I want to blink an led(0.75 seconds on and 0.75 seconds off) repeatedly. I was thinking of using an idea from a book called "But How Do It Know" by J. Clark Scott. A part of the book tries to explain how to make an oscillator to use in a computer(The book tries to teach the reader how to make a functioning computer)but doesn't really go into the details about how to change the frequency etc. A schematic to show a square wave generator is provided in the book. Here it is(Diagram 1):

schematic

simulate this circuit – Schematic created using CircuitLab

The author says that the output will be too "fast" to be used for anything. He then says that to slow it down you have to lengthen the wire like so(Diagram 2):

schematic

simulate this circuit

This is obviously over simplified and impractical because you can't extend the wire for a long length without running out of space. How can you "slow down" the output of the circuit in Diagram 1 to produce a desired frequency(0.75 seconds HIGH and 0.75 seconds LOW). Here is a clock digram of what output I want(in hertz I think this would be 0.75 Hz, Correct me if I am wrong):

schematic

simulate this circuit

My question is: How do I slow down the output of the "not gate oscillator" (Diagram 1), to the desired output of(0.75 seconds "on" and 0.75 seconds "off") and is this frequency 0.75 Hz?

  • 1
    It sounds like that book you are quoting is poo. – Andy aka Dec 19 '15 at 21:32
  • Well it does teach the reader how to build a fully functioning computer from logic gates, but doesn't really get specific into the electronic side of things. The rest of the book is detailed and descriptive. I don't think that the book was written to give out specific info on computer electronics, but rather to prove that building a computer is not as hard as people think. The author proves this by showing the reader how to build one and explains the various parts of the computer. If you want to know more about what's in the book go here:youtube.com/watch?v=cNN_tTXABUA – zack1544 Dec 19 '15 at 21:54
  • 2
    No thank you... – Andy aka Dec 19 '15 at 22:11
up vote 2 down vote accepted

schematic

simulate this circuit – Schematic created using CircuitLab

This tends to work better with a Schmitt-trigger NOT gate (one with some hysteresis, or difference between "voltage it changes state on when going down" and "voltage it changes state on going up.") Circuit-lab's logic selection does not appear to have any Schmitt-trigger parts. Incidentally a Schmitt-trigger NAND with the inputs tied together also makes a Schmitt-trigger NOT.

It's a crude solution in any case and the R, C values are highly unlikely to give the precise oscillation you want. A 555-timer or 555 timer into a flip-flop is a somewhat less crude approach.

The frequency is one over the time for one full cycle. A full cycle is 1.5 seconds, so the frequency is 0.666666666666666666666... Hz (0.66666667 in most calculators due to rounding.)

  • Just a question: Is the arrow on the bottom of the circuit ground or the output? – zack1544 Dec 19 '15 at 20:08
  • Ok, so the output of the circuit is in the top right corner? – zack1544 Dec 19 '15 at 20:12
  • 1
    This won't work with a standard NOT gate. You need a schmitt trigger input. – jp314 Dec 19 '15 at 20:13
  • @jp314 please read first sentence. Didn't see on in the circuit-lab embedded parts list. Zack - edited. – Ecnerwal Dec 19 '15 at 20:15
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    @zack1544 - Not only can you use a 74HC14, you MUST use a CMOS family such as 74HC/HCT or CD4000. Trying to use something like 74LS will not work with a 750k resistor. Also, I suspect that you'll get a faster oscillation than you expect, so be prepared to increase the capacitance. – WhatRoughBeast Dec 19 '15 at 20:58

use an RC in the feedback. Unless you use something like a 74HC14, the thresholds will not be as deterministic BUT it will facilitate generating a period slower than the devices propagation delay

You can add some components to the circuit as shown below to make your LED blinking oscillator. Make sure to use a chip like a 74HCT04.

enter image description here

Use R1 = R2 = 150K Ohms

Use C = 5uF

If the frequency does not come out just where you want it you can experiment with changing the capacitor value or the R1 and R2 value. Once again make sure to use a CMOS type NOT gate part for the three inverters. The part suggested above actually as six inverters on one package.

  • Does the 74HCT04 work with 5v? I was thinking of using the 74HC14, but it runs at 9v and I want 5v. – zack1544 Dec 19 '15 at 20:30
  • @zack1544 This datasheet says 2.0-6.0V for its 74HC14 diodes.com/_files/datasheets/74HC14.pdf - perhaps you need to shop around... – Ecnerwal Dec 19 '15 at 20:35
  • @zack1544 - You should really check the part data sheets to determine what range of voltage that a particular part will operate at. That said a common 74HC14 will operate OK at 5V. Since the 74HC14 has different type input voltage thresholds where it switches the output to high or low that what you would expect for an 74HCT04 the frequency of oscillation may be different. Yes a 74HCT04 will work at 5V. – Michael Karas Dec 19 '15 at 20:36
  • Ok thanks for the information. – zack1544 Dec 19 '15 at 20:37

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