Building a Guitar Tuner with a 555 Timer IC to Generate 440Hz Tone

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#31 21667758 26 Apr 2013 07:34

KRA5H KRA5H

Anonymous

Post #31
21667758 26 Apr 2013 07:34

I always expected some adjustment would be required which brings me back to my original question.

Right now I've got the circuit tuned to the open B string on my guitar (verified the B string tuning and 555 circuit tuning with a tuning fork app on my ipod) . I'll see if I can't check the resistance and instead of using the 100K pot use a single resistor. I might get lucky and have that particular value in my collection, but it will probably require more than one resistor in series.

Most of the resistors I have are gold band (5%) and I can swap various resistors in my collection and get as close to the sweet spot as possible. So, my question is might it be productive to use theCSS555C and program the internal trim capacitor of the CSS555C to compensate for the 5% errors in value of the resistors (let's say for argument purposes that "close to the sweet spot" is a single 68K resistor with a 5% variance)?

Recall that this was my blog post that prompted this discussion: http://www.science20.com/square_root_not/blog...licon_solutions_programmable_555_timer-108814

Here's the corrected circuit from my previous post:
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#32 21667759 26 Apr 2013 10:54

Steve Lawson Steve Lawson

Anonymous

Post #32
21667759 26 Apr 2013 10:54

Cool! I did not know about this part -- all of the things you wish the 555 could do, bundled into one little chip!!

So, here's my 2¢:

* consider if you want to be tweaking each one before it goes out the door
* BUT, if using the plain ol' 555, then, really, even 1% resistors and 1% capacitor will not be enough to get a consistent enough frequency -- would have to use a trim pot and tweak each one.
#33 21667760 27 Apr 2013 06:47

KRA5H KRA5H

Anonymous

Post #33
21667760 27 Apr 2013 06:47

"Cool! I did not know about this part — all of the things you wish the 555 could do, bundled into one little chip!!"

I know! The css55c is the neatest thing since sliced cheese...ok, I might be exaggeraitng a bit, but it is a pretty cool little chip.

"consider if you want to be tweaking each one before it goes out the door"

This circuit was never meant to go into manufacturing production--it's just a simple circuit for hobbyists.
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Topic summary

The discussion centers on building a 440Hz tone generator for a guitar tuner using a 555 timer IC, specifically the CSS555C, aiming for minimal component count and simplicity suitable for hobbyists. Concerns were raised about the temperature stability and frequency accuracy of the 555 timer circuit, given component tolerances in resistors and capacitors, and the purity of the output waveform, which is typically a square wave rather than a sine wave. Alternatives suggested include using crystal oscillators with frequency division (e.g., 32.768 kHz crystal with CD4060 and CD4046 PLL), microcontrollers (such as PIC16F series or PICAXE 08M) with internal timers and DACs or PWM outputs to generate more stable and sine-like tones. The CSS555C was noted as a specialized 555 variant with internal trim capacitor for frequency adjustment, potentially compensating for component tolerances. Filtering the square wave output with a low-pass filter was proposed to approximate a sine wave, balancing tone purity and circuit complexity. Practical tuning methods include using potentiometers or fixed resistors to set frequency, with suggestions to add series resistors to pots for stability. The project is intended as an educational, simple build rather than a production-level device. Additional technical details on PIC timer operation and oscillator configurations were discussed, emphasizing the trade-offs between simplicity, stability, and waveform quality in tone generation circuits.
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Building a Guitar Tuner with a 555 Timer IC to Generate 440Hz Tone

FAQ

TL;DR: At 440 Hz, human ears can detect ~2 Hz pitch differences; “The 555 is fairly stable,” but tolerances matter. [Elektroda, Steve Lawson, post #21667755]

Why it matters: This FAQ helps hobbyists build a minimal‑parts A4=440 Hz reference tone and avoid common pitfalls.

Quick Facts

- Goal: A4 reference tone at 440 Hz; ear detects ~2 Hz difference near this pitch. [Elektroda, Steve Lawson, post #21667755]
- Square wave’s next harmonic is 3× the fundamental; low‑pass filtering cleans it. [Elektroda, Steve Lawson, post #21667752]
- PIC16F1455/1459 include a 5‑bit DAC for crude sine output plus easy RC filtering. [Elektroda, Steve Lawson, post #21667747]
- PIC Timer1 can use a 32.768 kHz crystal oscillator independent of Fosc. [Elektroda, Steve Lawson, post #21667740]
- CSS555C offers a programmable trim capacitor to compensate R/C tolerance. [Elektroda, KRA5H, post #21667758]

Is a 555 timer accurate enough for a 440 Hz guitar tuner?

