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I want to switch an FTDI interface with a single mechanical switch towards two different endpoints. The FTDI interface has six lines, four two-way digital lines, plus power and ground.

My mechanical switch should be a single SPST or SPDT switch.

With relays, the answer would look like the schematics below. I would need six SPDT relays for that.

I would like to have a smaller solution, with a minimum of components and complexity. Voltage and currents of my circuit are low, about 3.3V and 10mA.

schematics

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like a cross port switch, as in any line can go to any other line? Or like a relay that you just want the electrical connect removed.... Also, they'll want to get specs voltage, current, etc... –  kenny Dec 11 '11 at 12:46
    
Like a relay, each line has to be separate. I will add the information to the question. –  henning77 Dec 11 '11 at 13:20
    
You need to be more specific. Are these signals or power? Analog or digital signals? –  starblue Dec 11 '11 at 13:35
    
All lines are digital signals. –  henning77 Dec 11 '11 at 13:38
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Is there a specific reason, you don't want to use relays? Solid state relays are fast, low power and quiet. –  0x6d64 Dec 12 '11 at 7:32

2 Answers 2

up vote 2 down vote accepted

Use the switch to control analog switches. The switch then just needs to be SPST with a pullup resistor. This will be the control signal to the analog switches. One switch line can easily drive many CMOS inputs, so fanning out to six won't be a problem.

Analog switches are sometimes called "bilateral switch" because, unlike "digital muxes", they work just as well in both directions -- much like a relay. Sometimes I use an analog switch to connect the output of one chip to either one of two "input" lines (with pull-up or pull-down resistors to set the "other" line to the appropriate state), much like a digital demultiplexer. Sometimes I use an analog switch to connect the input of one chip to either one of two "output" lines (no resistors are needed for this case), much like a digital mux. Sometimes I use an analog switch to connect a bidirectional line like the I2C data line.

Analog switches act much like relays, except (a) they can't handle much current -- some handle 25 mA, others can handle only 10 mA. (b) they can handle only a limited voltage range -- some handle 0 to +15 V, others handle only 0 to +5 V. (c) they are lower cost, can switch far more rapidly and more often, and require much less power than relays.

(This is a minor tweak of the answer from Olin Lathrop).

Some parts I would consider:

  • 74HC4066 quad SPST bilateral analog switch
  • 4053 triple SPDT analog switch
  • 74HC4353 triple SPDT analog switch
  • 4052 dual SP4P analog switch
  • 74HC4852 dual SP4P analog switch
  • 14551 quad SPDT analog switch

You might be able to do this with two chips, and perhaps some pull-up or pull-down resistors.

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I didn't know analog switches. Exactly what I need. Thanks! –  henning77 Dec 13 '11 at 11:10

Use the switch to control digital muxes. The switch then just needs to be SPST with a pullup resistor. This will be the control signal to the muxes. One switch line can easily drive many CMOS inputs, so fanning out to six won't be a problem.

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Can I also do it with a couple of simple transistors? These I have on my hand. –  henning77 Dec 11 '11 at 14:09
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Could you recommend a mux or range of muxes for the OP? –  Majenko - not Google Dec 11 '11 at 14:10
    
How would the mux solution work? As far as I understand, a mux would have one output line. Wouldn't I need six muxes then? –  henning77 Dec 11 '11 at 14:16
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@henning: Yes, you would need six 2:1 muxes. However, you can often find multiple muxes with a common control input in the same chip. For example, the 74x158 is four 2:1 muxes in a 16 pin chip. You would need 1 1/2 of those. –  Olin Lathrop Dec 11 '11 at 14:30
    
You might be able to do this with a couple of transistors (or, more likely, FETs), but we would need more info: what is connected to the inputs and outputs? (But the Muxes that Olin proposed are essentially a bunch of such FETs in a convenient package...) –  Wouter van Ooijen Dec 11 '11 at 21:25

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