In addition to a couple of 12V car phone chargers, you’ll also need a speaker for this project. You can use any salvaged wide-range speaker but in keeping with a mobile-phone theme, we used a boxed speaker from a hands-free car phone system. You can often pick these up at the same time as phone chargers.

Cost? Well at garage sales and the like, expect to pay only a few dollars for the lot. But what can you do with these bits and pieces? One answer is to make a very loud pulsing screamer.

The components

In this system, the two phone chargers perform different functions. The first is modified to produce a pulsing output voltage, which in turn powers the second charger. The second charger is modified to produce the audio output tone which is fed to the speaker. And the speaker? Well, it makes the loud noises!

The modifications to the charger boards are very easy and it takes only a few minutes to get the screamer up and running.

In addition to the chargers and the speaker, you’ll also need a selection of capacitors. You’ll only end up using two of them but having a range available makes it easy to get the sounds you want.

Fig.1: the pulsing screamer uses two slightly modified 12V phone chargers and a speaker. The first charger pulses the second charger which in turn produces the audio frequency that's reproduced by the speaker.

Building it

The first step is to modify one of the chargers to produce the pulsing output. Begin by removing the PC board from its cigarette lighter plug enclosure, then remove the output filter capacitor. This is the electrolytic capacitor that’s usually located near to the output leads (a typical value is 680mF). Just desolder it and place it in your parts drawer – you never know when it might come in handy for some other project.

The next step is to replace the timing capacitor. It’s dead easy to find – it’s the smallest disc-shaped capacitor on the PC board and typically has a value of 100nF. Carefully desolder this capacitor and temporarily replace it with a 100mF electrolytic capacitor (this can be tacked to the track side of the board).

Note that electrolytic capacitors are polarised, so be sure to connect the negative lead of this capacitor to the ground track of the PC board. You might have to do some track tracing to make sure you get this right.

Charger 2

The next step is to modify the other charger so that it will produce the sound (ie, an audio tone). As before, start by removing the output filter capacitor and placing it in your parts drawer. That done, remove the timing capacitor and temporarily replace it with a capacitor of around 1mF.

Next, connect the outputs of the "Pulsing" charger to the power supply inputs of the "Tone" charger, making sure that the polarity of the connections is correct – see Fig.2. You can then connect the speaker to the "Tone" charger’s output terminals.

Fig.2: here is a typical circuit for a 12V phone charger. The primary modification is to alter the value of the timing capacitor to dramatically lower the frequency at which the charger is operating. Shorting the output inductor and output diode increases the output level.

Testing and Tuning

Now for the smoke test – connect 12V power to the "Pulsing" charger and listen. It’s likely that the sound will not be quite as you want it – it may be too low in pitch and pulsing too slowly, for example (or vice versa).

The pulsing screamer is easily made from two modified car phone chargers and a speaker. In this case, we used a (brand new) speaker from a hands-free kit which we picked up at a garage sale but any wide-range speaker is suitable.

That’s easily fixed. To speed up the pulsing, decrease the value of the timing capacitor in the "Pulsing" charger. Similarly, to increase the pitch (frequency) of the sound, decrease the value of the capacitor in the "Tome" charger.

By making some simple capacitor changes, it’s possible to have anything from a deep, slowly pulsing foghorn to an ultra-piercing, frantically pulsing screamer - and everything in between! When you’re happy with the sound, properly solder the selected capacitors in place.

Now run the system for a while (you might want to wrap the speaker in a pillow!) and check the temperature of the two ICs. They are likely to be warm but they shouldn’t be too hot to touch. If they are, install a 5W 5W resistor in series with the 12V supply to the system. This will drop the audio output but the ICs will run cooler.

Incidentally, when testing, always power the system using the voltage that will be used in the final application. This is because the pitch and pulsing frequency will vary with supply voltage. Note that depending on the value of the capacitors used, the circuit will work down to about 4V.

Making it louder

If you want to increase the loudness of the output, solder a bridging wire across the inductor on each PC board (the inductor is placed near to the output and is simply a coil of wire). A second bridging link should also be installed across the output diode on each board (see Fig.2).

The prototype was configured to produce a very loud 200ms burst of 300Hz sound at 1-second intervals – so it was configured more as a "growler" than a "screamer"! This involved using a supply voltage of 12.0V, a 470mF capacitor in the "Pulsing" charger and a 47mF capacitor in the "Tone" charger. In addition, the inductors and output diodes were bridged on both chargers, as described above.

Housing your screamer

Many hands-free speakers use boxes that are held together with screws, allowing the enclosure to be easily opened. If that’s the case, the two modified chargers can be insulated (eg, by being wrapped in electrical tape) and then placed inside the enclosure, one each side of the speaker basket.

Alternatively, the chargers can be housed in a separate case.