Adding sensors - how difficult?

How difficult is it to add additional sensors (like a proximity or tilt sensor) to a car with an existing alarm if the alarm doesn't have any more input wires?

Can you wire two sensors (say a hood pin and a tilt sensor) in parallel? Series? Would it require some special diode or resistor?

Also, do proximity sensors (using sonar or radar) have 2-stages built into them or does the alarm need to support 2 stages? (2 stage support is: 1=warning 2=blast, right?)

Most sensors have 3 wires, right? (ground, 12volt constant input, and output (acts as a ground until it triggers then it's 12v)

Thanks guys

I found some information here https://www.the12volt.com/carsecurity/page5.asp and I understand the diode issue now, but specifically what applys to a hoodpin system where you piggy back a proximity sensor to a hoodpin?

(Do hoodpins have 2 or 3 wires?)

Would I just wire the proximity sensor off the hoodpins + and - and then wire the output with a diode to the hoodpin's - ??

You can't piggyback a (-) trigger to a (+) trigger device. You will need to either reverse the polarity of the device or hook directly to the other (+) trigger device with a diode. Hood pins are always (-) trigger and proximity sensors , impact sensors & tilt sensors are all (+) trigger devices.

ok, so to add a proxy sensor i can hook it's + up to some + and - up to ground and then hook up the output signal to the - of the alarm brain.. but if there is no - available on alarm brain, I can use an isolated diode and hook it up to, say the output signal (-) of a shock sensor? would that work? would I need to "isolate" both sensors with diodes or just the proxy?

Thanks!!

You have to check to see what your trigger wire is outputting on the device that you are trying to install. If the alarm requires a (-) input for the trigger input, and the proximity has a (+) trigger then you will need to use a relay to reverse the polarity to a (-) signal  otherwise you fry the device that your piggybacking and also the alarm trigger input. Diodes are only use as blockers... not polarity converters.

Most sensors like DEI's 508D(field disturbance) or 507T(digital tilt sensor) are negative triggering devices. RED is constant 12V+ , BLACK is ground(preferably ground when armed--- usually orange out of alarm diode isolate if sharing with starter kill output and/or other sensors) and BLUE and/or GREEN are negitive sensor outputs. Can diode isolate with any of the alarms negative trigger inputs (ie:negative door trigger input, hood/trunk trigger input, or external shock sensor inputs.

What is the make and model# of your alarm system and what kind of car?

So if I piggy-back a negative output device to another negative output device I should diod isolate the +, the -, and both - output (since this has two negative outputs (dual-zone proximity) right?

Will a 1amp diod suffice for me? (Well, 4 1amp diodes I guess is a better estimate since we're talking 4 piggy-backed wires)

I think the shock sensor is a negative output sensor with dual-zones so that might be the best to piggy-back off of....

HOW DO I TEST the output of something, say the shock sensor, with a voltmeter? Does "negative output" mean -12v? If so, then I'm sure my voltmeter will pick it up... that is, if my voltmeter supports negative voltage... hmmm that's a good question...