Picking transistor for arduino controlled switches

I am using an arduino in a project that simulates pressing buttons on the steering wheel in my car that control my stereo. Basically the way the buttons work is that they short two wires together through various resistances (a different one for each button) (from around 100ohm to about 1kohm). Right now I have a transistor for each button attached to the appropriate resistances to simulate pressing these buttons. The base for each transistor is connected through a 1kohm resistor to an arduino digital output.

This setup works well; the stereo responds as if I have pressed the buttons when the transistors are switched via the arduino as expected. However about 5% of the time, the wrong "button" is triggered. Most often when switching the transistor that connects through 100 ohms or 200 ohms, the stereo will behave as if the one that connects through 300 ohms or 500 ohms has been "pressed." (these are the next highest resistance values with other transistors)

I was thinking that the problem might be that the transistors I am using were switching too "slowly" and the stereo is seeing a different resistance as the transistor saturates.
I admit I know very little about this stuff...

The transistors I am using are nte194. Datasheet is here http://www.nteinc.com/specs/100to199/pdf/nte194.pdf and says they are called audio power amplifiers. Would I be better off using different ones?

Any help would be much appreciated!

Yes that is the wrong sort of transistor for what you are doing. You want a general purpose small signal transistor like a BC183. There are lots of different parts you can use.
However better than a transistor would be a logic level FET, these have a low turn on resistance, which is what you want actually, a transistor gives you a collector emitter saturation voltage drop which is part of your trouble.

Awesome, thanks!

Do you know of any specific logic level FETs that would work?

would this work?:

The on resistance seems a bit high compaired to some more modern devices but this should suite you fine for what you want to do.

Really? It says 5Ohm@10V. Is 5 ohms high for one of these? What is a better value? Or did you read that at 50 ohms like I did the first time I read it?

At any rate, thanks for the help, I appreciate it!

Really? It says 5Ohm@10V. Is 5 ohms high for one of these?

Seems to be in spec for this part ( Intelligent Power and Sensing Technologies | onsemi )

What is a better value? Or did you read that at 50 ohms like I did the first time I read it?

That all depends on the current you need to draw through the device. At the 200ma maximum continous current rating, that 5 ohms represents one volt lost to the load. This is a fine device for low current applications, however if you needed a logic level device to drive a 20 amp load you would be looking for a device with a much lower on resistance, say .05 ohms.


The last data sheet that crossed my path for a 20A power FET had an on resistance of 0.001 ohm.

But as I said 5R is perfectly fine for your application.