# using a transistor to convert ground pulse to positive

Joined Oct 4, 2013
472
hi.. I need to use a transistor to convert a negative(ground) pulse to a +12v positive pulse... thanks

Joined Jul 18, 2013
28,493
Sounds like a job for an inverter, ........or maybe this is too simple?
Max.

#### #12

Joined Nov 30, 2010
18,224
Maybe this?

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#### joeyd999

Joined Jun 6, 2011
5,162
Maybe this?
You'll want a resistor from that input to the base, #12.

#### #12

Joined Nov 30, 2010
18,224
Probably. If the opposite of zero is any significant voltage, a resistor will be necessary, but the OP didn't say so I can't calculate it.

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#### sheldons

Joined Oct 26, 2011
613
ORRRR at great expense this could be tried

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Joined Oct 4, 2013
472
thanks for the help guys.. I got it figured out.. all I needed was a pnp transistor. The npn wasn't right for the job...lol.. here is a schematic of how I built it. please reply with any input of anything you see wrongm or any way to better it.. I just happened to figure this out. im not sure why it does what it does. an explanation of how it works would be appreciated. and how much current do you think this will hold before it burns up? Im still really new at this but im trying to learn.

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#### MrChips

Joined Oct 2, 2009
30,439
The transistor symbol you have shown is for NPN transistor.

Point the arrow towards the base for a PNP transistor.

Joined Oct 4, 2013
472
The transistor symbol you have shown is for NPN transistor.

Point the arrow towards the base for a PNP transistor.
got it...thanks

#### #12

Joined Nov 30, 2010
18,224
It is a lot easier to figure out when you know what the load is and what voltages the input will be.

In fact, even now, we at AAC don't know what the input voltages will be, but we can see that you might put 30 milliamps through the LED, and most small Leds are rated at 20 ma, maximum. With 1.1 ma through the base of the transistor, it will not be in full saturation and might get a bit hot.

The 330 ohm resistor should be at least 500 ohms to keep the LED current at or below .02 amps and the input resistor should be about 5600 ohms.

Joined Oct 4, 2013
472
It is a lot easier to figure out when you know what the load is and what voltages the input will be.

In fact, even now, we at AAC don't know what the input voltages will be, but we can see that you might put 30 milliamps through the LED, and most small Leds are rated at 20 ma, maximum. With 1.1 ma through the base of the transistor, it will not be in full saturation and might get a bit hot.

The 330 ohm resistor should be at least 500 ohms to keep the LED current at or below .02 amps and the input resistor should be about 5600 ohms.
the voltage is 12v and this will drive 2 5v leds...or the input is 12v but when I measure the voltage at the led its 3.8v.. does this help?

#### #12

Joined Nov 30, 2010
18,224
Two LEDs of 3.8 volts each, in series, will leave 4.4 volts to be used by the resistor.
4.4V/.02A = 220 ohms minimum.

Of course, I'm guessing again because I don't remember using any 1.9 volt leds and you did not say whether you have them in series or in parallel with each other, or what color they are, or if the LEDs are labeled 5 volts because they come with their own limiting resistors, or why there is a numerical difference between, "2 5V leds" and, "measured the voltage at the led".

That's the best I can do right now.

Joined Oct 4, 2013
472
Two LEDs of 3.8 volts each, in series, will leave 4.4 volts to be used by the resistor.
4.4V/.02A = 220 ohms minimum.

Of course, I'm guessing again because I don't remember using any 1.9 volt leds and you did not say whether you have them in series or in parallel with each other, or what color they are, or if the LEDs are labeled 5 volts because they come with their own limiting resistors, or why there is a numerical difference between, "2 5V leds" and, "measured the voltage at the led".

That's the best I can do right now.
wow,, I need to learn to be more specific..lol.. ok.. the leds are blue, they are in a series. the package they came in said 5v max on it. I see no resistors on them, the leds are in a case separate from the board. I measured the voltage at the output on the board with the leds plugged in and on. I haven't measured the voltage "unloaded?" with the leds unplugged.., im assuming the leds don't come with their own limiting resistors because I hooked one straight up to the 12v supply and it popped right away..

#### #12

Joined Nov 30, 2010
18,224
It looks like I guessed pretty well. Let's assume that "5V" is just sloppy labeling because 3.8 volts is pretty typical of blue LEDs. What I want you to understand is that the voltage the LEDs don't need is used to place a resistor that will limit the current to the ability of the LEDs. In this case, 220 ohms or greater is a good answer.

Then, there is the input resistor. The, "right" way to do it is to have the transistor running at ten times as much collector current as it has base current. To get .002 amps through the base, you need about 5600 ohms. That will slam the transistor hard on and minimize its heating.