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Do you want to know what 110dB of noise sounds like? Well now you can easily find out. This circuit puts out between 108dB and 111dB at a distance of about one metre. Build a 6-12V alarm screamer module There are many applications where a low-cost alarm siren is required. This very effective unit from DIY Electronics certainly makes a racket and could serve as the siren in a house alarm system, in a car, or in many industrial applications. For exam­ple, you could R1 1.5M 1 D1 1N4148 R2 1.5M 2 4 14 IC1a 7556 6 C1 0.1 RED R3 15k 5 10 7 C2 .01 T1 C3 10 9 C 1 13 12 R5 27k E C VIEWED FROM BELOW D2 1N4148 8 C4 .01 T2 V+ Q1 BC639 R6 B 1k R7 1k Q2 BC639 B E B C5 0.1 R4 27k V+ 2 PIEZO 1 IC1b 11 3 3 BLACK wire it to a door switch in your car via an exter­nal on/off switch to serve as an intruder alarm. As shown in the photographs, the unit is housed in a spe­cially-designed plastic case fitted with a mounting bracket (which also forms the rear RED 3 2 PIEZO 2 C 1 BLACK panel). Its overall dimensions are 84 x 55 x 33m (L x W x D), not including the mounting bracket. What’s so special about the case? Well, to make the unit as effective as possible, it features two integral resonant cavities for the two piezo transducers that are used to V+ generate the noise. The unit is +5-12V supplied with these two piezo transducers pre-glued to the resonant cavities – all you have 0V to do is assem­ ble a small PC board, connect a few leads and a power supply, and stand back to avoid being deafened. It is interesting to note that without the resonant cavi­ ties, the sound generated by the piezo transducers in open air is barely audible. It’s a completely different story with the reso­nant cavities, though. E T1 AND T2: WINDING 1-2 1500T, 44SWG ENCU WINDING 2-3 220T, 44SWG ENCU "SCREAMER" ALARM Fig.1: the circuit employs two oscillator stages based on IC1a & IC1b. IC1a frequency modulates IC1b which in turn drives two piezo elements via transistors Q1 & Q2 and autotransformers T1 & T2. How it works Refer now to Fig.1 for the circuit details. In addition to the piezo transducers, it’s mainly based on a dual 7556 timer IC, two transistors and a couple of autotransformers. IC1b is wired in astable August 1995  75 Electronic Projects For Cars FROM NEW N CHIP O SILIC On sale now at selected newsagents Or order your copy from Silicon Chip. Price: $8.95 (plus $3 for postage). Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. ➦ Use this handy form Enclosed is my cheque/money order for $________ or please debit my ❏ Bankcard   ❏ Visa   ❏ Mastercard Card No: ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ 76  Silicon Chip The case is supplied with the two transducers glued to two internal resonant cavities. All you have to do is assemble the PC board, connect a few leads and apply power. configuration and oscillates at about the resonant frequency of the piezo transducers. This frequency is set by R4, R5, D2 & C4 and is about 2.7kHz. In order to produce a realistic siren sound, IC1a is used to frequency modulate IC1b at a low rate. It does this by apply­ing a triangular waveform voltage to IC1b’s control pin (pin 11). As with IC1b, IC1a functions as an astable oscillator but in this case its frequency of oscillation is only about 5Hz. Note that diodes D1 and D2 ensure that IC1a and IC1b both function with a 50% duty cycle. The output from IC1a appears at pin 5 and is a square wave. This is then converted to a triangular wave by R3 & C3 before being applied to pin 11 of IC1b. As a result, IC1b produces a modulated output at its pin 9 which constantly sweeps back and forth through the resonant frequency of the piezo elements. As well as modulating the frequency to produce a siren sound, this technique means that no trimpot is required for frequency cali­bration. The pin 9 output from IC1b drives transistors Q1 and Q2 with the modulated 2.7kHz signal. Note that these two transistors are driven in phase. Q1 in turn drives piezo 1 via auto­ transform­er T1, while Q2 drives piezo 2 via autotransformer T2. In greater detail, each time the transistors turn on, cur­rent flows in the 220-turn winding of each auto­ trans­former. Con­versely, each time the transistors turn off, the magnetic field in these windings collapses and this induces a much higher signal in the 1500-turn windings. As a result, the