NAND Gate Oscillator

Discussion in 'General Electronics Chat' started by abesanity, Nov 12, 2012.

  1. abesanity

    Thread Starter New Member

    Nov 2, 2012
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    Hey everybody, I have been working on the NAND Gate Oscillator circuit from the book "Electronic Circuits for the Evil Genius" and I have hit a brick wall. The circuit calls for a 4011 Quad NAND Gate but due to limited resources, I am using an SN7400 Quad NAND and I believe this is where the problem lies. When the push-button is pressed, the circuit should flash the LED once a second for about 8 seconds. When I apply voltage, my LED comes on immediately without activating the push-button.

    I attached the circuit and may try to put the circuit together today with AND/NOT to see if I can get it working. Any help/suggestions would be appreciated.

    VALUES:
    VDC = 5V
    R1 = 100kΩ
    R2 = 10MΩ
    R3 = 470kΩ
    R4 = 2.2M
    C1 = 1μF
    C2 = 0.1μF
    IC1 = 4011 (I am using a 7400)
     
    Last edited: Nov 12, 2012
  2. MrChips

    Moderator

    Oct 2, 2009
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    You cannot substitute an SN7400 for CD4011. The input currents are completely different and hence all the resistance values are drastically different. This would of course change all the timing parameters.
     
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  3. abesanity

    Thread Starter New Member

    Nov 2, 2012
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    Thank you, I was thinking it had something to do with the characteristics of the chips but was unable to properly identify it. Now with some timing parameter changes I'm assuming I can get it to work, or is there something else I'm missing? Again, thank you for the help and quick response.
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    You cant use them with 9V.

    Maybe simulate it in LTSpice etc.

    Forget about resistors larger than 20 KOhms.
    You need some 100uF for 8 seconds using 74xx.
     
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  5. abesanity

    Thread Starter New Member

    Nov 2, 2012
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    I apologize for not including it in the original post, I was using 5V.
     
  6. MrChips

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    Oct 2, 2009
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    You have to change a number of things.

    1) TTL 74xx00 series require 5V. 9V will not work.
    2) The pinouts for SN7400 are different from CD4011.
    3) All component values have to change.

    Do you have anything else besides SN7400?
    For example, 7414, 74LS14, 74C00, 74HC00 etc.
    or even a 555 chip.

    I can come up with a new circuit for you. Just let us know what you have available.
     
  7. takao21203

    Distinguished Member

    Apr 28, 2012
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    I've built a ring oscillator only using 1489 serial receivers (they are kind of TTL inverters).

    -I had to use high efficiency LEDs.
    -Transistors were needed as well to get clean flashes. Replaced the actual LEDs with 1n4148 except 1 LED!

    So it is flashing every few seconds.

    Before building the circuit on breadboard, I simulated it using LTSpice.

    Not to say it is a trivial circuit, and better suited for microcontroller.

    But I think hobbists, students, beginners, children etc. actually like these ring oscillators.
     
  8. abesanity

    Thread Starter New Member

    Nov 2, 2012
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    At this time, I only have the SN7400. I do have LM555 available but I'd like to try to construct this circuit without it first if possible. Was thinking about it in the shower (where most of my critical thinking gets done:p), maybe if I use 100μF and some ~15k. I should put it in multisim when I get to the lab.
     
  9. MrChips

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    Oct 2, 2009
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    You can use three of the 7400 NAND gates to make a ring oscillator with a 1kΩ resistor and 470μF capacitor.

    The problem will be coming up with the 8-second delay.

    I will post a schematic later.
     
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  10. abesanity

    Thread Starter New Member

    Nov 2, 2012
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    Is it normal to be reading 1.6 V from inputs to ground on the sn7400 when the IC is energized?
     
  11. MrChips

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    Oct 2, 2009
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    Yes. Unconnected TTL inputs float up to a logic high.
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Here is a 1Hz oscillator using an SN7400:
    [​IMG]

    I show two ways of connecting the LED. You may want to experiment which to use and which gate output to connect to since the signal is not symmetrical, i.e. not 50% duty cycle.

    Creating an 8-second delay is not so easy if you want to avoid using a ridiculously large value capacitor such as 47000μF across 220Ω.

    There are various solutions using:

    1) CMOS gate
    2) analog comparator
    3) 555 timer
    4) counter chip
    5) microcontroller

    Which one would you prefer?
     
    Last edited: Nov 12, 2012
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  13. Audioguru

    New Member

    Dec 20, 2007
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    The SN7400 has a maximum input low current of 1.6mA. The highest value resistor to ground to cause a valid input logic low is 500 ohms. 15k won't do anything.

    The maximum input low current for a CD4011 is 1uA. The highest value resistor to ground to cause a valid input logic low is 1.5M ohms.
     
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  14. takao21203

    Distinguished Member

    Apr 28, 2012
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    This is why people don't like 74XX.

    If you don't really totally care, you end up with a mix of LS,HC, and HCT.

    Sometimes circuits just don't work properly. Then you swap out one HC with another HC (different manufacturer), and suddenly it works!

    I went through that a few times. So when I came accross microcontrollers, I was more than happy to abandon this 74XX stuff.

    Still use the shift registers and LCX244 buffers...
     
  15. abesanity

    Thread Starter New Member

    Nov 2, 2012
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    You guys have been a great help, don't worry about the 8 second delay.
     
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