74HC595 - resistor position affects whether LED will turn on - why?

Hi,

I’m testing 75HC595 and I thought that when protecting an LED with a resistor, it doesn’t matter which side you put the resistor on. But when I change the resistor position with the 74HC959, it will determine whether the LED will light up. (See the hookup below)

Could anyone please explain why is this happening? What am I missing?

Many thanks!

Why? Because you are not using your breadboard properly.

In the left example the Resister is shorted out and doing nothing to protect the 74HC595 and the LED is getting it's full output power. You may fry your 74HC595 if you keep doing this.

In the right example the LED is shorted out and so cannot light up.

Based on the way you have your examples laid out I will call what's connected to pin one of the 74HC595 Row 5. Row 5 is one bus line so in the left example connect the LED from the GND to Row 4 and the Resister from Row 4 to Row 5. In the right example connect the Resister from GND to Row 4 and the LED from Row 4 to Row 5.

Hope this helps.

Oh crap, that's quite a stupid mistake to make:) It all makes sense now - many thanks! Good learning for me.

Also you don't have decoupling. All logic chips require decoupling right at the power and ground pins.

Here's a link to an old article that talks about decoupling for older technologies like TTL Logic.

This is a great article because it explains why decoupling is so important. Even with the advent of low current Logic IC's decoupling is still emensely important.

http://www.vagrearg.org/content/decoupling