Help Choosing a transistor

[kieron_kohlmann]

Im new to this site but am wondering on how to choose a transistor. I plan on using it in my automobile. basically i have a 5 volt digital signal coming out of the computer. I need to turn this 5 volts into 12 and figured this would be a good application of a transistor. Im not pushing much amperage but i have no idea how to figure out which transistor or how to hook up this circuit. Any help would be appreciated. Thanks!

 

[MikeMl]

What does the "transistor" drive? A LED? Lamp? Relay?

 

[kieron_kohlmann]

Its going to drive a tachometer.

 

[Mikebits]

This part would work.
https://www.electro-tech-online.com/custompdfs/2010/11/mc14504.pdf
In stock at Digikey
Logic - Translators | Digi-Key

 

[kieron_kohlmann]

Dont make fun of me but i have no idea how to utilize something like that. I can do a typical 3 leg transistor. Sorry im an automotive Technician not an electrician.

 

[KeepItSimpleStupid]

That's really a good choice, other wise you would need two transistors and a few resistors. The problem is, the transistor by itself inverts the signal.

Part is trivial to use. Vcc connects to +5 on the computer end and Vcc connects to +12 on the vehicle. Tie all unused inputs to ground.
Connect the MODE pin to Vcc.

There is a dot on the IC. That dot is pin #1. If you orient that pin to the bottom left, Pin #2 is the next pin going in the clockwise direction and continuing around the IC.

The problem may be the package availabe. DIP or Dual Inline Package has pins.

Now we are only guessing that the part may work. we know little about the computer's signal and little what the tach expects.

The tach may expect "a contact closure to ground" or a "switched +12 signal" and sometimes is selectable.

We know little about the currents available sinking and sourcing on the computer side and the amount of current the tack is going to consume. It's only a guess whether the circuit will work.

We also don't really know the locic power supply for the computer, but are asuming it's 5V.

I'm sure we can walk you through some ideas, but it's difficult because the constraints aren't well known.

 

[kieron_kohlmann]

What kind of information would be helpful? The tach is looking for a +12V signal but the computer is only putting out a +5V digital signal. would amperage out of the computer be helpful?

 

[KeepItSimpleStupid]

I guess I'm asking because tach's used when cars had points, the +12 went to the coil and the points switched the other end of coil to ground. When the points opened, the field collapsed. That's like an example of a tach input signal.

The actual tach could have any number of input circuits. It was expected to work with the above kind of system.

The computer if it's embedded might be connected directly to a pin or through an interface circuit. It would be nice to know the equlivelent circuit of the output. TTL compatable output is OK because TTL is well known.

 

[MikeMl]

Here is how I would do it. Commonly available parts. Pinout of a 2N3904 (use the TO-92 package).

 

[kieron_kohlmann]

There is no coil on this vehicle. Its actually a diesel so theres really no ignition. The computer(PCM) is reading a crank sensor and from that making its own 5 volt signal. It was never actually used they just put it there from the factory incase someone wanted to add an after market tach. So the PCM is not providing ground but 5V in this case.

 

[kieron_kohlmann]

Alright this is the circuit i plan to build this weekend! Ill let you know how it goes! Thanks!