I am having an issue with using a 74150 in a circuit

If I set one up on a breadboard as follows:

The address lines connected to GND or VCC
GND and VCC pins connected
Strobe connected to GND

Logic probe connected to Output

The output goes HIGH with no input

If I then connect the relevant input to VCC (as per the address lines) then the output goes low....

Now in my circuit (an arduino with the address lines and output connected to analogue pins but being used as digital) and with the inputs floating normally and then connected to VCC the output never changes.

Also when the scope is attached to the floating input the 74150 is making it hover at 0.5v when that input is selected.

I created a sketch that runs all 16 addresses and displays the output value.... The address lines change perfectly at 0V and 4.96V so no issues there.

I have tried pulling all the address lines low using a 10K resistor network, disconnecting the IC's output and connecting the logic probe to it and also tried replacing the IC with the one from the breadboard.

 

Hmmmm,

Just grabbed a spare arduino and re-created the circuit on a breadboard. The circuit still does the same so I moved the I/O lines to digital pins and it now works...

PLUS

Now connected the address lines back to analogue pins and it works again. Just will not work as input via an analogue pin.

 

The 74150 is a digital TTL IC. If it works with digital I/O pins on the Arduino, why are you trying to use the analog I/O pins?

I'm not familiar with Arduinos, but the old TTL requires a fair amount of source/sink capability for whatever is driving the inputs; around 1.6mA for sink (logic 0/low), and up to 1mA for source (logic 1/high).

It's not a good idea to leave any logic IC's inputs floating (no current path to Vcc/Vdd or GND) as your results can vary significantly.

You need a 0.1uF ceramic or poly metal film across the supply pins of any IC as a minimum requirement.

10k is far too high of a value to use for a standard TTL pull-up or pull-down resistor. You should be using 330 Ohm to 470 Ohm resistors. 10k works fine for 4000 series CMOS for pull-up or pull-down resistors.

You really need to post a schematic, and indicate what Arduino you are using.