Problems with 74HC595N Output Enable

Hi, I've got two sensor IC's connected up to a 74HC595 for control. It seemed to be giving me some odd outputs on the output pins, so I checked the datasheet and realised that my output enable pin was set high, connected to 5 volts, whereas the datasheet says it should be low for enabling the outputs. After changing it around to low, now all the outputs are permanently low. I'm a bit confused. Any suggestions for debugging steps I could take? Thanks,


OE/ must be low for outputs to change, otherwise they will float, unless pullup/pulldown resisters are in place.

Post your schematic, maybe something else is miswired as well.

Something sounds wrong here, if you have sensors wired to a 74HC595. That chip is normally used to generate 8 digital output signals. It's not useful as a way to read inputs from sensors.

Sorry about the delay, took me a while to figure out eagle. Here is how I designed it and how I believe it is wired. I should add it was also previously working on a breadboard, I’ve only been having trouble since moving it to a protoshield. Thanks,


How are you driving pin 12, RCK? If it's floating (unconnected), then that could be your problem.

It isn't connected to anything. The tutorials indicated only 3 digital i/o lines were required to use the shift register I thought.


Yes - SER, the SerialData going in SCK, the clock to put data into the input registers RCK, the cock to move the data in parallel to the output register.

SCL clears the input registers

I’ve double checked and it is actually RCK on Pin 12 going to pin 6 of the Arduino. I drew the diagram incorrectly. See attached. Thanks,


So, SCL, pin 10, is left unconnected? In that case, it will be floating and will pick up mains noise (50 or 60Hz), radio signals (mobile phones) and so on. No input pin on the 74HC595 should be unconnected. Looks like this one needs to be tied HIGH (to the 5V supply).