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Topic: opposite of transistor switch (Read 3937 times) previous topic - next topic


I have been looking into using transistors as switch but I was wondering if there is something opposite to that function. where as the transistor gives power to a circuit when powers given to it I want a device that cuts the circuit when powers given to it. I've looked into optoisolators with no luck and was wondering if there is any digital component that can accomplish this task. I don't want to use a relay since it requires too much power to be run off of a coin cell. Anything with low power consumption would be appreciated.



npn transistor, when no current is at the base the circuit is off?


Can you rewrite your question? I don't understand it.

Perhaps a "logic level" mosfet ? The gate of the mosfet requires no current, and power mosfets are able to switch high currents.


A PNP transistor gives no power when the base is high, and gives power when the base is low.
Is that what you mean?
You are using the wrong words you know.



Scroll down to the bottom of the page, Transistor NOT gate


Thanks for the link, Lakes. Out of all the pages I've read about transistor operation, I found that to be the most understood. - Scotty


Sorry guys, I knew that posting a question at 2:30 in the morning wasnt a good idea haha.

So what I was trying to say is that I wanted a component that acted similar to a transistor but rather than connecting the emitter and collector when the base is powered, I wanted it to disconnect the emitter and collector when the base was powered. As Grumpy_Mike said, a PNP transistor seems to do exactly that, which does exactly that.

The problem I am trying to solve seems simple. In my project I have a male and female audio jack. I have an arduino that is reading the signals from the male audio jack, however when the arduino goes to sleep and its not capturing the audio, I want the male audio jack to connect to the female audio jack so essentially the microcontroller is bypassed and thus still allows the audio to flow through. This seems easy to do with a mechanical toggle switch to break the wire to the female jack from the male one, but I want to be able to do this through the arduino. A relay also seems like a viable solution, however, relays are generally big and since I want my final project to be run on a ATTINY85/84 on a coin cell, a relay would take too much energy to keep flipping the connection. Thus I would like to use a transistor or optoisolator or something of the like to accomplish that goal and preferably would like something that requires low current to activate.

Since my project will be bypassing the audio most of the time I want to use something that will only require energy to BREAK the bypass to save battery in the long run.

I have two questions though:
1. What is the difference between the PNP transistor and a Transistor Inverter (NOT GATE) as shown in the link that Lakes posted? Why would you use one over the other? Which would be more battery efficient and would either work for the AC current of an audio jack?
2. If they wont work for AC current, what are my options? I though about a two coil latching relay since they dont require current to keep their state, but I am not sure I could drive the relay multiple times with a coin cell and the relay noise would not be nice in my final project. Would i want to use something like a latching reed switch? Is there anything smaller, lower power, and quieter?

Thanks alot guys, and please excuse my lack of competence in the wee hours of the night.

Din See


A transistor is not sutiable for switching audio signals.
You need to look at something like an analogue multiplexer like a 4051 or similar.


You could use an FET as an analog switch
for your application you would need to drive it (N-Channel FET) with the transistor NOT gate or use a P-Channel FET.

Or use this type of IC
or the CD4051 whichever is cheapest or available.


You could use the NC poles of a relay.

Rob Gray aka the GRAYnomad www.robgray.com


So after watching that video and finding this amazing overview about semiconductors:

I finally learned some of the differences between mosfets, jfets, and transistors. It seems like the best thing for me to use is a P-channel mosfet or a P-channel JFET since I want the circuit to break when I provide the signal voltage to the gate. Now I am wondering which one would have less of an effect on the audio that is being passed through. Do either of these have a voltage drop across the drain and source? Also do either work for an audio signal which is AC current? As I said, the main goal is just to connect the audio collected from the male jack, to a female jack in order to bypass the microcontroller while its not in use.


Get a CD4066 IC it's called a transmission gate and is Perfect for doing as you asked, run from a coin cell and be switched on/off by the Arduino, Very easy circuit to built, very inexpensive components and best has a VERY small current drain and works well at 3V (from the Coin Cell) It is a pert designed to control/switch Audio Signals, There are 4 gates in the package 2 for a 2 Channel Audio switch (Or a SPDT Switch in case you wanted to 'send' the audio elsewhere when you so desire... REALLY EASY Part to use too. At leas it was for me, I built a whole Audio patch panel/level and equalization device for a home stereo many years ago. Crossbar switching with the ability to send anything to any place with or without gain or EQ.

--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard


Although the 4066 fits the bill in working with AC current and low power consumption, is there a way to use it so that current flows between the circuit when there is NO power given to the IC and when power signal is given by the arduino to the IC it stops conducting? Essentially use it like a P-fet rather than a N-fet?

Also is there any benefit to the 4066 vs using the mosfet system? And is there an IC like the 4066 that only has one analogue switch since i really dont need 4 switches and I want to make my project as small as possible?

Thanks alot everyone, this is really helping out.


Jul 23, 2012, 08:08 am Last Edit: Jul 23, 2012, 08:59 am by Graynomad Reason: 1
I want to make my project as small as possible?

So I guess my relay idea is not a goer then. Although you can get very small relays.

Do you mean it has to transmit when there is no power at all to the chip or just the control input? If it's the first I don't see how any ICs will work.

If it's the second have a look at analog switches like the ADG884, DG9415DQ and MAX4741/MAX4742/MAX4743 they all have NO and NC connections and are in very small packages.

Rob Gray aka the GRAYnomad www.robgray.com


I finally learned some of the differences between mosfets, jfets, and transistors. It seems like the best thing for me to use is a P-channel mosfet or a P-channel JFET

Yes, a P channel Jfet will work nicely.

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