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I have actually a solution based on a multiplexer (CD4051) which drives 8 outputs, either at 0V or 3.3V (the input is at 3.3V). Each output of the multiplexer drive a NMOS and the goal is to have a signal behind the NMOS driven to 0V or floating. In an another words I drive 7 signals at 0V and one signal is not driven (floating). The multiplexer is used to extend the gpio of my microcontroller (3 to 8 and could be more by adding more cascading multiplexers). My question is could I do the same without the NMOS? I mean, is it possible to have a component (multiplexer) which has an output either at 0V or floating? To be more accurate, could it be possible to have 7 outputs at 0V and one floating?

I guess it is not feasible but I prefer to ask, in case of :)

Thanks in advance

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  • \$\begingroup\$ Maybe not with a multiplexer, but could you use an open collector shift register like the SN74LS596? Then you can have any combination of floating and 0V you like! \$\endgroup\$ – jfowkes Apr 6 '17 at 15:55
  • \$\begingroup\$ SN74ALS156 does what you want but is a 5 V part. It only needs 2 V on the inputs so if you have a 5 V rail handy it should work with your micro's output pins. \$\endgroup\$ – Andrew Apr 6 '17 at 16:01
  • \$\begingroup\$ @Andrew the LS156 is open collector, but the logic still needs inverting, no? \$\endgroup\$ – jfowkes Apr 6 '17 at 17:25
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You could use a 74HCT138 and drive a ULN2803 transistor array from its outputs.

The 74HCT138 is a 3-to-8 decoder. All outputs are driven high except the code-selected output which is driven low.

The ULN2803 contains 8 Darlington transistor drivers with input base resistors and open-collector outputs. A logic high input drives the output to 0.8..1 V. A logic low input lets the output float at high impedance.

If the ULN2803 'on' voltage is too high for your application, you could look for a FET equivalent or consider using discrete logic-level N-channel MOSFETs.

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