how to trigger strobe with alarm/switch

Depending on where you put them, and a few other factors, you may be able to power them right off the siren output of the alarm.  However, keep in mind that they strobe may or may not flash when you arm and disarm the system.  If the caps in the strobe can be charged in the short time that the siren is beeping it may be enough to flash once or twice.  If this acceptable, then that would be a good route to go.

If you want to add an external switch you will need to diode isolate your siren(s) from your strobelight(s).  Once they are diode isolated you can put a toggle switch between the diode and the strobe.

Yes, two seperate diodes would be needed.  Both would attach to the siren output, with the stripe facing away from the brain (towards the devices). 

those 90watt strobe pull 10-15 amps... How big of diode's would i need?

use your siren ouput and wire it to an external relay.

85 - ground
86 - 12v siren wire
70 - high current 12v fused @ 20 amps
30 - to strobe light 12v

Use 1A diodes, but as tedmond says, you will need to use a relay.  Diode isolate the relay from the siren, just like you would have the strobe.  The relay will draw less then 1A, so the 1A diode will be plenty.

You can still put the switch in the same place and make it all work!

tedmond wrote:

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use your siren ouput and wire it to an external relay.

85 - ground
86 - 12v siren wire
70 - high current 12v fused @ 20 amps
30 - to strobe light 12v

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Where would the diode go?? also where would the switch go?

From pin 30. place a diode on each end of the wire you want to connect to pin 30. With diodes cathode band facing away from relay. also, the switch will sit after the relay so you have the option to turn it on or off. im sure viper has the ground when armed wire. if so, you can use 2 relays to allow the strobes to ONLY work when the car is armed. if you wish to do so heres how.

Relay to control lights

85 - ground
86 - 12v siren wire
70 - high current 12v fused @ 20 amps
30 - to pin 70 of relay 2 and one to siren with a diode, cathode (white band) away from pin 30.

Relay to allow strobe flash on ONLY when armed (must wire 2 relays together)

85 - constant 12v
86 - ground when armed from alarm
70 - wire from Pin 30 of relay 1 with diode inline cathode away from relay 1
30 - to strobe light 12v in

^^^ Not a good idea.

Most automotive relays will draw around 150mA of current to keep them energized.

With a relay connected to the ground when armed wire the battery will be dead in 4-7 days of not driving the car.

You should NEVER have a relay constantly energized when the car is off.

The siren wire should go to two diodes (non striped side).

The striped side of one diode should go to the siren.

The striped side of the other diode should go to pin 85 of the relay.

Ground Pin 86 of the relay

Pin 30 of the relay should go to a fused wire to the battery (fused at battery end).  This wire should be large enough to carry the current you need

Pin 87 will be your strobe power wire.

One "change" you might consider is instead of hooking pin 86 to ground hook it to (-) when armed wire like mentioned above.  With the + being supplied by the siren the relay will ONLY latch when the siren is chirping AND the system is armed.  However, this may not be necesarry depending on how long it takes the strobe to start strobing.

No they don't.  At least not for consecutive days without having a dead battery.

The general acceptable 'at rest' current draw from an automtotive electrical system is 50mA.  Most stock cars will be below 25mA at idle.

A single relay will be around 150mA.  Add that to the stock load of 25mA and you are way over the acceptable amount of current.

Find out what your batterys reserve capacity is and then calculate how long it will take to drain it with a 175mA load - it will surprise you.

In all my years of installing alarms I NEVER installed a starter/fuel pump/ignition kill that was always activated when the alarm was armed.  I rewired a few here and there that were wired that way after customers complained of dead batteries.  Every effective starter kill system I've seen connects to the (-) when armed signal for ground and the ignition for (+).  So, in reality, they are only actually interupting the starter when the ignition is on.

Here is a thread talking about reserve capacity and how long a vehicle can sit with certain loads: https://www.the12volt.com/installbay/forum_posts.asp?tid=57224&get=last