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Low cost siren for burglar alarms Want a really loud siren which will wake the dead? Well, maybe this one won't wake the dead but it will certainly disturb the living. It's built on a tiny printed board that takes up hardly any space at all. By LEO SIMPSON Since it runs from 12V DC, this siren could be used in a car or home burglar alarm system. Its current requirements are fairly modest, at 1.25 amps, so it could be switched into circuit with a relay or a power transistor. It does not have any time out circuitry so that will have to be added, if required. What it does do is make a very loud sound with a minimum of components. It is a classic siren sound, heavily modulated in frequency at about a one second rate. It uses just one CMOS hex Schmitt trigger IC and six low cost transistors. The way in which these components are hooked together to produce this alarm circuit is quite interesting. First, there are two oscillator circuits. ICla is the low frequency oscillator. Its operating frequency of about lHz is set by the lMO resistor and lµF capacitor. The second oscillator is based around IClb and its basic operating frequency is set by the 2.2MO resistor and .OOlµF capacitor. It is modulated up and down in frequency by the RC network connected to the output of ICla. Its range of fre- quency modulation is from about 600Hz to 3.6kHz; ie, about 6 to 1. What happens is that ICla oscillates at about lHz and produces a square wave at its output. This is filtered to a rough sinewave by the 220kn resistor and its associated lµF capacitor. This signal is then fed to a 2.2MO resistor and clamped to 4. 7V by zener diode ZDl, after which it is fed to IClb via a .OOlµF capacitor. Thus ICla frequency modulates IClb. Hence, the output of ICl b (pin 12) is a square wave of varying frequency. Its amplitude is almost exactly equal to the input supply voltage minus the voltage drop across diode Dl. Dl is there merely to prevent damage if you connect the supply around the wrong way. The square wave output of ICl b is inverted by IClc and fed to the base of transistor Ql and to paralleled inverters ICld and ICle. These then drive the base of transistor Q2. H-pack output Just one CMOS IC and 6 low-cost transistors make up t~is nifty little siren , circuit. Don't bolt the tabs of the TIP31/32 power transistors together - you II create a short circuit if you do. 48 SILICON CHIP Four transistors, Q3, Q4, Q5 and Q6, drive the horn speaker which is what makes all the noise for this project. The four transistors are connected in an "H-pack" arrangement which effectively doubles the drive voltage fed to the horn speaker. It works in the following way. When the output of IClc goes high, it turns on Ql. Ql 's collector is connected to the base of Q4 while its emitter is connected to the base of Q5, via a 1000 resistor. So when Ql turns on, so do Q4 and Q5. The same arrangement goes for Q2. Its collector is connected to the base of Q3 while its emitter is connected to the base of Q6, via a 1000 resistor. So when Q2 turns on, so do Q3 and Q6. D1 1N5402 r 100{) +12V 1.2A 02 BC337 B 1M 2.2M + - ZD1 , J 1N7504A .,. ~ B EOc VIEWED FROM BELOW SUPER SIREN Fig.1: ICla is a lHz oscillator & this is used to frequency modulate IClb from 600Hz to 3.6kHz. IClc-lCle then produce complementary signals which drive Qt, Q2 & the H-pack output transistors. Because of the way in which the bases of Ql and Q2 are connected to complementary square wave signals (derived from inverters IClc, ICld and ICle), they turn on alternately for respective half cycles of the square wave signal. So Q4 and Q5 turn on together to pass current in one direction through the horn speaker. Then Q3 and Q6 turn on to pass current through the speaker in the opposite direction. At any one time then, the speaker has virtually the full supply voltage across it, continually being switched in one direction and then the other. This maximises the power output and therefore gives maximum scream for minimum component count. By the way, while the power in the horn speaker is maximised, the SPEAKER Fig.2: here's how to mount the parts on the PC board. Be sure to use the correct transistor type at each location. power dissipated in the four output transistors is very low since they spend their time either fully switched on (and saturated) or switched off. Either way, their power dissipation is minimal so they can be operated without any heatsink. Neat, huh? Another neat feature of this economy circuit is the use of just one current limiting resistor (1000) each for Ql and Q2. Other "Hpack" drive circuits we have seen usually use more transistors and need more current limiting resistors. Construction As we noted before, the printed board for this neat little number is teensy weensy. It measures just 32 x 45mm - about twice the size of an average postage stamp. It comes with green solder mask on the underside and a screen printed component overlay on the topside. Assembling it is hardly a major job and probably won't take much more than about 15 minutes for most people. There are a couple of points to watch though. First, the oscillator has been designed to suit Philips 40106 ICs. If you use 40106s or the equivalent 74C14s from another manufacturer, the chances are that their different hysteresis levels (ie, continued on page 69 BCIE PARTS LIST 1 PC board, 32 x 45mm, with screen-printed overlay 1 80 horn speaker Semiconductors 1 40106 (IC1 - Philips, see text) 2 BC337 NPN transistors (Q1,Q2) 2 TIP32 PNP transistors (Q3,Q4) 2 TIP31 NPN transistors (Q5,Q6) 1 1N5402 silicon diode (01) 1 1N750A 4. 