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I have several shift registers and only one "read" pin.

I would like to know if I can use PNP transistors to switch between the registers ?

So every emitters will be connected to the data output of his associated shift register, every collectors connected to the single input and the base connected to some current source allowing me to switch between the registers. I'm a bit scared about signal integrity. Does it sound ok for you or a total non sense ?

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(Don't take in account the selected parts number, just for the example)

EDIT : A simple remark, clocks and latches are all "synchronized" on the same two pins. So this would allow me to drive virtually as many register as I want with only 3 pins used on the arduino.

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  • \$\begingroup\$ Can this read pin be an ADC? \$\endgroup\$ – Daniel Grillo Nov 28 '13 at 3:31
  • \$\begingroup\$ It can be. In fact the input pin is located on an arduino. I would prefer to use the digital ones. \$\endgroup\$ – Emmanuel Istace Nov 28 '13 at 3:38
  • \$\begingroup\$ 4021 is a parallel to serial shift register. You don't have to worry about inputs with this chip, there are 8. HEF4094BP is a serial to parallel shift register. By daisy chaining them you can access each individual bit. \$\endgroup\$ – jippie Nov 28 '13 at 7:24
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    \$\begingroup\$ Conditional yes. Vbe will be reverse biased in some states. As Vdd is increased you may get unexpected current flows. |You can use eg CD4016 or CD4066 transmission gates in place of transistors, or CD4051 family which have selection multiplexer built in. \$\endgroup\$ – Russell McMahon Nov 28 '13 at 8:22
  • \$\begingroup\$ @Russell McMahon : That's what I was looking for thank you ! \$\endgroup\$ – Emmanuel Istace Nov 29 '13 at 5:39
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No, this won't work as drawn. The PNP transistors will pull the Arduino input pin high but there is nothing to pull the pin low. You could add a pulldown resistor where you connect to the Arduino, and that will work at low frequencies. It seems that a 4:1 multiplexer would be a much better choice.

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