I used 74HC138 in the first circuit and 74LS138 in the second circuit.
Only the one with the 74HC138 is working.
Why?
Electronic – The circuit with 74HC138 is working, but with 74LS138 it is not, why
digital-logicintegrated-circuitproteusttl
Related Solutions
A zener diode needs some current (a few mA) trough it to 'drop' its designed volatge.
If you want the + input to be the zener voltage below the positive rail, connect the resistor from the + input to ground.
If you want the + input to be the zener voltage above ground, connect the zener between + input and ground, and the resistor between + input and the positive rail.
74LS32 is a LSTTL series chip (really DTL). If you leave the inputs unconnected they are effectively at "1". You need to ground each input to get a "0".
Each input has a 20K typical pullup resistor.
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Best Answer
The 74ALS138 device that you found was unable to drive the output LEDs is operating (more than likely) correctly - the output high drive current is only specified at 400 uA in the data sheet.
Compare this with the 74HC138 - it can drive a high output current of 4 mA.
ALS device: -
HC device: -
So, given that the ALS device is pretty good at pulling down its output (note that 8 mA can be obtained), I would recommend that you turn the diodes round and have the anodes up to + 5 volts instead BUT, don't forget to use current limiting resistors like 220 ohms or thereabouts.