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I was planning to use the SN74HC595 shift register in association with the ULN2803A and MIC2981 to switch on and off some loads based on the data of the shift register.

What happens if I disable the output of the shift register (using the OE line) and lines Qa to Qh go into high impedance state ?

Would the ULN or MIC chips read the high impedance input as randomly high/low, ie, switching on and off the loads randomly ? 

 ----------     Qa Qh    ----------    
|          |  --------  |          |  -------- Load A
|  Shift   |  --------  | ULN2803A |  -------- Load B
| Register |  --------  |    or    |  -------- 
|          |  --------  | MIC2981  |  -------- 
|          |  --------  |          |  -------- ...
|          |  --------  |          |  -------- 
|          |  --------  |          |  -------- Load H
 ----------              ----------
     |OE
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  • \$\begingroup\$ You happen to have chosen bipolar input drivers, where high Z input is OK, and behaves as 'off'. There also exist cmos input drivers, where the same does not apply. \$\endgroup\$
    – Neil_UK
    Commented Jul 20, 2018 at 8:49

2 Answers 2

Reset to default
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ULN2803 will read low input, see the equivalent circuit from the datasheet:

enter image description here

The resistors will pull the inputs down as you see.

MIC2981 will also read low since there is no current to drive the input transistor base. Cautions must be taken because here there is no pull down resistor and the input is noise sensitive.

enter image description here

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  • \$\begingroup\$ Thanks ! What would be a good way to reduce noise for the MIC2981 ? \$\endgroup\$
    – Fredovsky
    Commented Jul 20, 2018 at 8:56
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    \$\begingroup\$ @Fredovsky A 10 kohm pull down resistor and short traces will do. \$\endgroup\$
    – Dorian
    Commented Jul 20, 2018 at 8:59
  • \$\begingroup\$ Per driver I guess, so I believe 8 10kOhm resistors per MIC2891 if I’m using all the lines? \$\endgroup\$
    – Fredovsky
    Commented Jul 20, 2018 at 9:39
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    \$\begingroup\$ @Fredovsky Yes, each input to ground \$\endgroup\$
    – Dorian
    Commented Jul 20, 2018 at 9:44
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According to to ULN2803A datasheet, the internal transistors have pull down resistors so the transistors will switch off if the shift register outputs go high impedance -

enter image description here

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