Project 6b: The Two-Tone Siren (Nee-Naw with Two 555s)

Text written by Claude Fable 5.

Project 6b: The Two-Tone Siren (Nee-Naw with Two 555s)

The one idea: in Project 6 you built a tireless tapper that switches thousands of times a second. Now build its lazy twin (the exact same circuit, just with a capacitor a thousand times bigger, so it taps about once a second) and let it poke the fast chip. Every poke makes the pitch jump: nee-naw, nee-naw. A real two-tone siren.

This is a direct follow-up to Project 6: start with that circuit still working on the breadboard. The only shopping is a second NE555 and a 100 µF electrolytic, both already in the sensing-arc parts order from the Read Me First, so you may well have them.

A companion explainer for the parent: the computation explainer picks up the question this project tends to raise among adults (our chip has 25 transistors, so how many would a real computer take?) and follows it from Turing’s universal machine to the first CPUs. Nothing in it is needed for build day.

Same circuit, stretched in time

There is genuinely nothing new to learn about the second chip. Recall the pitch formula from Project 6:

f \;\approx\; \frac{1.44}{(R_1 + 2R_2)\,C}.

Keep the same resistors (R1 = 1 kΩ, R2 = 10 kΩ) and swap the capacitor: C = 100 nF gives ≈700 Hz: a tone. C = 100 µF, a thousand times bigger, gives ≈0.7 Hz: a beat, one flip every second and a half. Too slow to hear as a pitch, but exactly right for switching between two pitches. That’s a lovely teaching beat in itself: the same machine, run a thousand times slower, does a completely different job.

The secret doorknob: pin 5

How does the slow chip change the fast chip’s pitch? Through the one pin Project 6 left unconnected: pin 5, the control voltage.

Inside the 555, a chain of three equal resistors sets the two “finish lines” the capacitor races between: ⅓ and ⅔ of the supply voltage. Pin 5 is a wire soldered straight into the middle of that chain. Leave it alone and it sits at ⅔ of the supply. Pull it up and the upper finish line moves up, so the capacitor takes longer to get there: lower pitch. Pull it down: higher pitch. So a slow square wave wiggling pin 5 toggles the tone between two notes.

Build it

Leave the Project 6 circuit untouched: that’s your fast 555, buzzer and all. (If the pitch knob is in, just park it at a pitch you like.)
Plant the second 555 a few columns away, and wire it identically: pins 8 and 4 → +, pin 1 → GND, R1 = 1 kΩ from + to pin 7, R2 = 10 kΩ from pin 7 to pins 6 + 2 tied together, except that the capacitor from pins 6/2 to GND is now the 100 µF electrolytic. It’s the polarized kind: + leg to pins 6/2, stripe (−) to GND. Nothing on its pin 3 yet; its pin 5 stays free.
The one new connection:
```
slow 555 pin 3 ──[10 kΩ]──► fast 555 pin 5
```
The 10 kΩ coupling resistor is not optional: pin 5’s internal divider is stiff (≈3.3 kΩ), and a bare wire would slam it to the rails and choke the tone. Through 10 kΩ you get a polite nudge instead.
Optional but great: an LED with a ≈470 Ω resistor from the slow chip’s pin 3 down to GND. Now the invisible conductor becomes visible: the LED blinks exactly in time with the pitch jumps.

Power up and it goes nee-naw all by itself.

Tuning: the coupling resistor sets how far apart the two notes are. 4.7 kΩ = dramatic siren; 47 kΩ = subtle warble. Swap the slow chip’s 100 µF for the 10 µF and the beat jumps from ~0.7 Hz to ~7 Hz: a frantic alarm rather than a stately fire engine.

What to say to the child

“Remember the tireless tapper, too fast to see? This new chip is its lazy twin brother. It’s built exactly the same, but its bucket is a thousand times bigger, so it fills up slowly and it only taps about once a second. See, this light blinks every time it taps. And instead of tapping the buzzer, it taps the fast chip on the shoulder. Every tap makes the fast chip sing a different note. Nee-naw, nee-naw: you built the siren the fire trucks use!”

For you: the physics

Pin 5 sets the comparator thresholds to $V_{\text{CV}}$ and $V_{\text{CV}}/2$ (instead of ⅔ and ⅓ of $V_{cc}$ ). A neat asymmetry falls out: the discharge half-cycle runs from $V_{\text{CV}}$ down to $V_{\text{CV}}/2$ through R2, a fixed 2:1 ratio, so $t_{\text{low}} = R_2 C \ln 2$ regardless of the control voltage. Only the charge half changes:

t_{\text{high}} \;=\; (R_1 + R_2)\,C \, \ln\!\frac{V_{cc} - V_{\text{CV}}/2}{V_{cc} - V_{\text{CV}}}.

The coupling resistor forms a divider against pin 5’s internal Thevenin resistance (5 kΩ ∥ 10 kΩ ≈ 3.3 kΩ, sitting at ⅔ $V_{cc}$ ). At 4.5 V supply with 10 kΩ coupling, the slow chip’s output (≈3.9 V high, ≈0.1 V low) swings pin 5 between roughly 3.2 V and 2.3 V. Plug those into the formula with R1 = 1 kΩ, R2 = 10 kΩ, C = 100 nF and you get about 630 Hz and 870 Hz, close to a musical fourth apart, which is why it sounds like a proper European siren rather than a gadget.

And name the concept for yourself, because it echoes later: wiggling one oscillator’s parameter with another, slower signal is modulation: the same move that puts a song onto a light beam in Project 14, and onto every radio carrier ever broadcast.

If it doesn’t work

Steady tone, no nee-naw: the slow 555 isn’t oscillating. Check the electrolytic’s polarity (+ leg to pins 6/2) and that its pin 4 is tied to +. The optional LED tells you instantly whether the slow chip is beating.
It switches, but the two notes sound almost the same: coupling resistor too big; drop toward 4.7 kΩ.
The tone stutters or glitches at each switch: two bipolar 555s spike the supply rails when they flip. Park the 10 µF capacitor straight across + and GND near the chips.

Try next

Wail instead of nee-naw: instead of the slow chip’s pin 3, take its capacitor voltage (pins 6/2) through a resistor to pin 5; that node ramps up and down like a triangle wave, so the pitch glides: an American-style wail. One caveat: this loads the timing node, and pin 5 sits at exactly the voltage the capacitor is trying to reach, so use ≥47 kΩ or the slow oscillator crawls near the threshold and can stall. The pin-3 version is the robust one for build day.
Hand-cranked siren: put the pitch knob back in as the fast chip’s R2; now the child steers the pair of notes up and down while the siren keeps switching between them.
Light-controlled siren: the LDR from Project 12 in place of the coupling resistor makes the siren’s interval depend on the room’s light.

← Back to Project 6 or the Read Me First.