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Author Topic: 12v current on led indicator on power cord  (Read 781 times)

Offline bullpuppy

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12v current on led indicator on power cord
« on: February 26, 2015, 11:02:14 pm »

I am fairly new at electronics but I am stuck on what I would think would be simple. I have a theremin and I like to add a power on LED light. I do not want to open the box but only want to add it utilizing the power cord only and the light is activated when the theremin is turned on. I was told that I could do this with a transistor or a relay. I am trying the relay approach now but to get it to work the relay is constantly  activated. I have a 5 pin relay. I have not tried the transistor approach. The simple approach would be to wire the led (with the resistor) in series with the theremin, however the theremin requires the full 12 volts 30ma so this will not work. Ideally i like to connect something in series using the power cord only with no decrease in voltage to theremin or the to led (plus resistor). The only trigger is when the theremin is turned on There must be a simple way of doing this.

Any feedback would be most appreciated.

Offline RexBeckett

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Re: 12v current on led indicator on power cord
« Reply #1 on: February 27, 2015, 03:51:00 am »
First, are you sure you intended to post on the Vera Home Automation forum? I'm sure there are better forums for basic electronics questions.

Second, anything you wire in series with the dc supply will reduce the voltage. To select a solution you need to determine how much of a drop the Theremin will tolerate.

An LED and resistor will drop about 2.2 volts.

A simple transistor & resistor circuit will drop about 0.7 volts.

A more-complex comparator circuit could work with drop of about 0.1 volts.