mosfets will give a combined RDSon of approx 1.5mR, and at 30 amps dissipate approx 1.35 watts, virtually no heat-sink required. The 8 pin Picaxe cpu has the brains to pulse the horn and switch off when the input signal stops or the battery starts going flat.

Will create a small pcb next.

Cheers
Mike

Davo99
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Joined: 03/06/2019
Location: Australia
Posts: 1577

Posted: 05:41am 02 Apr 2020

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Great Idea.

People pretty much Ignore anything that sound like an alarm now.
Perhaps you could put some horns Inside the house as well. Even normal electric car horns would work.

Mate of mine did that with those Piezo screamer things. Hid them behind the Ducted AC grills in the roof and just poked the wire through through the flexible ducting.
He has demonstrated and it would be impossible to stay in there with them going.

Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 1122

Posted: 09:35am 02 Apr 2020

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Davo99 said
Mate of mine did that with those Piezo screamer things. Hid them behind the Ducted AC grills in the roof and just poked the wire through through the flexible ducting.
He has demonstrated and it would be impossible to stay in there with them going.

Yes we have them (Piezo) inside, unbearingly loud; but not so much outside, thus the truck air horns on the external laser diode beam sensor circuits.

Mike

Solar Mike
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Joined: 08/02/2015
Location: New Zealand
Posts: 1122

Posted: 02:21am 03 Apr 2020

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PCB, 100 x 76mm
Q1,2 have drains joined on pcb, which acts as a heat sink, center pin not connected to pcb.

Have added provision to run off other higher voltages by adding pads for 72-12v psu at the end of the board, maybe useful if we want to power stuff off 48v battery.

Cheers
Mike

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