Have fun and drive your family mad at the same time! This neat little siren can be added to your favourite toy or used on its own, and you can change the sound too.

BUILD TIME: 1 HOUR
DIFFICULTY RATING: BEGINNER

The standard Kids Basics rules apply: no soldering or special tools required, and we’ll take you through step by step.

Assumed knowledge:

Using The 555 Timer IC:

Issue #19, Page 80

Did you build the flashing emergency vehicle lights in issue 19 and wish that you had a siren to go with your creation? Now you can, and to make it, we’re revisiting the 555 timer. This time, however, we’re using it a bit differently, and we’re using two of them. One makes an electronic wave that gets sent to the next, to change the sound it makes.

You can add this to the flashing lights cardboard artwork from issue 19, or you may even want to build this and the lights into a toy police car, fire truck, or ambulance. But ask your parents before trying to open up the toy someone probably bought for you.

ELECTRONIC PARTS REQUIRED:	Jaycar	Altronics	Core Electronics
1 × Small Prototyping Breadboard	PB8820	P1002	CE05102
1 × Breadboard Wire Links	PB8850	P1014A	CE05631
2 × 555 timer IC	ZL3555	Z2755	COM-09273
1 × 10Ω Resistor*	RR0524	R7510	COM-10969
1 × 1kΩ Resistor*	RR0572	R7558	COM-10969
1 × 2.2kΩ Resistor*	RR0580	R7566	COM-10969
1 × 4.7kΩ Resistor*	RR0588	R7574	COM-10969
1 × 10kΩ Resistor*	RR0596	R7582	COM-10969
1 × 15kΩ Resistor*	RR0600	R7586	-
1 × 33kΩ Resistor*	RR0608	R7594	-
1 × 82kΩ Resistor*	RR0618	R7604	-
1 × 10nF MKT Capacitor*	RM7065	R3013B	FIT0118**
1 × 100nF MKT Capacitor*	RM7125	R3025B	FIT0118**
1 × 47µF Electrolytic Capacitor*	RE6100	R5102	CE05130
1 × 470µF Electrolytic Capacitor*	RE6194	R5163	CE05130
1 × BC548 NPN Transistor	ZT2154	Z1042	-
1 × Small Speaker	AS3000	C0610	ADA1890
1 × 9V Battery Snap	PH9230	P0455	CE05205
1 × 9V Battery	SB2423	S4970B	CE05337

*Quantity shown, may only be available in packs. **Ceramic capacitors. MKT not available from this supplier.

The Build:

Step 1:

Put the breadboard on the table in front of you with the blue (-) negative power rail closest to you.

Step 2:

Insert the two wire links on the left that join the set of power rails at the top with the ones at the bottom. Be very careful that (+) goes to (+) and (-) goes to (-).

Step 3:

Gently insert the two 555 timer ICs, making sure the notches face left. Some versions have a dot instead that should go at the bottom left.

Step 4:

Insert the two wire links shown here one each from the upper (+) rail (red) to pin 8 of each IC.

Step 5:

Insert the two wire links shown here one each from the lower (+) rail (red) to pin 4 of each IC.

Step 6:

Insert the two wire links shown here one each from the lower (-) rail (blue) to pin 1 of each IC.

Step 7:

Insert the three wire links shown here that join pin 2 to pin 6 of the left-hand IC.

Step 8:

Insert the three wire links shown here that join pin 2 to pin 6 of the right-hand IC.

Step 9:

Insert the wire link shown here from the lower (-) rail (blue) to the spot just beside the right-hand IC.

Step 10:

Insert the three wire links shown from pin 5 of the right hand IC to the spot in the middle of the two ICs on the half of the board closest to you.

Step 11:

Insert the 15kΩ (brown-green-orange or brown-green-black-red) resistor as shown here between the upper positive (+) rail (red) and pin 7 of the left-hand IC. Resistors do not have a direction, but if you place them so the colour codes read top to bottom, then life is easier when you have to troubleshoot or change anything.

Note: We have trimmed the leads of some of our components and bent the leads so that they fit neatly onto the board. This helps us in photography, but it will help you in checking, avoiding mistakes, stopping components touching each other, and making changes more easily.

Step 12:

Insert the 33kΩ (orange-orange-orange or orange-orange-black-red) resistor between pins 7 and 6 of the left-hand IC. Note that you will have to fold one lead over beside the resistor, as it stands on one end.

Step 13:

Insert the 2.2kΩ (red-red-red or red-red-black-brown) resistor between pin 3 of the left-hand IC and the wire link from step 10.

Step 14:

Insert the 10kΩ (brown-black-orange or brown-black-black-red) resistor between the upper (+) rail (red) and the resistor you placed in step 13.

Step 15:

Insert the 4.7kΩ (yellow-purple-red or yellow-purple-black-brown) resistor between the upper (+) rail (red) and pin 7 of the right-hand IC.

