Oscillating with the 74HC14

Posted by on Apr 9, 2011 in Blog, Etc | 7 Comments

Today, I built an oscillator with the 74HC14 hex inverter Schmitt trigger.
Here’s the datasheet.
I thought this was interesting, the inventor of the Schmitt trigger, Otto Herbert Schmitt (1913 – 1998), was a scientist who worked both in physics and biology. He figured out the trigger while studying neural impulse propagation in squid nerves.

Pretty

I built this circuit with a capacitor 0.01uF and resistor 14.8k, which were just lying on the bench. I calculate a frequency of about 6.75kHz. Freq (Hz) = 1/ (R*F)

Which is pretty close to what shows on the oscilloscope … I’m looking at the output signal on pin 12.
Time (seconds) = 1/Frequency (Hz)
osc

I then used all the gates available and made three oscillators. By using different values for R (Mohm range) and C (uF range) on each oscillator, and therefore creating three out of phase oscillators, I was able to get a nice color cycling effect on an RGB LED.
osc



7 Comments

  1. lamin ceesay
    November 4, 2011

    very interesting little project. i just have a quick question on how u got your frequency.

    you stated freq = 1/(C*F)
    am guessing c = capacitor and F = resistor and * = multiplication yes/no . if yes then

    1/(0.01uF * 14.8K) = 1/(148^-6) = 6.75KHz

    please help me correct my way if am wrong. thanks

    Reply
    • Sophi
      November 6, 2011

      hello-
      Good catch- you are correct, and I have corrected the 2 typos.
      If you look at the oscilloscope, each square is 100us.
      You can see that a full cycle is just under a square and a half, say 150us.

      Frequency = 1/cycle = 1/(150*10^-6) = 6666.66Hz =6.7kHz

      Reply
  2. ed
    August 27, 2014

    i understood the formula to be in this case: 1.2/(RC) and in that case it would be 8.1 kHz

    Reply
    • fuqthegovt
      August 29, 2015

      It depends on the k factor, which in turn depends on the power voltage to the chip. It could be anywhere from 1.7 down to .8 within the datasheet limits of 2-6v (and the relationship isn’t linear at all). So if the supply varies by even a few mV (from a typical LM7805) your frequency will vary as well by a few parts per thousand (even when using 30-50 ppm RC components). This is why quartz crystal oscillators are used if you need a relatively stable frequency.

      Reply
      • fuqthegovt
        August 29, 2015

        I guess I should also mention that temperature variations have an even bigger effect, close to 1% per C.

        Reply
  3. Lee
    June 24, 2015

    So what does the input require to get it to oscillate?

    Reply
  4. Amit
    March 11, 2017

    You’ve stumbled upon the basis of the old BEAM robotics Nv neurons, central pattern generators like the microcore and bicore and “nervous system”. There should be a lot of old websites around with robots built around this.

    Reply

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