SN74LS221NE4 output

Mohamed Ali25

Hello all,

I wanted a DIP chip that will generate a a low pulse output, with a high at 3v3 and a low= gnd. I bought the SN74LS221NE4 chip and set the pins as follow:

Pin 1: GND

Pin 2: Input from a photto-interrupter, signal goes from ground to 3v3

Pin 3: Vcc= 5.1 Volts

Pin 4: output attached to a sparkfun Esp8266 RST pin (contains a pull up resistor to 3v3, and resets the processor by an active Low)

Pin 5,6,7: Empty

Pin 8: GND

Pin 9,10,11,12,13: Empty

Pin 14: capacitor= 4.4 microFarad, connected to Pin 15 node

Pin 15: Resistor= 100k connected to Vcc.

Pin 16: Vcc= 5 Volts

Even though my input signal rises to 3v3, much faster than 1 V/Second at Pin 2, I am still unable to generate a low pulse. Can someone tell me what could I be doing wrong? I am following the datasheet as much as I can. I noticed the function table states that bulses are created with the external resistor = 2k ohms and external capacitor= 80 pico Farade, is this necessary? What if I need a longer pulse width?

Any help would be appreciated, even if it means to buy another chip that can serves my purpose and creates a negative pulse output between ground and 3v3.

-Mo

over 9 years ago

0 Daniel Hartung over 9 years ago

TI__Mastermind 28455 points

Hi Mohammad,

I have a question about the Pins that are labeled "empty"
Are those pins tied to GND or Vcc? Any unused inputs of the device must be tied to Vcc or GND to ensure proper operation of the device.

Thanks,
Daniel

0 Mohamed Ali25 over 9 years ago in reply to Daniel Hartung

Prodigy 80 points

Hey Daniel, thank you for responding. Even if all the the "empty" pins are associated with the other multi-vibrator? It's a dual multivibrator chip. I'll do that regardless, and let you know. Best, -Mo

0 Daniel Hartung over 9 years ago in reply to Mohamed Ali25

TI__Mastermind 28455 points

Hey Mohammed,

If the other pins are being used for another multi-vibrator then they are considered to be "used" and thus don't need to be tied to either GND or Vcc. There is an applications note that goes over it in detail in "Implications of Slow or Floating CMOS Inputs"

Thanks,

Daniel

0 Mohamed Ali25 over 9 years ago in reply to Daniel Hartung

Prodigy 80 points

Daniel,

Great, I grounded all of the floating pins except for pin number 13, which is output Q, the output I am using is Q dash (pin 4). Should I ground pin 13 or leave it floating?

Mohamed

0 Daniel Hartung over 9 years ago in reply to Mohamed Ali25

TI__Mastermind 28455 points

Hey Mohamed,

Since that pin is being used as an output (whether or not you use it is a different story) that pin should be left floating.

Thanks,
Daniel Hartung

0 Mohamed Ali25 over 9 years ago in reply to Daniel Hartung

Prodigy 80 points

Great, is there anyway I can output a "low pulse" where the high is a 3V3 and the low is GND? I have Vcc at 5.15V, and my output has a high of 4.10V and low is GND. If not, what would you suggest me to add after my output that would keep the integrity of my pulse without a huge amount of delay. Would taking Vcc down work? Is there a similar Monostate multivibrator that does that (That is made by Texas Instrument or any other company)?

-Mohamed

0 Daniel Hartung over 9 years ago in reply to Mohamed Ali25

TI__Mastermind 28455 points

Hey Mohamed,

Attached below is the specs for VOH and VOL for the device

It seems that your VOH is a bit high for the device, but I noticed that the Vcc test spec on this datasheet is the minimum recommended value for the device. The first thing I would try doing is supplying the device with 4.5 V and see how that changes the VOH. I would do this because 4.5V is the minimum recommended operation voltage. As far as VOL is concerned there will always be some small voltage no matter what device you use, however it will always be very close to ground.

Thanks,

Daniel

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SN74LS221NE4 output