Difficulty Rating

Purpose

This is a simple mod to the previous circuit. Instead of blinking an LED, it makes a clicking sound in a speaker.

Bill of Materials

We need the built circuit from the previous article, Hack01-02, plus these few parts:

Label	Description	Image
R4	Resistor, 10 kΩ (= 10 kilohm or 10,000 ohms). Color stripes: brown-black-orange.
SPKR	Speaker, miniature, 8-ohm to 32-ohm

The speaker should have wire leads that can fit in the breadboard holes. But if it doesn't, you can use clipleads.

Assembly

Click on the image to see a larger version.

Starting from the last project, remove the LED, and connect the speaker in its place.

Your speaker may be smaller or larger than what I show here. Just about any size should be OK.

The resistor R4 was 100kΩ (100,000 ohms) in the last project. But for this project, a value of 10kΩ will work better.

Success is when you can hear clicking from the speaker, and its rate varies as you adjust the potentiometer.

Schematic

Click on the image to see a larger version.

The speaker is the load in this circuit, instead of the LED in the last project.

Images

Click to see larger.

Here are voltage waveforms of nodes n2 thru n6 from the circuit simulator software. (Nodes n0 and n1 waveforms are simply flat lines — not too interesting). In this set, the potentiometer is set to 100% (fully clockwise). This circuit should produce a tempo range from around 46 to 160 beats per minute.

How Does It Work

The circuit is mostly the same as the circuit in the last project. It is still a narrow pulse generator. The biggest change is using a speaker instead of an LED, so we can get a sound pulse instead of light pulse.

We also use a different value of R4, 10 kilohms, so the pulse is not too wide. If we kept R4 at 100 kilohms, the pulse would be wider and not give us a clear "click" sound — we would hear both the rising and falling parts of the pulse. We humans can hear very short time differences between clicks! (shorter than what we can see).

History

The first metronome was mechanical, and patented in 1815 in England.

The earliest speaker (proper name is loudspeaker) was invented in 1861. The moving coil dynamic loudspeaker — like the kind we use today — arrived in 1898. More specifically, the miniature style used in this project became common in the late 1930's and 1950's when small portable radios were widely sold.

Simple Mods

Widening the Adjustment Range — I discussed how to widen the range in the previous article, Hack-01-02. You can try decreasing R1 to 2.2 megohms, or maybe 2.0 megohms. But don't expect much improvement.

Shifting the Overall Range — You can easily change how fast or slow the circuit oscillates by changing C1. Using about half the value — 100nF instead of 220nF — will double the speeds and give a range of about 92 to 320 beats per minute. But maybe something in between would be more musically useful!

Changing the Tone Quality — If the "click" sound is too weak, what can we do to get a deeper sound? Generally, a bigger speaker allows deeper tone, while a smaller speaker allows a higher, sharper tone. You may not a variety of speakers on hand to try, though. Some other ways to alter the tone are:

Add a Capacitor Across the Speaker — Connect a capacitor in parallel with the speaker — that is, from nodes n1 to n2. This will tend to make the "click" more mellow or deeper. By adding this capacitor, you are shifting the resonant frequency ("favorite tone") of the speaker lower. Try a value like 1μF (1 microfarad) as a start. But as you use larger values, you may find that the "click" gets too soft to hear.
Change R4 — The 10 kilohm value is a good starting place, but you can try higher and lower values. This will also shift the tone, somewhat. It will have more effect if you also do the above change.
Change R3 — The 68 ohm value is a good starting value, but you can try higher and lower values. Lower values will tend to make the "click" stronger, and speed up the oscillation. But as you use lower values, the effect may diminish, and the circuit may not work at all. I would not go below 10 ohms, since T2 needs some resistance to protect it from excessive current.
Increase Voltage — You can try operating the circuit from a higher-voltage power supply — 3 or even 4 AA-cells — which is 4.5 or 6 volts. This will definitely have a noticeable effect!

Known Weaknesses and Their Fixes

Same as what I discussed and addressed before: see the previous article, Hack-01-02. The circuit's frequency range is limited, and not all of the potentiometer's adjustment range is used.

Next Steps

I titled this article "Project 02: Sound, a Metronome, and More" so what's "more"? We'll hack this circuit some more, but that deserves a separate article... up next!

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Transistorize Me

Sep 3, 2015

Project 02: Sound, a Metronome, and More

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