autotransformers significantly step-up the signal voltage that’s used to drive the piezo transducers. In fact, a potential of over 200V is PIEZO 1 PIEZO 2 3 2 1 0V V+ T1 0.1 D1 1 2 3 .01 0.1 1k 27k 27k 1 IC1 7556 1.5M 1.5M .01 15k 1k Q1 D2 10uF T2 Q2 Fig.2: follow this wiring diagram when installing the parts on the PC board and take care to ensure that the two autotransformers are correctly oriented (see text). The photograph at right shows how the PC board is installed in the case. induced which can give you quite a shock if you are careless enough to touch the auto­transformer leads or the transducer terminals. This also means that the piezo elements are overdriven and this has been done deliberately to give maximum noise output. This causes no harm to the piezo elements and test circuits have been run for several hours at a time without component failure. Power for the circuit can come from any 5-12V DC source; eg, batteries or a 9V DC plugpack supply. Do not use a 12V DC plugpack as this could deliver more than 16V when lightly loaded. Assembly The parts for the Screamer Alarm are all installed on a small PC board measuring 78 x 48mm. This board features screened lettering to show where all the parts go and should only take about 10 minutes to assemble. Fig.2 shows the assembly details. Install the resistors first, followed by the two diodes and all the capacitors. Take care to ensure that the diodes and the 10µF electrolytic capaci­tor are installed with the correct polarity. The remaining ca­pacitors can be installed either way ar­ound. Next, install the two transistors and the IC socket. The IC can then be plugged into the socket, taking care to ensure that the notch in the IC body goes towards the 0V & V+ supply termi­nals (ie, pin 1 must be adjacent to the two 1.5MΩ resistors). Pin 1 will also generally be indicated by an adjacent dot in the IC body. Now complete the board assembly PARTS LIST 1 case with two piezo elements plus 4 screws 1 PC board (DIY Kit 15) 1 16-pin IC socket 2 autotransformers 2 150mm-lengths of hook-up wire (red, green) Semiconductors 1 GLC556, 7556 dual CMOS timer (IC1) 2 BC639 NPN transistors (Q1,Q2) 2 1N4148 diodes (D1,D2) Capacitors 1 10µF 16VW electrolytic 2 0.1µF monolithic 2 .01µF greencap Resistors (0.25W, 5%) 2 1.5MΩ 1 15kΩ 2 27kΩ 2 1kΩ Where to buy the kit A complete kit of parts for the 12V Screamer Alarm (DIY Kit 15) is available from: DIY Electronics, 22 MacGregor St, Nu­murkah, Vic 3636. Phone (058) 62 1915. The price is $23.50 plus $3.50 p&p. by installing the two auto­transformers (T1 & T2). These are oriented in opposite directions to each other and must have their leads bent through 90 degrees so that they lie flat against the PC board. Note that in each case, the centre terminal must be towards the top of the device (see photo). Two “tie-down” pads have been provided next to the body of each autotransformer and you can loop wire links over the auto­transformers at these locations. In practice, the leads on the autotransformers will usually be strong enough to stop them from moving. Finally, solder the six leads to the PC board at the desig­nated locations. There are two each for the piezo transducers (red to positive, black to negative), plus two more for the power supply connections. This done, the board can be mounted upside down inside the case, with the supply leads exiting from the notch, and the cover secured using the screws supplied. Testing Before applying power, wrap the unit in a towel to muffle the sound level (so that you won’t be deafened). After that, all you have to do is connect the power supply and the unit should immediately start. It’s best to start with a supply of about 5V and then test the unit at higher supply voltages – up to 12V. Exercise caution, though – this unit puts out ear-splitting sound, so keep it well wrapped up. If the unit doesn’t work, the most likely reason is poor soldering. Check all solder joints carefully under a good light and reheat any that appear suspect (disconnect the power supply before starting work). Next, check that all the parts are in their correct locations and that the IC, electrolytic capacitor, tran­sistors and autotrans­formers are all correctly oriented. If only one piezo transducer works, then check the transis­tor and auto­ trans­ f ormer associated with the SC non-functioning transducer. August 1995  77