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SN74123: SN74123N - Dual Retrigg. Monostable Multivibrator w/ Rest to create nanosecond pulses

Hans Herath
Prodigy 90 points
Part Number: SN74123

I am using the SN74123N monostable multivibrator to create nanosecond pulses using an Arduino microcontroller. I am hoping I could get some help with the connections of the pins - I am following this schematic from behindthesciences.com . And I want to know whether pin 16 (Vcc) and pin 8 (GND) get the +5V input and is grounded (respectively). Also, I want to know what is an appropriate input pulse width from the Arduino.

I require ~50 nanosecond pulses that is +5V.

I would greatly appreciate any help on this matter.

*The schematic shown below doesn't give me nanosecond output, I want to know what I am doing wrong (I am using a breadboard for the connections)

monostable multivibrator

  • 0
    TI__Genius 16725 points

    Hi,

    I am a bit confused on your question. Vcc is where you connect your supply voltage (power) and yes you connect ground to GND. The inputs are A, B and CLR. The output is Q.

    For the pulse width calculation, you have to use the equation given in the datasheet. https://www.ti.com/lit/ds/symlink/sn74123.pdf

    If you read page 8 of the datasheet, you need to change the value of Rt and Cx to get the width of the pulse you desire.

    If you read through the following appnote, you will get an idea of how to design with a monostable multivibrator. http://www.ti.com/lit/an/slva720/slva720.pdf

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 90 points

    Hi Karan,

    I really appreciate your reply!

    My first question was whether it was possible to create nanosecond pulses with the monostable multivibrator I chose, I believe you answered that.

    My main question is how and which pins should be connected to what in order to get the nanosecond output pulse (Does the schematic work?)

    The schematic I attached shows no supply going into Vcc nor the ground attached to GND pin 8 - so I was wondering whether this is an issue with the schematic I have shown or whether it is not needed.

    Also, I am guessing the resistor-capacitor combination shown in the schematic will not work for my set up because I am using a different TI monostable multivibrator. 

    The below figure was obtained from the datasheet of the monostable multivibrator I am using - therefore to get an approx 50 ns pulse, I am supposed to use 5k ohm resistor and a 1 to 4 pF capacitor, am I right?

  • 0 in reply to Hans Herath
    TI__Genius 16725 points

    Hi,

    Please read the appnote I attached on the designing with the monostable multivibrators. It will answer all your questions. http://www.ti.com/lit/an/slva720/slva720.pdf

    Vcc is the power pin. Without power, it will not work. The pin connection is usually not shown as it is a given. Without a GND connection, your circuit will not be closed.

    You seem to have figured out the resistor, capacitor combination. You don't have to refer to the chart, the equation in the datasheet is T = .33*R*C where C is in pF, R is in KOhms and T is in ns. If you read the datasheet you will see R cannot be smaller than 5KOhms. So for 50ns you can do the math.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 90 points

    Hi Karan, Thanks again for the fast reply - I really appreciate it! I brought up that graph because the datasheet for the monostable multivibrator I selected points out if the capacitor I am working with is less than 1000pF I should refer to the graph.

    Since you made it clear I think that answers that question.

    About the pinout connections for my application - which is obtaining nanosecond pulses from Arduino input. Which schematic should I focus on? Because I do not believe I should focus on the Debounce Application Schematic.

    Finally in order to obtain nanosecond pulses, what frequency should the input pulses from the Arduino be inputted?

  • 0 in reply to Hans Herath
    TI__Genius 16725 points

    Hi Hans,

    I understand the schematic is for a debounce circuit but it gives you a general idea on the external component connections.

    I don't know what frequency you want to input. This depends on what you need to do. The appnote shows exactly how the monostable multivibrator works. The capacitor discharges to make the output go high and once the capacitor charges back to 63%, the output gets low. The datasheet for the device even shows example operation to show that you can keep re triggering the device and there is a truth table to understand how you need to connect your inputs so that your output will be what you desire.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 90 points

    I can't thank you enough for your help! I really appreciate it

    Hans