# 555 and loudspeaker - Beginner question

First of a disclaimer - I'm a hobbyist and quite the beginner at that, so bear with me.

What I'm trying to build is a metronome with an astable 555 oscillator that is driving an 8 Ohm loudspeaker. I have gathered some experience with 555's now and have already completed this project with a piezo.

In that case I used two 555 oscillators, one that produces the square wave and one that uses a transistor to turn the first one on and off at the desired beats per minute.

Now I've looked into how to do it with a loudspeaker and have found these guides:

https://www.555-timer-circuits.com/metronome.html

https://www.instructables.com/How-to-Make-a-Metronome/

https://www.instructables.com/How-to-Make-an-Electronic-Metronome/

Safe to say I'm quite confused, baffled even. They all just use one astable 555 that represents the bpm. Don't loudspeakers need an AC signal? My piezo does.

How do they turn on the loudspeaker with just the "on period" of the duty cycle?

I suspect it has something to do with the capacitor in series with the loudspeaker but it would be nice if someone took the time and explained it to me.

PS: My confusion in a simple picture:

EDIT: As requested, I added a schematic of the mentioned piezo circuit below.

• Show a schematic of your actual circuit, there's a drawing tool for that available when you edit your question. If the frequency of the beep is too high (for your ears and/or speaker), you would only hear it switching on and off and that sounds as clicking. Commented May 5, 2021 at 13:39

every time the output of the 555 changes state, it will cause a pulse of current into the speaker as the cap charges or discharges through the speaker. That pulse of current will give a loud audible "click" in the speaker.

On a positive edge, the cap will charge via the speaker, while it is charging the speaker cone will push in one direction for a fraction of a second, then return to the rest ("zero") position. On a negative edge, the cap will now discharge, causing the cone to push in the opposite direction and then return to "zero".

For an 8R speaker and a 22uF cap, the charge/discharge time is about 5CR or about 880us, which is a bit less than one ms. A sine wave with a period of 880us would have a frequency of about 1.1kHz - nicely in the audio band. It's going to be quite a loud click for a 9V pulse.

• Aha! Thank you very much. So the frequency is both determined by the impedance of the speaker and the capacitance of the cap and the bpm just by the 555? Can you tell me how you calculated that frequency? Commented May 5, 2021 at 13:48
• Sure. If you take a square wave of 1ms period it has a frequency of 1/T which is 1kHz. That is for a continuous wave. If you take a single cycle of a square wave (i.e a single pulse of 500us) it gives a whole bunch of frequencies around 1kHz which appear for a moment and then decay quickly. This is not a square wave but an exponential, so it has a different set of harmonics, but they will be centred near to 1kHz. This is not at all a rigorous analysis, but enough to know that the frequencies of the resulting sound will be at very audible frequencies. Commented May 5, 2021 at 13:54
• Also: Depending on the impedances in the circuit, there may be some ringing at each transition (as the speaker overshoots and settles), which might be perceived as a brief "ping". Commented May 5, 2021 at 15:28
• You do not have an 8 ohm speaker and you do not have a 22uF output capacitor. Instead you have the second oscillator causing the piezo to be beeping. BUT your schematic has no power supply voltage and no transistor part number. I agree that the reset pin should be used to turn the beeps on and off. Commented May 6, 2021 at 20:40

Typically, the sound of a metronome is not a short tone burst, like a beep or chirp. It is a click or tick or tock - sonically it is an impulse, not a discernible note with a frequency. That is why the circuits you have found send a simple, short DC pulse to the speaker.

AND - The "really happening" drawing above is incorrect. There will be a sound impulse at both the rising edges and the falling edges of the 555 output waveform. The two impulses will sound slightly different, because the main part of the sound is different. One is a compression wave (the speaker cone moves toward the ear) and the other is a rarefaction wave (the speaker cone moves away from the ear). There are harmonics, speaker cone oscillations, and other contributors to the overall sound, but the main part is what the cone does when the signal edge comes along.

If you really want to send a tone burst to the speaker, post your schematic for us to review. In general, it is better to enable/disable the second 555 by driving its Reset input rather than switch its power on and off.

• Updated the response. Commented May 5, 2021 at 15:04
• All metronomes I have heard make clicks or a knocks. I have never heard one that makes beeps. My smoke alarms make beeps. Commented May 5, 2021 at 16:17
• I know, but the initial question was beeps. Commented May 5, 2021 at 17:05