Hi,
I'm trying to use standard 2N3904 transistors to switch analog signal lines in an audio mixer. But I'm running into this issue where the 2N3904 will still be partially open when the input is HIGH but seemingly completely closed when floating which I find really weird because it should be more closed when driven HIGH according to the datasheet. This is a problem because I'm trying to control the transistor with an ATTINY85 and I'm trying to drive it HIGH to close it completely.
Does anybody know why this is happening?
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Schematic please
You are somewhat confusing electronics terminology. A switch may be "open" or "closed", the latter meaning it completes a circuit, but a transistor (including FETs) either conducts (to some degree) or does not.
Please have another go at explaining your project and provide a schematic.
It may be useful to read the instructions.
Here is a schematic. What I mean by "partially open" is that the transistor seems to still conduct some energy across itself when pulled high noticeable because the audio is still present but just much lower in volume but when the transistor base is kept floating this is not noticeable.
The switching transistors I am referring to are the ones on the left side of the image
Hi, @maxall455
Welcome to the forum.
Unfortunately your schematic resolution is very low, and hard to read.
A hand drawn schematic would be better, just one ATtiny module will do and an output module connected to it?
Please include component names, pin labels and power supply.
Where are the analogue line you are switching, ie audio in and audio out?
Thanks.. Tom.... 
Yes, the diagram is very difficult to read. However, you appear to be driving the base of a transistor directly from an Arduino pin. You must have a current limiting resistor (say 1k) in that path or you risk destroying something.
If you want to build an audio mixer, you'll probably find it much easier to use ICs designed for that purpose.
JFETs are used for switching low level audio signals.
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Please post a photo of a hand drawn schematic of the transistor circuit in question.
Higher Res schematic:
Note: The transistors here are not used for amplification there is another transistor for that but they are used for things like muting channels.
BJT does not work on AC signals.
You want a 74HC4066 Quad analog switch.
Potentiometers in that circuit do not make sense!
I fixed the pot issue. Can I use a JFET for this or do I have to use a specialized chip like that?
The CD4053 (SPDT) could be a better choice for audio.
They have a -VEE pin for a negative (-5volt) supply voltage.
So they can switch unbiased (ground referred) audio.
Leo..
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Excellent point- I was thinking about the negative voltage matter.
Actually, nowadays it is the 74HC4053. The plain "CD" series is generally obsolete (unless you need to operate at 15 V).
Yes, I see the 74HC4053 can do 10volt peak/peak (VCC - VEE).
And a charge-pump with Schottky diodes could make -4volt for VEE from a 5volt Arduino pin driven at high frequency with tone().
Leo..
I was able to fix this by using a JFET transistor instead of a BJT
Normally if using a transistor for this you need to have a bias circuit so that it can cope with the positive and negative excisions , you can then also clamp it off .
Google transistor amplifier, active region, biasing
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