Resistor alarm key
Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=63397
Printed Date: July 06, 2025 at 4:31 PM
Topic: Resistor alarm key
Posted By: dualsport
Subject: Resistor alarm key
Date Posted: September 28, 2005 at 1:12 AM
Anyone try making a circuit to make use of a resistor "key" to act as a switch to disarm an alarm?
I was considering using two voltage comparators to detect the insertion of a specific resistor, where it would have to be within a certain tolerance to trigger the output and disarm the security device.
If I combined it with a momentary switch that will only allow one trigger attempt with a 20 second delay to lock out attempts at sweeping a potentiometer, would that be fairly foolproof?
Thought maybe using a blade type fuse body with the resistor soldered in would serve as a key, with the fuse holder acting as the keyhole.
Normal process would be to insert the fuse-key, then push the momentary switch to disable the alarm. If the momentary switch is pushed with the wrong resistor value in there, it wouldn't trigger, and an RC circuit would have to charge back up before a subsequent try.
Sort of a simplistic version of the OEM resistor encoded ignition keys- might be interesting to try out; but wanted to see if it's be done and what the reliability might be.
I was considering it for a motorcycle, so if it got rained on, it wouldn't be a good thing if the resistance dropped out of range as a result...
Replies:
Posted By: lanman31337
Date Posted: September 28, 2005 at 9:18 AM
You could always get one of those waterproof fuse holders with the cap, so if it does rain, you won't have any water to deal with.
Posted By: dualsport
Date Posted: September 28, 2005 at 9:30 AM
Yep, that ought to do it-
Also, if I keep the tolerance wide enough on the allowed range, it should make sure it's not a problem. Using lower values would help too, but I'd like to keep the standby current draw at a minimum, since the bike battery doesn't have as much reserve capacity.
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