Jump to content
Electronics-Lab.com Community

NPN transistor circuit - Is this right? I doubt it...

Recommended Posts

You need a resistor in series with the LED. 220 Ohms will do.

Without this modification, your circuit will kill the LED when the switch is closed.
With the modification, your circuit will light the LED when the switch is closed.

Positive signal? Controls a negative signal? What does that mean?

Link to comment
Share on other sites

Without the current-limiting resistor in series with the LED then the LED, the transistor and maybe even the 5V power supply will all blow up.

A logic high at the input produces a logic low at the output. Nothing is negative in that circuit.
The input uses a low current and the transistor's output current can be much higher.

Link to comment
Share on other sites

Point taken, I forgot about the resistor in series with the LED...

By a positive signal that controls a negative signal, I mean, I want to take the logic high signal from a ship, and use that to connect the ground, or the negative terminal on my battery, to a device.

I really cant explain it well....

But if you guys say the circuit is right, with the exception of the led, which wont actually be their, then it should do what I want it to do.


Link to comment
Share on other sites

Did you select a transistor that has a high enough current rating so it can pass the amount of current for the device? Which transistor? How much max current?

Can the "ship" drive the 1k resistor in series with the base of the transistor with at least 3.5V/1k= 3.5mA?

Is the current gain of the transistor high enough for only 3.5mA input?

Link to comment
Share on other sites

The "ship"!? What is a "ship"?

I have no idea what transistor I need to do this. I only have a couple of them... I know I need a NPN, but other than that, I know not.

Here's my circuit:


Problem with this circuit, with the 5v power supply out of my PS2 mouse controller in my computer, I don't have enough juice at the end to close the relay. I'm hoping that the the transistor wont need so much juice to close... Not that it really closes.

What would you suggest for a transistor in this case?

I have two transistors...

I can get others from radio shack too...

Link to comment
Share on other sites

You need to know how much current is used by "the device" that will have one wire grounded by the transistor, so you can use a suitable transistor.

The 4017 has a minimum output current of only 2mA when it has a 5V supply. A 2N3904 transistor will turn on pretty well with only 2mA of base current if its load current is only 100ma or less.

Link to comment
Share on other sites

Well, how much current do you think is passed when a mouse clicks? I will check...

Ok, This meter reads .1mA...

I don't know if it is right. I don't know how to test a meter to see if the amp setting is correct. My multi meter is correct, but sadly, it is at school. :(

Just real quick how do I make sure my meter is reading amps accurately?

Does .1mA sound right to you?

Link to comment
Share on other sites

So your transistor is not turning on a powerful motor nor a lightblb. It is the low current output of a pc's mouse?
I never looked, but I would think that a mouse has a logic output that is either high or is low. Your transistor will make an output that is only low, never high. Connect a 1k pullup resistor from the collector of the transistor to +5V. Then the output will be high when the transistor is turned off.

Link to comment
Share on other sites

Well, I don't know what you just said..  ???  :-\

However, when the mouse button is suppressed, one of the pins of the IC is connected directly to ground. What I am trying to do is create a signal (done, with your help...) and use it to connect that pin of the chip (not "ship"  :-[) to ground.

So, because I don't know what you just said, I don't know if it is needed?

Oh, and what value resistor do I need at the base of my transistor, if any?

Link to comment
Share on other sites

It looks like you are going to connect the collector of your transistor to a button on a PC's mouse. Then it will already have a pullup resistor so a collector resistor won't be needed.
A resistor in series with the base isn't needed but use a 10k resistor anyway in case you want to measure the output of your circuit or use it to drive something else.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...