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post #1 of 2 Old 10-09-2006, 07:03 PM Thread Starter
Join Date: May 2005
Location: Ottawa (Kanata), Canada
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Question for electronics gurus

'm trying to hook up a visual indicator (blinking LED) to my alarm system that does not have such an option.

With a multimeter, all I've been able to find is a +12V that is on when the alarm is off, and vice versa. This is used by the perimeter sensor.

At first I thought I could use this line as ground, and connect the positive lead of my LED to an always-on source (the battery). Then, when the alarm was off, the alarm lead would drop to 0v and the current would flow. Yes, this works, but the blinking LED caused a fluctuation in the current which triggered the alarm, as if it was the perimeter sensor. The alarm wasn't triggered with a steady LED.

So what I did is build an inverter out of some NPN transistors and resistors. It works just fine - just ugly with all the leads. But with the LED/alarm off, the battery needed to be charged after a two week hiatus.

Can someone offer an alternate solution?



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post #2 of 2 Old 10-09-2006, 09:01 PM
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Join Date: May 2006
Location: Nashville, TN
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You could use a small normally-closed relay to accomplish the same thing. Transistors work just fine, and are the solid-state way to do it, but sometimes reverting to mechanical devices is just easier. Using a normally-closed relay would cause the relay to be open when +12 is on the coil and closed when there is no voltage to the coil. Since there is no current draw from a relay that isn't energized, that might be slightly more energy-efficient than your transistor relay.

I'd also put a current-limiting resistor in series with the LED to limit the current draw (current = voltage / resistance so the higher the resistance the lower the current flow). If, for example, the LED draws 10 mA you would need a 1200 Ohm resistor (resistance = voltage / current, or 12 (volts) / 0.01 (10 milliamps) = 1200 Ohms. That should help reduce the current flow through the LED and somewhat prevent the battery from draining as quickly. Nothing is going to stop it though. The LED is using a certain amount of current whenever it is on, and sooner or later it will drain the battery. Additionally, the electronics of the alarm system are using a certain amount of current. You might want to put a switch somewhere in the circuit so that if you know you are going to be away from the bike for a couple of weeks you can just turn the LED off.

Scott Craig - Nashville, TN
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