7V zener diode (ZD1) Capacitors 1 4 7 µ,F 16VW PC elelectrolyic 2 1µ,F 16VW PC electrolytic 1 .001 µ,F ceramic Resistors (0 .25W, 5%) 2 2.2MO 1 1MO 1 220k0 3 1000 Where To Get The Kit Kits for this project are available from Oatley Electronics. The basic kit, containing a PC board and all parts except the horn speaker, is $10.40 plus $3.00 packing and postage. The 80 1 OW horn speaker is an extra $13.50. You can contact Oatley Electronics by phone on (02) 579 4985 or by post at PO Box 89, Oatley NSW 2223. OCTOBER 1990 49,. The PC board'is housed in a small plastic case measuring 82 x 54 x 31mm. As shown in the photographs, the voltage adjust potentiometer (VRl) and the on/off switch are mounted on the lid, while the output banana terminals are mounted on one side. To install the hardware in the case, first drill the holes in the lid using the front panel artwork as a guide, then drill holes in the side for the output sockets. These sockets should be 19.5mm apart and should sit as close to the bottom of the case as possible. This done, attach the front panel artwork, install the potentiometer and switch, and complete the wiring as shown in Fig.4. When installing the wiring, sit the PC board on the back of the lid next to the switch and pot as shown in the wiring diagram. The PC board is then installed upside down in the case when the lid is screwed down and can be held in position using a small piece of foam rubber. Testing is straightforward - just connect the output to your DMM, switch on and check that the output voltage can be varied from 0-ZOV using the 10-turn pot. If you strike trouble, check for 25.8V between the cathode of DZ and pin 5 of ICl. This will tell you whether the fault lies around ZDl or around ICl and the voltage doubler. ~ Burglar Alarm Siren upper and low threshold voltages) will result in the oscillator frequencies being different - you may have to change some resistor values. Second, watch the component polarities, both for the electrolytic capa citors and the semiconductors, particularly the TIP31s and TIP32s. It is all too easy to put these in the wrong way around and then you have a very dud project. Testing Don't be an idiot when you hook this up to your power supply. At the very least, put the horn speaker face down on your workbench when testing it - it is extremely loud and it will just about blow your F• • iI LL LL 0 NI 0 I- en => -, C <( UHF REMOTE CONTROL EA April 89 Our latest UHF transmitter • proven reliable unit • Complete Tx kit and PCB with components for Rx kit. Tx Battery included . UNBEATABLE PRICE $49.90 for the pair EXTRA Tx $17 .20 Because of demand we will hold th is price until December 1990 XTAL LOCKED ULTRASONIC MOVEMENT DETECTOR ALARM SC July 1989. No false triggers with this proven commercial unit. supplied in kit form. Easil y con nected as a self-standing alarm, with bonnet and boot protection. Instructions provided . Gan be activated by a hidden switch or you can use it with our UHF remote control. Complete PCB and components kit, and th e prewi red transducers. ONLY $29.90 INCREDIBLE PRICE DON'T YOU MISS OUT 'POWERHOUSE BARGAIN' NEW INDUSTRIAL GRADE, 6.5 Ah seal ed Gel battery. Only a fraction of their real value. ONLY (10+ $27 .90 $24 .90) Dimensions 150 x 95 x 65 mm. We ight 2.54kg. INQUIRE about our reasonable shipping cost. Fig.5: these are the full-size artworks for the front panel & the PCB. ctd from page 49 head off if you cop the full blast. Better still, do your initial testing with a fair sized resistor connected in series with the speaker. For example, we used an 8200 5 watt resistor when we tested the unit on the bench. However, any value from a few hundred ohms up to say, 2k0 will do the job and protect your ears. What a bout different supply voltages? Yes, you can increase the supply up to 15V which is the limit for the 40106. And the circuit will operate, with reduced power, down to about 9 or 10 volts. Below that, it's not worth bothering and you would have to change resistor values to make the oscillators work correctly. ~ MASTER SLAVE SWITCH EA JANUARY 1990 Remotely switch your non remote TV off an d on via your remote controlled VCR, many oth er uses etc . One switch operation, mains filter and overvoltage protection incl uded .Gom_plete PCB an d all on board components. ONLY $24.90 HALF THE PRICE OF MOST MAINS FIL TEAS . HE-NE LASER WITH DC POWER SUPPLY This is not a kit , complete unit includes laser head with a beam sh utter and instruct ions. Ideal for leveling sights, special effects or scientific experiments ate.Power 0.7 to 0.95mW maximum. DC _Power re ~l.rem_eQ_t_s,_1 0-14V <at>_ 1A O N LY $309.00 We also have laser (mains and battery operated) kits, laser tubes, laser diodes and night viewer kits still available. OATLEY ELECTRONICS PO BOX 89, OATLEY, NSW 2223 Telephone: (02) 579 4985 Fax No: (02) 570 791 0 Certified p&p $4-$6Aust. NZ add $2 OCT0 BER1 990 69