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The starting point for the multi-tone warning module is this toy cellular phone. It cost just $4. A low-cost multi-tone dashboard alarm An audible alarm in a car is a useful indicator. It could accompany an oil pressure warning light, it could remind you to turn off the fullhouse car alarm or provide an engine overtemperature alarm. Here, we show you how to organ­ise a 9-tone alarm module for about $4! By JULIAN EDGAR The basis of the alarm is a toy cellular telephone – yes, you read that right! Toy phones have an integrated circuit, sound transducer (often a small speaker) and a battery holder all combined into an incredibly cheap package. Pressing the phone’s button makes noises and these can be useful for more than just entertaining 2-year olds. However, before throwing down this magazine and charging off to the local discount store, read on. During the extensive research for this feature, I investi­gated three different toy phones – and beware, only one was suit­able. Suitability of the phone for this 22  Silicon Chip application requires the following: (1) continuous sound when a button is held down. Some phones emit the tone only for a short time, irrespective of how long the button’s pressed. (2) a number of different tones. Some phones have very few different tones, even though there’s lotsa buttons! (3) tones which are appropriate in a car warning situa­ tion. But then again, maybe you’d like your dash to yell in monkey-talk “Sorry the line is busy now”. (4) as loud a sound output as possible. These toy phones vary in price from $2 to $4, too – so shop around. The phone which I used was unbranded but don’t let that worry you. Any toy phone which satisfies the above criteria will be adequate. Pull it apart OK. You’ve got the phone home and extracted it from the grasp of your little brother/kid who lives across the road/ intel­ligent dog/your own baby. You’ve put up with the sweet smile of your partner who has decided that your movement back towards childhood has become extreme and you’re waiting breathlessly for the next piece of invaluable advice. It’s pretty simple: pull the phone apart. Mine took a couple of turns of a Phillips head screwdriver; others inspected pulled apart with brute force. Once opened, you should be able to see the sound transducer, the keypad and the integrated circuit (no, it’s not a neat little package with legs but instead a blob on the board). Talking about the keypad, if it seems to have fallen apart don’t worry. On the printed circuit board there should be a pattern of tracks, with the tracks coming close together in a meshed The toy phone pulls apart to reveal a speaker, sound generator integrated circuit (the blob on the board) and a keypad. pattern under each key. When the keys are pressed, a conductive material on their ends squashes down onto the printed mesh, bridging the circuit and making the thing work. It’s just a cheap switch – and a wetted finger will often work in the same way. Now this bit’s for Serious Modifiers only. You can change both the loudness of the tones and their pitch by making some electronic modifications. The simplest way of increasing the sound volume is to connect the module to a 200 watt amplifier ... er, just kidding. The cheapest way of increasing the sound output level is to use a more efficient speaker than the one provided. I happened to have an old 8-ohm cone tweeter lying around and that worked fine. If you don’t, then investing $2.50 in a 57mm speaker will almost certainly lift the sound output level. If it doesn’t, then give the newly-bought speaker to the little brother/kid who lives across the street/intelligent dog/ your baby to eat. Changing the tone and speed of the recital on my phone was as easy as changing the value of about the only component which was accessible – a resistor. It started off as 268kΩ but experi­ mentation showed that add- ing a 1MΩ resistor in parallel both increased the speed with which the sounds were played (the guy inside the blob went into overtime) and also increased the pitch at which it happened. Next up is the decision about which tones you want to use. The keypad pushbuttons will still work while pulled apart if they’re pressed against the PC board or alternatively, you can simply bridge the conductors by using a screwdriver to replay all of the sounds. Pick the keypads which give the right sounds and then carefully solder two wires to the PC pads, with each wire soldered to the different conductors on the intermeshed grid. Check that when these wires are joined the wanted tone sounds continuously. The power supply can be either de- The keypad uses these conducting buttons which, when pressed, squash down on a PC grid pattern, joining the two conductors. February 1996  23 The resistor seen here can be changed in value to modify the sound output. Adding a 1MΩ resistor in parallel made the man inside the blob go into overtime! Pairs of wires are soldered to the selected switch pads, to trigger the different sounds. The pads are picked on the basis of the sound generated – you pick which warning tones you want to use. The finished warning module. The wire pairs on the left are used to trigger the different sounds, while an old cone tweeter has been substituted for the original speaker to increase the output volume. 24  Silicon Chip rived from the original button cell or from AA batteries (both last a very long time in this application), or a trimpot can be used to provide the supply voltage from the car battery. You don’t need to supply an exact (regulated) 3V to power the thing; anything around that value will work fine. Making the connections So how do you connect the Sound Module (you don’t call it a toy phone any more) to the engine? If you’re mon­itoring tempera­ture sensors which trigger warning lights by switching to ground, then wiring the switch output directly to a module input (with the other module input wire earthed) will trigger the sound at the same time as the light comes on. If the trigger is an output voltage, for example, when monitoring an ECU “Check Engine” light, then a low-current relay (less than $3) can be wired into the warning circuit to work the module. The module can be mounted in a Jiffy box or simply wrapped in electrical tape and mounted under the dash. The speaker does­n’t need to be close to the module, meaning that it can be locat­ed where the sound will be heard the loudest. When you consider the horrible possibilities of a missed warning light, $4 and a few hours’ work doesn’t seem SC too bad, does it?