Is there a way to keep an LED turned on even when the power source is off?

In summary, a PNP transistor circuit may be able to achieve the desired result of allowing current to flow from the Collector to the Emitter when the Base voltage is 0V, and preventing current flow when the Base voltage is +0.7V. This can be accomplished with a circuit that connects the GPIO pin of a Raspberry Pi to the Base and an LED to the Emitter. This system should be solid state to withstand vibration, so a mechanical relay switch may not be suitable.
  • #1
andrew700andrew
11
0
Hi, I'm looking for a device like a transistor which preforms the opposite. E.g.

On an NPN when Base is +0.7V current can flow through the Collector to the Emitter and when the voltage at B is 0V (i.e. a switch connected to base is open) no current will flow from C to E. I want the opposite of this to happen. So when the Base voltage is 0V (i.e. a switch connected to Base is open) current CAN flow from Collector to the Emitter and when the Base voltage is +0.7V no current can flow from C to E.

Does a device like this exist, or can I do something to make it work? For some context, this will be on a circuit with the GPIO pin of a raspberry Pi connected to Base with an LED connected to the Emitter. I want it so that even if the raspberry Pi turns off (and the GPIO pin turns off), the LED will remain ON, only when the GPIO pin of the Raspberry Pi is high will the LED be turned off.

Also, I will need the system to be solid state as it will be exposed to serious vibration so any mechanical parts like a mechanical relay switch (which would work I assume) will likely fail.

Thanks.
 
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  • #2
PNP transistor circuit will likely do the trick
 
  • #3
Try this circuit:
upload_2015-2-6_17-24-59.png
 

1. What is an Opposite NPN transistor?

An Opposite NPN transistor is a type of NPN transistor that has its base, collector, and emitter regions reversed, resulting in opposite polarity compared to a regular NPN transistor. This means that the base is connected to the negative terminal of the power supply, while the collector is connected to the positive terminal and the emitter to the negative terminal.

2. How does an Opposite NPN transistor differ from a regular NPN transistor?

The main difference between an Opposite NPN transistor and a regular NPN transistor is the polarity of the base, collector, and emitter regions. In an Opposite NPN transistor, these regions are reversed, resulting in an opposite flow of current compared to a regular NPN transistor. This also affects the way the transistor is biased and its amplification characteristics.

3. What are the applications of Opposite NPN transistors?

Opposite NPN transistors are commonly used in electronic circuits for switching and amplification purposes. They are also used in inverters, oscillators, and power regulators. They can also be used in high-frequency applications due to their fast switching speed.

4. How do you test an Opposite NPN transistor?

To test an Opposite NPN transistor, you can use a multimeter in diode mode. Connect the positive lead of the multimeter to the collector and the negative lead to the emitter. If the transistor is working properly, the multimeter should show a voltage drop of around 0.7V. Then, reverse the leads and check the voltage again. If there is no voltage or a very small voltage, the transistor is functioning correctly.

5. What are the advantages and disadvantages of using Opposite NPN transistors?

The main advantage of using Opposite NPN transistors is their ability to handle high voltages and currents, making them suitable for power applications. They also have a fast switching speed and low saturation voltage. However, they have a lower current gain compared to regular NPN transistors and can be more difficult to bias correctly due to their opposite polarity.

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