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Author Topic: inhibit pin high without power?!  (Read 2292 times)
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Newcastle, UK
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I'm building a simple audio switch using a HC4052 (mux/demuix). Pin 6 is an inhibit pin that needs to be low for normal operation.
I did plan on adding a toggle such that it's high for a mute function which is easy enough, but when there's no power to the circuit there's connectivity across (at least one of the) input/outputs.

It's not desirable that there be any connectivity when the circuit is off, but I'm a bit stumped being a bit of a noob... Is this the job of a suitable transistor such that the two output +s for L and R are blocked until there is a +5v rail available?
Will this be an expensive way to do it bearing in mind that I'm trying to keep the audio distortion to a minimum (I'm not an audiophile, perhaps minimum isn't the right word here...)

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Fort Lauderdale, FL
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I'm a little bit confused.

It sounds like your question is:  "how I do I keep the Pin HIGH while there is no power in the circuit?"  If there is no power, why does it matter?
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Manchester (England England)
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but when there's no power to the circuit there's connectivity across (at least one of the) input/outputs.

When there is no power to a circuit there must be no signal into it. That is the way electronics is designed. Putting signals into a chip with no power applied is a good way of destroying a chip or at least latching it up.
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Newcastle, UK
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Ok, ok... I was trying to give as much information and background as I could - perhaps I could have been more succinct and saved some confusion!

My question is not really how do I have the inhibit pin high when there's no power; I understand that this wouldn't work. But when there's absolutely no power to my circuit, the 4052 allows current to flow between inputs/outputs which means I'll get sound coming out my speakers from and and all input sources. I've proven that this happens in a test circuit, and is not good.

I was really asking for alternative solutions, hense my direct question about using transistors.
The real requirements is that the outputs (two of them - L and R) from the 4052 should be blocked when there is no power, but allowed when there is a supply. This part of the circuit needs to not interfere with audio signals. So finally, my question is what is the easiest / cheapest way to achieve this?

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Manchester (England England)
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When no power is applied to a chip it looks like a collection of diodes. Therefore to block audio going through you will have to reduce it to below 0.7v and then amplify it up. Probably using op-amps.

Anyone else got any other ideas?
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Interesting. I didnt know the signal(s?) got through those analog multiplexers with no power.

Maybe use opto-couplers on the inputs. Digital signals for the opto input (on/off), analog inputs with some circuit for the "other side".. Which may be as simple as just a resistor to GND. But then you might not need the HC4052 smiley

Or, keep it powered but inhibited.

It gotta be possible somehow! (Without resorting to relays).
(FYI, I'm not an audiophile either).
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Maybe use opto-couplers on the inputs.
Problem is that opto couplers are not linear devices so you need an opto with one LED and two photo transistors and an op-amp to get a liner response.
But basically passing signals through unpowered equipment is a rather odd thing to want to do.
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