Electronic – Shift register Tri-State and ULN2803A

high-impedanceshift-register

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

Best Answer

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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