Going nuts testing gates

[gkmaia]

I've been able to test gates this way in the past. But yesterday nothing worked.

The LED in the right is connected to VCC.

The one in the middle is connected to pin 12 therefor should be HIGH, but it is not. Instead outputs 150mV

The one in the left is LOW. And should be as pins 9,10 & 11 are connected to VCC.

I did this test with several different gates types. The one on the photo is a 74ls12 and I tested 5 different ICs brand new. Same result.

My gate outputs no mater what the input is always output around 0.150V.

I am certainly missing something here. It is not possible almost 20 brand new ICs are faulty.

[rstofer]

I'm not entirely clear on where pin 1 is but I'm guessing lower left corner of the DIP as shown in the photo.

You pulled pin 1 high and left pins 2 & 13 floating which means high (probably).  All 3 inputs are high, the output should be low.

You should never leave pins floating.

[RoGeorge]

The datasheet says 7412 has open collector outputs.  That means the output of the gate can only pull down (GND), it can not pull up (to Vcc).
You need a resistor to V+ for each gate output pin (6, 8, 12)

Alternatively, for testing you can connect the LED to the Vcc instead of GND (with LED anode to Vcc), so you won't need extra resistors for each gate output.

[gkmaia]

The datasheet says 7412 has open collector outputs.  That means the output of the gate can only pull down (GND), it can not pull up (to Vcc).
You need a resistor to V+ for each gate output pin (6, 8, 12)

Alternatively, for testing you can connect the LED to the Vcc instead of GND (with LED anode to Vcc), so you won't need extra resistors for each gate output.

Thanks so much! Will try that when I am back home!

[gkmaia]

It worked! Thanks guys!