Delaying current to a relay

This is my first post so please bear with me.  I am using the 12 volt positive siren lead from my Viper alarm to trip a relay in addition to powering the siren.  I do not want to energize the relay when the alarm is armed/disarmed and the siren chirps, but I want the relay energized when the alarm is tripped and the siren is sounding.  Is there a way to delay the current to the relay {but not the siren} for a second or two?  I have a JPEG of the diagram if anyone can assist me with posting it.

Regards,

Well, as for delay you could just use the RC time constant:

T (seconds) = R (ohms) * C (farads)

For example, if you wanted a .5 second delay:

T = 10Kohm * 47uF

= 10x10^3 * 47x10^-6 ~= .47s.

You would connect the (+) to the resistor, the resistor to the capacitor's (+), and the capacitor's (-) to whatever you wanted to power on that delay. 

However if you wanted the relay tripped when the alarm is tripped and the siren is sounding, you could also just use a $1 boolean AND gate (or 2 NAND gates) from radio shack, then have it output to your relay.

nand gate at radioshack:  http://support.radioshack.com/support_supplies/doc14/14800.htm.

Just remember to use resistors where applicable so you don't blow things out

-Derek

Derek, you went WAY over my head!  I have zero experience or knowledge of resistors and capacitors  {but I have relays and diodes down pat!}

Could you put it in layman's terms for me?

Radkonn, I will do a little searching as you suggested

haha.. sorry...

http://www.tpub.com/neets/book2/3d.htm

Basically if you put a resistor in series with a capacitor, when you send a pulse to that little circuit, it will delay that pulse by the amount of time formulated by that equation i gave you.

I don't know how much delay you're looking for (in seconds), but you can still use that equation.  Just make sure to use a large (47uF would be fine) capacitor so you can have a good-sized delay. 

Most places (radioshack... local parts stores) should sell ceramic and electrolytic capactiors.  Electrolytic are better.  You can probably pick up a 47uF 50V Electrolytic capacitor for something like 80 cents.  The resistor would probably be around a dollar for a pack of 10 10K (or whatever resistance you want to use.. the higher the resistor's value... the longer the delay).  

If you want to calculate for a given amount of time, lets say, 2second delay (assuming you use a 47uF capacitor):

2s = R * 47uF

R = 2s / 47uF

R = 2 / (47x10^-6) ~= 42K resistor (radioshack sells 33K resistors and 10K resistors, so you could buy one of each and just put them in series... that would make a total resistance of 43K going into the capacitor... which is a little over 2s).

By the way, If i remember correctly, there are relays with built in delays... 

-Derek

Oh, and just a quick run down of what capacitors and resistors do:

capacitors store a charge

resistors limit the flow of electric current

Therefore the RC time constant sort of works like filling up a glass of water.  If you put your faucet on full blast it will fill up the glass (our capacitor) really quick (lets say, .5 seconds).  However, if you LIMIT that flow of water by turning the knob (knob limits the flow of water just like a resistor limits the flow of electric current.. so in this case our  knob is a resistor) your "glass" will fill up much slower, taking a  longer time to get charged.

Once a capacitor reaches it's charged level, it discharges its stored charge gradually.

Hope this helps.

-Derek

Do you know where I might get one of these relays with a 1 to 3 second delay?I think I am catching on here - please correct me where appropriate:

Capacitors have a polarity and resistors do not?

I connect the switched 12volt+ siren lead to either side of a {what value?} resistor{s}

Then connect the remaining resistor lead to the positive side of a {47uF?} capacitor

Then connect the negative side of the capacitor to the relay's contact {85 or 86 - does it matter?} and the other relay contact to ground.

How am I doing?

I would rather buy a time delay if I could find one.  The places I called wanted a minimum of $75 for what I wanted.  Any suggestions on where I could find a 12 volt dc time delay or a relay with a built in time delay?  I will also check on alternative ouputs from my Viper alarm.