Yes for home use, but component tolerances set the limit. Use tighter R/C parts or a trim method. “The 555 is fairly stable,” yet you still hear ~2 Hz errors near 440 Hz. Consider a regulator to reduce supply drift. [Elektroda, Steve Lawson, post #21667755]

How do I get a cleaner (more sine‑like) tone from a 555 square wave?

Add a low‑pass filter. Because a square wave’s next harmonic is 3× the fundamental, even a simple RC can help. Set the cutoff below 440 Hz for better purity, then add gain if level drops. [Elektroda, Steve Lawson, post #21667752]

Can I just low‑pass filter the 555 output?

Yes. It’s a trade‑off: lower cutoff yields purer tone but less amplitude. You may follow with an amplifier. This approach works because the dominant unwanted harmonic sits at 1320 Hz. [Elektroda, Steve Lawson, post #21667751]

What resistor strategy makes tuning easier than a big pot?

Use a fixed resistor in series with a smaller pot. Example: a 42 kΩ series resistor with a 20 kΩ pot narrows the range, eases dialing, and reduces drift risk. [Elektroda, Steve Lawson, post #21667757]

My piezo beeper detunes the 555. How can I buffer it?

Insert a voltage‑follower (emitter follower) between the timing network and the piezo. The piezo then becomes the emitter load, isolating the oscillator from variable piezo loading. [Elektroda, Steve Lawson, post #21667757]

Can the CSS555C fix 5% resistor tolerances without trimming parts?

Yes. Program the CSS555C’s internal trim capacitor to offset resistor or capacitor spread and lock the frequency. This lets you keep simple fixed values and still hit pitch. [Elektroda, KRA5H, post #21667758]

Do I need crystal stability if I only use it indoors?

Not necessarily. If temperature is stable (typical room use), a 555 solution suffices. Large outdoor temperature swings would argue for a crystal or MCU. [Elektroda, KRA5H, post #21667753]

What microcontroller options give a simple 440 Hz tone?

A PICAXE 08M can generate 440 Hz with a few lines of code. It simplifies timing and can drive a buzzer or an RC‑filtered PWM for near‑sine output. [Elektroda, KRA5H, post #21667753]

What is PWM and why use it for audio?

Pulse‑width modulation varies duty cycle at a high carrier rate. Filtered PWM reconstructs a near‑sine at 440 Hz. Pick a carrier well above audio for cleaner output. [Elektroda, Steve Lawson, post #21667740]

What is a DAC and is 5‑bit resolution usable here?

A DAC converts numbers to voltage. Even 5‑bit DACs can make a rough sine that filters well when only 440 Hz is needed; add an RC filter. [Elektroda, Steve Lawson, post #21667747]

What is PLL and when would I use it?

A phase‑locked loop locks a variable oscillator to a reference. You can lock a sine oscillator to a crystal‑derived 440 Hz for high stability and low drift. [Elektroda, Steve Lawson, post #21667729]

How does PIC Timer1’s 32.768 kHz crystal help tone accuracy?

Timer1 can run from a 32.768 kHz watch crystal independent of Fosc. Use timers or divisors to synthesize precise periods for 440 Hz timing tasks. [Elektroda, Steve Lawson, post #21667740]

What prescaler options exist on classic PIC timers?

Typical prescaler steps run 1:2 to 1:256. These ratios help derive convenient interrupt rates to time square waves, PWM updates, or DAC steps. [Elektroda, Mark Harrington, post #21667732]

Is a crystal oscillator always better than a 555 for a tuner?

A crystal offers superior temperature and long‑term stability. For minimal parts, a 555 works; for precision or wide temp ranges, use crystal‑locked or MCU‑based synthesis. [Elektroda, Steve Lawson, post #21667729]

How do I set a 555 to exactly 440 Hz without lab gear?

Use a smartphone tuner app as the reference. Quote: “set the tuner with a fork/app, then trim parts.” Verify pitch while adjusting the pot, then measure and replace with fixed values. [Elektroda, KRA5H, post #21667758]

Quick How‑To: Trim a 555 to 440 Hz with minimal parts

Start with R in series plus a small pot across the timing path.
Power the circuit and monitor pitch with a tuner app; adjust to 440 Hz.
Power down, measure the pot’s setting, and replace with nearest fixed value(s). [Elektroda, Steve Lawson, post #21667757]