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Powering an LED only when there is not a voltage on a lead?

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Powering an LED only when there is not a voltage on a lead?

Postby darthjeff2 » Thu Oct 15, 2015 11:52 pm

I am not incredibly familiar with circuits, a lot of what i have learned has been from simple online resourses so i am not very advanced what so ever. But essencially what i am trying to do is power an LED (and a small motor once i have the circuit logistics down), but only when a signal lead does not have a high voltage.

schemeit-project.png
schemeit-project.png (13.71 KiB) Viewed 7418 times


this is what i have come up with so far. might not be correct but it made sense at the time. essentially what i was going for is when the base of the transistor is powered, there is equal voltage on either side of the LED, and without the difference in voltage from the annode to the cathode it shouldn't light up. and once the transistor's base is no longer powered, the LED can pass current through it normally.

But all i am getting by trying this out is the LED blinking from a bright light to a dimmer light, even when the base is not powered. if i am missing something in this circuit or if there is a simpler circuit to do what i am trying to do i would appreciate the help! thanks :)
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Re: Powering an LED only when there is not a voltage on a le

Postby pebe » Fri Oct 16, 2015 4:32 pm

Referring to the transistor, what are the voltages on the base (relative to battery+) when the transistor is powered and un-powered?
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