Two-Tone Siren
A police-car sound that rises and falls — built from two timer chips.
What you’ll learn
- How to cascade two oscillators so one modulates the other.
- Voltage-controlled frequency on a 555.
- The difference between an audio-rate oscillator and a sweep-rate one.
What you need
| Item✓ | Qty | ~Cost | Where to buy | |
|---|---|---|---|---|
| 9V battery + snap clip | 9V battery + snap clip | 1 | GHS 23.00 | Any supermarket or hardware store. |
| 220 Ω resistor | 220 Ω resistor Look for red-red-brown stripes. | 1 | GHS 0.75 | Electronics shop or online (Amazon, Adafruit). Look for red-red-brown stripes. |
| Red 5 mm LED | Red 5 mm LED Any colour works; red is brightest. | 1 | GHS 1.50 | Electronics shop or online. Any colour works; red is brightest. |
| Half-size breadboard | Half-size breadboard | 1 | GHS 23.00 | Online (Amazon, Adafruit, AliExpress). |
| Jumper wires | Jumper wires | 4 | GHS 30.00 | Online — usually sold in packs of 30. |
| 555 timer IC (NE555 / TLC555) | 555 timer IC (NE555 / TLC555) Bargain even by the bag of 10. | 1 | GHS 7.50 | Electronics shop or online. Bargain even by the bag of 10. |
| 10 kΩ resistor | 10 kΩ resistor Brown-black-orange. | 1 | GHS 0.75 | Electronics shop or online. Brown-black-orange. |
| 100 kΩ resistor | 100 kΩ resistor Brown-black-yellow. | 1 | GHS 0.75 | Electronics shop or online. Brown-black-yellow. |
| 10 µF capacitor (electrolytic) | 10 µF capacitor (electrolytic) | 1 | GHS 1.50 | Electronics shop or online. |
| 100 nF capacitor (ceramic) | 100 nF capacitor (ceramic) | 1 | GHS 0.75 | Electronics shop or online. |
| Extra 555 timer IC | Extra 555 timer IC | 1 | GHS 7.50 | Online. |
| Piezo buzzer (passive) | Piezo buzzer (passive) | 1 | GHS 15.00 | Online — make sure it says "passive" not "active". |
| Assorted resistors (10 k, 47 k, 100 k) | Assorted resistors (10 k, 47 k, 100 k) | 3 | GHS 6.75 | Online resistor pack. |
| Extra capacitors (100 nF, 10 µF) | Extra capacitors (100 nF, 10 µF) | 2 | GHS 4.50 | Online. |
| TotalTotal | ~GHS 123.25 | |||
Build it step by step
Build the slow timer (U1)
A 555 in astable mode at ~2 Hz — same shape as Project 6, with a 100 kΩ + 100 kΩ + 10 µF.
Build the audio timer (U2)
A second 555 in astable mode at ~600 Hz — 10 kΩ + 47 kΩ + 100 nF.
Connect U1 output to U2 control
U1 pin 3 → U2 pin 5 through a 10 kΩ. The slow square wave wobbles the audio frequency up and down.
Drive the buzzer
U2 pin 3 → buzzer + ; buzzer − → − rail. Snap on the 9 V — wee-woo-wee-woo.
How it works
Pin 5 of a 555 is the "control voltage" input. When you push that pin around, the chip’s threshold moves with it, which slides the output frequency. Hook it to another 555’s output and you get a frequency that sweeps up and down — the classic siren sound.
If something’s not working
Buzzer just clicks
- · You’ve got an active buzzer (built-in oscillator) instead of a passive one. Active buzzers ignore frequency input.
Sound is steady, no sweep
- · U1 isn’t oscillating, or pin 5 isn’t connected. Probe pin 3 of U1 — it should swing up and down.
Try this next
Swap U1’s capacitor for a smaller value. The sweep gets faster.
Tie pin 5 to a potentiometer instead — manual pitch control.