Step 16:

Insert the 82kΩ (grey-red-orange or grey-red-black-red) resistor between pins 7 and 6 of the right-hand IC. Note that you will have to fold one lead over beside the resistor, as it stands on one end.

Step 17:

Insert the 1kΩ (brown-black-red or brown-black-black-brown) resistor between pin 3 of the right-hand IC and the spot on the right-hand side of the IC.

Step 18:

Insert the BC548 transistor with the flat side facing you and the middle pin in line with the resistor you placed on step 17.

Step 19:

Place the wire link shown here between the right-hand leg of the transistor and the lower (-) rail (blue).

Step 20:

Place the 10Ω (brown-black-silver or brown-black-black-gold) resistor between the left-hand leg of the transistor and a spot on the other half of the board, as shown.

Step 21:

Take the small speaker and turn it over so that the terminals are facing up.

Step 22:

Twist the ends of two long breadboard wire jumpers through the holes in the speaker terminals as shown.

Step 23:

Insert the other end of one jumper wire into the upper (+) rail (red).

Step 24:

Insert the free end of the other speaker wire into the breadboard at the end of the 10Ω resistor from step 20 as shown.

Step 25:

Insert the 100nF (marked 0.1 or 104 with other printed characters) MKT capacitor between the upper (-) rail (blue) and pin 5 of the left hand IC.

Step 26:

Insert the 10nF (marked 103 followed by a letter and other numbers) MKT capacitor between pin 2 of the right-hand IC and the wire link placed in step 9 as shown.

Step 27:

Insert the 47µF electrolytic capacitor between the lower (-) rail (blue) and pin 2 of the left-hand IC. Note that the negative side of the capacitor is shown with a strip, and must go to the (-) rail (blue).

Step 28:

Insert the 470µF electrolytic capacitor between the lower (-) rail (blue) and the spot where two resistors and a wire link met earlier, as shown in the picture.

Step 29:

Insert the battery snap wires, red into the upper (+) rail (red), and black into the upper (-) rail (blue).

Step 30:

Warn anyone around you, and connect the battery! If you don’t hear noise straight away, disconnect the battery immediately, and check all of your connections.

What’s going on?

There are two parts to this circuit. The left-hand half is a 555 set up to turn on and off, at roughly one cycle per second. It isn’t perfect, but turns on for a bit over half a second, and off for a bit under half a second. The output pin 3 is connected to a capacitor (470µF) and resistor (1kΩ) via another resistor (2.2k). As soon as power is applied, the capacitor starts to charge, but not too quickly as the 1kΩ resistor slows things down. When the output at pin 3 is on, or ‘high’, current flows through the 2.2kΩ resistor to the 470µF capacitor as well.

When the output at pin 3 turns off, or goes ‘low’, the electricity stored in the capacitor leaves slowly via the 2.2kΩ resistor, through the 555. This creates a gradually rising and falling level which is sent to pin 5 of the second 555.

The second 555 is set up in a similar way, but with different components so that the rise and fall is much faster, and without a resistor and capacitor network to make it gradually rise and fall. It actually switches on and off, but it happens so fast that it makes a sound instead of clicks.

Pin 5 of a 555 is called the Control Voltage input. While the resistors connected to pins 6 and 7, and the capacitor on pin 2 all control the overall timing, pin 5 can be used to make the timing vary a bit within the other limits set by those components.

So, the gentle rising and falling from the resistor and capacitor network we’ve been discussing changes how fast the second 555 switches. Try it – turn on the device, and pull out a jumper that connects the resistor/capacitor network to pin 5. It’s one of the ones we put in during step 10. What happens to the sound?

WHERE TO FROM HERE?

As you’ve just read, the values of the components around the two 555s do a lot. Why not change them and see what sounds you get? Try values close to the ones already there first, but you could go wild too.

On the first 555, the 15kΩ resistor from (+) to pin 7, the 33kΩ from pin 6 to 7, and the 47µF capacitor to pin 2 all control the timing. Try a 10kΩ, 47kΩ, and 100µF on those places. Then choose some really random values. Try changing one thing at a time when you get really radical.

On the first 555, the 4.7kΩ resistor from (+) to pin 7, the 82kΩ from pin 6 to 7, and the 10nF capacitor to pin 2 control the timing. Try changing them one at a time for larger or smaller values, and listen to the sound.

As in the issue 19 article, you can go to this website to calculate the timings with different components: http://www.ohmslawcalculator.com/555-astable-calculator

It is important to disconnect power before making any changes. Short-circuits can damage some components.

By the way, the BC548 transistor allows the small currents involved in the 555 to control the bigger current needed by the speaker, so leave that section as-is.

By way of sound though, try different small speakers, or better still, cut a hole in the bottom of a plastic or paper cup, or make a paper cone. How does this affect the sound?