Mux switch -- I need to build a circuit board that will change inputs on demand.

Thread Starter

nonefyourfingbis

Joined Jan 3, 2021
12
Hi, I have zero understanding in electrical engineering, and a friend of mine said that I need a mux switch. I was wondering if he is correct and if yes, what components do I need to buy.

I need to build a circuit board that will change inputs on demand.
I have 2 sources with 4 wires each, I need only one source to be active at a time.
I have a signal (in a form of 3.3v/5v). If I have current (in the signal) it will choose the first source and if not it will change to the other. In other words 8 wires going in, 4 coming out.
 

Ya’akov

Joined Jan 27, 2019
6,260
Welcome to AAC.

Yes, it does sound like you need a multiplexer (mux) but to help properly it would be much better if you could explain the actual application not just some parameters of it. There may be something that changes the advice considerably that are omitting with your simplified case (without knowing it).

Could you describe the problem you expect switching the between the inputs will solve?
 

DickCappels

Joined Aug 21, 2008
9,067
What are the sources? In other words, what are the signals. One is a tachometer signal -can you describe it electrically? Same question for the PWM.

The simple answer unless more is known about the signals, would be to use a 4PDT relay or the equivalent made of SPDT or DPTD relays.
 

Thread Starter

nonefyourfingbis

Joined Jan 3, 2021
12
What are the sources? In other words, what are the signals. One is a tachometer signal -can you describe it electrically? Same question for the PWM.

The simple answer unless more is known about the signals, would be to use a 4PDT relay or the equivalent made of SPDT or DPTD relays.
I have a dual system chassis with two separate computers in it (the one is a PC and the other is a server). They share a power supply, and as a result, the PSU is always on (because the server always stays on). My fan hub is connected to the fans, PWM source (the PC), and the shared PSU. When I turn off the PC the hub doesn't get a PWM signal, so it defaults to 100% speed on the fans. I want to design a board that will be connected between the PWM source and the hub, and when it senses that the PC is off it lets the server's PWM signal go through.
 

Thread Starter

nonefyourfingbis

Joined Jan 3, 2021
12
Have you considered how you would do it using old-fashioned (and reliable) relays?
So, I'm a chemist and don't have much knowledge in designing circuits and picking components but I do know how to solder. if you think that relays are the way to go, I'm in. If you can tell me which components are the correct ones for me (preferably from amazon because I'm not from the US) and tell me how I should connect them, I'll be grateful.
 

Ya’akov

Joined Jan 27, 2019
6,260
If you want to use the logical control then you can choose the ic below:
74HC257_74HCT257 Quad 2-input multiplexer 3-state
The Rohm BU4551 seems suited as well, and offers high voltage operation (12V supply would work).

But, @DickCappels idea of a relay is attractive based on simplicity of implementation for a neophyte.

I wonder, though, if it would just be better to disconnect the fan's supply when the power is off its host rather than feeding it a PWM signal that will vary because of some other host's state. (I might have the problem a bit wrong, if so please excuse the confusion.)
 

ScottWang

Joined Aug 23, 2012
7,272
The Rohm BU4551 seems suited as well, and offers high voltage operation (12V supply would work).

But, @DickCappels idea of a relay is attractive based on simplicity of implementation for a neophyte.

I wonder, though, if it would just be better to disconnect the fan's supply when the power is off its host rather than feeding it a PWM signal that will vary because of some other host's state. (I might have the problem a bit wrong, if so please excuse the confusion.)
I like to use ic to build the ee stuffs, low power, small size, no sound, in this case is depend on the TS, he has no experience, so probably to use relay is a easy way.
 

BobTPH

Joined Jun 5, 2013
5,458
You say the power supply is the same for both computers, so why sre you switching ground and +12V, which are already connected together? Unless I misunderstand something, you only need to switch two signals.
 

Thread Starter

nonefyourfingbis

Joined Jan 3, 2021
12
You say the power supply is the same for both computers, so why sre you switching ground and +12V, which are already connected together? Unless I misunderstand something, you only need to switch two signals.
Well, you are probably correct, but as I said I have no experience and I'm not well versd in how the motherboard deals with electricity. I thought maybe it can trip something if it doesn't detects current coming back, I just didn't want to risk it, especially if I can just double the number of components or get ones with more inputs and outputs.
Correct me if me assumption is totally wrong.
 

Thread Starter

nonefyourfingbis

Joined Jan 3, 2021
12
The Rohm BU4551 seems suited as well, and offers high voltage operation (12V supply would work).

But, @DickCappels idea of a relay is attractive based on simplicity of implementation for a neophyte.

I wonder, though, if it would just be better to disconnect the fan's supply when the power is off its host rather than feeding it a PWM signal that will vary because of some other host's state. (I might have the problem a bit wrong, if so please excuse the confusion.)
Well, I can disconnect the fan hub when I turn off my pc, but there are two problems, one is it's in a hard to get place so it's a hassle to connect and disconnect then every time, the second problem is that the server needs the air flow of the fans, I would connect them only to the server but when the pc is on, he is the hottest part and it needs more airflow than the server. I can just increase the fan curve, but that is also a problem because I want the fans as quite as possible.
 

Ya’akov

Joined Jan 27, 2019
6,260
The +12V and 0V ("ground") lines can just stay connected. They don't have to be switch if they are already common between
Well, I can disconnect the fan hub when I turn off my pc, but there are two problems, one is it's in a hard to get place so it's a hassle to connect and disconnect then every time, the second problem is that the server needs the air flow of the fans, I would connect them only to the server but when the pc is on, he is the hottest part and it needs more airflow than the server. I can just increase the fan curve, but that is also a problem because I want the fans as quite as possible.
I was suggesting using a different signal from the PC to sense it has been turned off to switch off the 12V supply that keeps the PC fan operating.
 

Thread Starter

nonefyourfingbis

Joined Jan 3, 2021
12
The +12V and 0V ("ground") lines can just stay connected. They don't have to be switch if they are already common between


I was suggesting using a different signal from the PC to sense it has been turned off to switch off the 12V supply that keeps the PC fan operating.
That is exactly what I wanted to do at the beginning, but I realized that the server would heat up without the fans.

So what you are saying is that I need to switch between only two wires from each source?
 

BobTPH

Joined Jun 5, 2013
5,458
The 12V and ground for the fans can come directly from power supply, or, if you insist, from the server since it is always on.
 
Top