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Blog 555 Sawtooth Oscillator
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  • Author Author: jc2048
  • Date Created: 6 Jun 2022 10:10 PM Date Created
  • Views 8784 views
  • Likes 10 likes
  • Comments 6 comments
  • sawtooth waveform
  • constant current
  • timer
  • oscillator
  • analogue design
  • 555
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555 Sawtooth Oscillator

jc2048
jc2048
6 Jun 2022

The ST datasheet for the 555 has this circuit for generating a linear ramp. The linearity comes from the use of a transistor to give a constant current source for the capacitor charging.
image
As shown, it's a monostable (it's designed to generate the ramp when it's triggered), but a trivial change will adapt it to an oscillator. Here's the same circuit, in a simulator, with the trigger input connected round to the threshold pin. I've also put some resistance between the discharge output and the capacitor to slow it slightly.
image
image
The result, then, is a sawtooth waveform on the capacitor. (Strictly, I should say almost a sawtooth, because it's only the upward ramp that's linear, but it's good enough for what I want from it.) If you wanted to use the sawtooth waveform to drive another circuit, you'd probably need to add a buffer of some sort to avoid affecting what's happening when the capacitor charges.

To see if that works in practice, I built one on a breadboard.


image

Here are the waveforms (yellow capacitor, blue output)


image

Other than the output of the real chip managing a noticeably higher 'high' level than the simulation model, it's all very similar.

If you want to experiment with it, there are three ways to change the slope of the ramp and thereby change the period of the oscillation. One is to change the value of the timing capacitor, the 47n in my circuit. Increasing the capacitance slows the ramp. A second way is to change the value of the emitter resistor. Increasing the value will lower the charging current and slow the ramp. The final way is to change the voltage at the base of the transistor, which alters the voltage across the emitter resistor. Increasing it (by adjusting R1 and R2) will lower the voltage across the emitter and thus the current and again slow the ramp. Be careful if you change the base voltage, though, as if you bring it much below the upper threshold of the 555 (2/3 of Vcc) the current source won't regulate all the way up to the threshold and the oscillation will probably stop.

Following blog: /technologies/555-timers/b/blog/posts/555-voltage-controlled-oscillator 

If you found this interesting and would like to see more blogs I've written, a list can be found here: /members-area/b/blog/posts/jc2048-blog-index-new-version

References
[1] https://www.st.com/en/clocks-and-timers/ne555.html
[2] https://www.ti.com/lit/ds/symlink/ne555.pdf

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  • scottiebabe
    scottiebabe over 3 years ago in reply to jc2048

    It wasn't intended to detract from your post, I was just curious how non-linear the RC response is over the 1/3, 2/3 VCC thresholds of the 555. I suppose that quick plot is slightly better in that matlab got to pick the best fit strait line, where as in a real circuit the error would be onside like the following

    image

    image

    Interesting observations, thanks for sharing this circuit!

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  • shabaz
    shabaz over 3 years ago in reply to jc2048

    Hi Jon,

    I only mentioned it because I thought the PWM add-on is handy, but actually you explained the sawtooth circuit behaviour better than me : )

    When I used the circuit, it was buried as part of a project so wasn't as reusable-building-block friendly as your blog post.

    Looking forward to seeing how it gets used.

    My blog post was regarding dimmable xmas lights.

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  • jc2048
    jc2048 over 3 years ago in reply to shabaz

    Sorry, I didn't realise you'd done something with a similar circuit (or I've simply forgotten it - my memory isn't too good anymore). You should add a link to it here. Fortunately, I was going to go on and use it for something other than PWM, so hopefully we'll diverge when I do the next blog.

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  • jc2048
    jc2048 over 3 years ago in reply to scottiebabe

    That's interesting. I didn't realise it was that close.

    Wouldn't the error be slightly more than you show? One time constant is 0.63 whereas the threshold is at 0.66.

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  • scottiebabe
    scottiebabe over 3 years ago

    Nice little circuit:)! It is also interesting that the RC exponential response from 1/3 VCC to 2/3 VCC is relatively linear

    image

    image

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