Moderator's Note:
This thread was split from -- 74HC08N always outputs 1 .


Okay I got another problem.

I got a SN74LS136N (XOR), connected to +5V Vcc, and ground, output to 100 ohm to LED to ground,
and whatever input I gave it output was always zero (when one of inputs is 1 and other 0 output should be 1)

So I thought maybe its because of the floating pins, so I connected all unused input pins to gnd, (5 of them with normal
wires and 1 of them with a 100 ohm resistor because I dind't have any more wires left) but for whatever A and B inputs it was still 0.

So I thoguht maybe I should check datasheet for and and datasheet for 74HC08N (the one which I got working fine) and compare.

And I saw that for 74LS136N I should use 40 uA current for high-level input current (did find high-level input current in 74HC08N, just input current which should be 1000nA? it works when 5V are connected directly so I don't know what current I'm inputing in it), so I connected (Vcc and gnd) of 74Ls136N to (5V and gnd), output to LED, 2nd input to gnd to 0 ( B = 0 ) and well for first input, I got from +5V to 10 000 ohm resistor, then to 1000 ohm and then to another 1000 ohm ( so current is then 41.6 uA ) and then to 1st input ( A = 1).

And yet LED doesn't shine

EDIT: Oh just realised that 5 / 12000 = 4.16 * 10tothepwerof -4 -.-
So I should really use 120 000 ohm of resistance for HIGH_LEVEL input ?

 

74LS00 gates don't drive LEDs very well with ACTIVE-HIGH signal. Put the LED on the high side.

Never take a logic signal from a gate output that is already connected to an LED.
The proper way to monitor a gate output is to buffer the signal via a 74LS04 and use the inverter to sink current from the LED.

Here is an example:


For feeding the signal to subsequent circuits, use the node labeled "LOGIC SIGNAL" and not from the output where the LED is being driven.

 

I got a SN74LS136N (XOR), connected to +5V Vcc, and ground, output to 100 ohm to LED to ground,
and whatever input I gave it output was always zero (when one of inputs is 1 and other 0 output should be 1)

So I thought maybe its because of the floating pins, so I connected all unused input pins to gnd, (5 of them with normal
wires and 1 of them with a 100 ohm resistor because I dind't have any more wires left) but for whatever A and B inputs it was still 0.

For the 74LSxx series, put the input pin floating is equal to pull high.
For the CD40xx and CD45xx and 74HCxx series, put the input pin floating is similar connecting to a noise generator.
You can check the internal structure of above ICs to know more details.

 

Okay, So I used a voltmeter, from wanted pin to gnd pin of 74LS136N, 1st pin Vcc 4,97V, 2nd pin 0 V ok, 3rd pin 4.97 V ok,
4th output pin 0 V not ok :/

 

Okay, So I used a voltmeter, from wanted pin to gnd pin of 74LS136N, 1st pin Vcc 4,97V, 2nd pin 0 V ok, 3rd pin 4.97 V ok,
4th output pin 0 V not ok :/

Are you talking about the pin-out of 74LS136?
What I saw the pin-out were different from your description.

 

Why not read the datasheets?
The datasheets of old SN74LSxx ICs from Texas Instruments say the recommended output high current is only 0.4mA (makes a red LED look very dim) and the spec's say with an output current of 0.4mA then its minimum output high voltage is only 2.4V which is not enough to light white, blue or bright green LEDs.

Why were you limiting the input current with resistors? The MAXIMUM input current of a 74LSxxx is 40uA so simply connect it to Vcc (+5V) and it draws no more than 40uA without a resistor.

Yeah, pin 4 of a 74LS136 is an input of the 2nd gate that you connected to ground so of course it is 0V.
You are completely confused with the pins. Pins 1 and 2 are the inputs of the first gate and pin 3 is its output, not pin 4.

 

Oh my fault,

14th pin Vcc 4,9 V, 13th pin 0 V ok, 12th pin 4.97 V ok, 11th output pin 0 V not ok :/

Those voltages are between those pins and 7th pin (GND)

 

Oh my fault,

14th pin Vcc 4,9 V, 13th pin 0 V ok, 12th pin 4.97 V ok, 11th output pin 0 V not ok :/

Those voltages are between those pins and 7th pin (GND)

The output is wrong for a 74LS136 but is correct for a 74LS08. Which IC are you measuring?

 

SN74LS136N (beneath that it says K8330 if that means anything : )

 

I bought 2, tried the other one and the other one also doesnt work.

On both it says SN74LS136N, there is nothing connected on my board except IC and LM7805 which is connected to the battery.

From LM7805 - to GND on IC, from LM7805 + to Vcc. 13th input to + same row as Vcc,
12th input to GND same as IC gnd, and 11th is 0 V

 

Did you buy cheap fake ICs from ebay, Amazon or Aliexpress?

 

No I bought a few of ICs in my local electornic shop, 74HC08N works fine

Also got a few more like 74LS00N and 74LS02N they work great...

Does that mean that my 2 74LS136N 's are broken?

 

No I bought a few of ICs in my local electornic shop, 74HC08N works fine

Also got a few more like 74LS00N and 74LS02N they work great...

Does that mean that my 2 74LS136N 's are broken?

Are you trying to use 74HC and 74LS in the same circuit? That's asking for trouble if you don't know what you're doing.

 

No, 74LS136N as stated before alone doesnt work :/

 

No, 74LS136N as stated before alone doesnt work :/

Since this is unrelated to the original problem, you should have started a new thread...

 

Maybe you do not know which breadboard hole connects to what breadboard hole?
Maybe the contacts on your breadboard make poor contact?
Then maybe you should try soldering the pins together on a perf board.

 

Post a photo of your breadboard setup.

 

Okay here is a pic how I connected it:


As you can see I have 2 of them, and for both of them 11th pin is 0V when 12th and 13th are 0V and 4.97V :/

11th pin is the one to which nothing is connected near the blue wire. I used multimeter to get voltage on it (Measured it from that pin to gnd pin)

EDIT: Also when I put 74HC08N (AND)on the exact same spot on the breadboard and both 13th and 12th pins are 4.97V, output is also 4.97 V, so breadboard is probably fine.

 

74LS136 has open collector outputs; you need pull-up resistors.

 

Still not owrking, one switch is off, goes to the 13th pin, 13th pin is 4.95 V, one switch is on, goes to the 12th pin, 12th pin is 0V, 11th pin (output) is 0V. I used 4.7 kilo ohm resistors

EDIT: also another one doesn't work either