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Fig.8: the Doorbell Sounder is based on a 7555 to produce a Ding Dong tone. You’ve had your fun – now convert it to a wireless doorbell T HIS PROJECT is also based on Jaycar’s Secret Farter. After you’ve had your fun with it, turn it into something useful – a wireless doorbell. You can place the door-bell receiver anywhere in your house or backyard so that it can be easily heard where you are located. There are plenty of times when it is impossible to hear if the doorbell rings. You may be entertaining or just relaxing and miss hearing someone is at the front door. In fact, it is often invited guests who have the most trouble in making their arrival noticed. A wireless doorbell can help solve this. The receiver is battery-powered so that it is portable and this makes it ideal for occasional use. When used as a permanent doorbell receiver, it should be powered from a 6V DC plugpack rated at 100mA. No modifications are required for the remote transmitter fob. It can be installed at the front door like any other doorbell switch but it is easier to do than with a wired doorbell, as no wires need to be passed through siliconchip.com.au OK, you’ve shocked Aunt Martha, mortified your granny and severely embarrassed your fairy godmother! Now that you’ve had your fun, you can convert your Secret Farter to something really useful – this Wireless Doorbell. September 2004  37 Fig.9: the component layout for the Doorbell Sounder. Fig.10: this is the full-size etching pattern for the PC board. the wall. The transmitter is powered by a 12V lighter battery. A Doorbell Sounder PC board needs to be added inside the receiver case. As mentioned earlier in this article, Q2 and Q3. These in turn drive the loudspeaker via the 10µF capacitor and 10Ω resistor. At the same, the positive control signal turns on transistor Q1 which pulls pin 5, the modulation input, down to a level set by trimpot VR1. This sets the initial oscillation frequency. When the transmitter button is released, the control signal goes low, Q1 is switched off and pin 5 rises back to a level set by the resistors at pin 5. This reduces the frequency produced by IC1 and its tone continues until the voltage at pin 4 drops to about 1V. The result is the characteristic DingDong tone. the Secret Farter receiver produces a positive voltage whenever the transmitter switch is pressed. This is used to control the Doorbell Sounder circuit which is shown in Fig.8. It comprises a single 7555 timer (IC1) that produces two separate tones, one when the transmitter button is pressed and the second when the button is released. Initially, when the control input is low (0V) pin 4 (reset input) of IC1 is low and so the output at pin 3 is low and the loudspeaker is mute. When the transmitter button is pressed, the receiver’s control input goes high and pin 4 is pulled high via diode D2 so that IC1 can oscillate. Its tone output at pin 3 drives the complementary buffer comprising transistors Construction The Doorbell Sounder PC board is coded 15109043 and measures 57 x 42mm. It fits into the receiver case behind the loudspeaker. Its component layout is shown in Fig.9. The proce- Table 2: Resistor Colour Codes o o o o o o No.   3   1   1   2   1 38  Silicon Chip Value 1MΩ 470kΩ 330kΩ 100kΩ 10Ω 4-Band Code (1%) brown black green brown yellow violet yellow brown orange orange yellow brown brown black yellow brown brown black black brown 5-Band Code (1%) brown black black yellow brown yellow violet black orange brown orange orange black orange brown brown black black orange brown brown black black gold brown siliconchip.com.au Parts List This shows the assembled PC board. Note how some of the parts are bent over to provide clearance. dure for installing it into the receiver is the same as for the Bed Wetting Alert with two exceptions, First, capacitor C16 on the receiver board is changed from 4.7µF to 22µF (not 100µF as used in the Bed Wetting Alarm). And second, there’s no need to parallel R23 with a 100kΩ resistor. The receiver can be placed anywhere within a 15m radius of the doorbell. For long term use, the receiver should be powered from a 6V DC plugpack rated at 100mA or more. If you do this, remove the batteries and attach a DC socket to the receiver suitable for the plugpack connection. Finally, the receiver and doorbell circuit are protected against reverse polarity connection so if you reverse connect the plugpack, it will not cause SC any damage. 1 PC board coded, 15109043, 57 x 42mm 1 Secret Farter (Jaycar GH1088) 5 PC stakes 1 100mm length of red light-duty hookup wire 1 50mm length of white lightduty hookup wire 1 50mm length of brown light duty hookup wire 1 50mm length of 0.7mm tinned copper wire 1 1MΩ horizontal mount skeleton trimpot (VR1) REAL VALUE AT $12.95 PLUS P & P Semiconductors 1 7555 timer (IC1) 1 BC547 NPN transistor (Q1) 1 BC337 NPN transistor (Q2) 1 BC327 PNP transistor (Q3) 1 1N4004 1A diode (D1) 1 1N4148 switching diode (D2) These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. Capacitors 2 100µF 16V PC electrolytics 1 22µF 16V PC electrolytic (required for Secret Farter receiver) 1 10µF 16V PC electrolytic 1 1µF 16V PC electrolytic 1 1nF ceramic (.001µF code 102 or 1n0) H 80mm internal width Resistors 3 1MΩ 1 470kΩ 1 330kΩ 2 100kΩ 1 10Ω Car Projects, Volume 2 THE PROJECTS: High-Energy Universal Ignition System; High-Energy Multispark CDI System; Programmable Ignition Timing Module; Digital Speed Alarm & Speedometer; Digital Tachometer With LED Display; Digital Voltmeter (12V or 24V); Blocked Filter Alarm; Simple Mixture Display For Fuel-Injected Cars; Motorbike Alarm; Headlight Reminder; Engine Immobiliser Mk.2; Engine Rev Limiter; 4-Channel UHF Remote Control; LED Lighting For Cars; The Booze Buster Breath Tester; Little Dynamite Subwoofer; Neon Tube Modulator. Mail order prices: Aust – $14.95 (incl. GST); NZ/Asia Pacific – $18.00 via airmail; Rest of World – $21.50 via airmail Or 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. siliconchip.com.au Silicon Chip Binders H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A12.95 plus $A7 per order. Available only in Australia. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 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_______ September 2004  39