Fullscreen HTML5Med Res (??MB)HTML4

M-u-u-u-m . . . he’s been in my room again! Do you have children or grandchildren who are very territorial? Do they want extra security against invasion of their rooms by pesky siblings? Why not build a Personal Security alarm so they can be alerted when their room is about to be invaded? It will sound an alarm as soon as the door knob is touched and possibly avert any noisy squabbles. The kids will love it! Of course, you may then have to make an alarm for each kid who wants one. Build the watchdog alarm        (for peace in your home!) By JOHN CLARKE A kid’s domain is sacrosanct – especially if they are fortunate enough to have their own room. But no amount of threats or retribution will keep a sibling (or parent!) out when they’re uninvited! Kids are sneaky that way. . . This Watchdog Alarm is effective and certainly preferable to more drastic measures such as trip wires, buckets precariously balanced above doorways, trapdoors, Gatling guns and other schemes which war-like adolescent humans are likely to dream up to protect their room, cave, cubby house or lair. Having an audible warning means the hostile room occupant will immediately be alerted to an undesirable alien (eg, little brother!) touching the doorknob, even before they open the door. That warning may or may not be sufficient to prevent the mischief maker from creating mayhem by opening the door, knowing they 76 Silicon Chip have been detected, but it will certainly be a deterrent against further incursions. (OK, we’re not seriously suggesting this alarm as a proper security device . . . but it could have other applications where a change in the capacitance of a metal object needs to be detected.) The idea for this project came from an article in the April 1981 edition of Electronics Australia for a portable burglar alarm. Powered from a 9V battery, it used a hex Schmitt trigger connected as a couple of oscillators and a timing circuit to drive a siren whenever the doorknob was touched. That article suggested it as being suitable for use in hotel rooms but that is not practical today for modern hotel and motel rooms, which are usually entered by swiping a card through a magnetic scanner, or even via an RFID tag. Nevertheless, with child territoriality in mind, Australia’s electronics magazine siliconchip.com.au we have updated this concept using a low power microcontroller. The resulting Watchdog Alarm uses an 8-pin micro on a small PCB which can be easily assembled and set up within an hour. It is powered from a small, onboard 3V cell and is presented as a bare printed circuit board (PCB) assembly to minimise cost. A sensor loop is placed over the doorknob (no electrical connection required) to detect when this is touched. The Watchdog Alarm is turned on and off with a toggle switch and an indicator LED flashes to show that the Fig.1: the Watchdog’s microcontroller feeds a 2MHz signal via trimmer capacitor VC1 to the doorknob sensor loop. If someone touches the doorknob, doorknob is being monitored. their body capacitance shunts this signal to ground. The micro senses this and When the Watchdog Alarm is first sounds the piezo alarm or siren. switched on, the indicator LED flashes rapidly for about 10 seconds during which time the doorknob can be an off-board piezo siren. This is more 2MHz signal which is applied to the touched without sounding the alarm. likely to wake more comatose room doorknob which is monitored at the Touching the doorknob during this occupants (no guarantees, though!). same time. period will cause the LED to light fully. If a pesky human touches the You can use the 10-second period doorknob, the person’s body cato check the operation of the pacitance will effectively shunt Watchdog Alarm. We’ll explain away the 2MHz signal and this • Detects the presence of a hand on a doorknob this later in the setup section. will cause the micro to sound Once the 10-second period has • Option of piezo transducer or louder siren for alarm the alarm. The circuit requires expired, the LED will flash about a “counterpoise”, made from • Testing period during initial power on without once per second to indicate that three lengths of wire and this sounding alarm the Watchdog Alarm is armed. serves to provide a virtual ground Touching the doorknob then will • LED indicator shows initial test period, normal reference. cause the alarm to sound. The circuit, shown in Fig.2, monitoring and alarm There are two alarm options. is based on a PIC12F617 8-pin One is a on-board piezo transducer Block & circuit diagrams microcontroller (IC1). that beeps twice every three seconds Switch S1 connects the 3V button Fig.1 shows the block diagram. (about 1.5Hz) if the alarm is triggered. As already noted, this alarm uses a cell and diode D1 provides reverse poFor a much louder alarm you can use microcontroller and it produces a larity protection. If the cell is somehow Features Fig.2: the micro provides two alarm options. The first is a piezo transducer driven with anti-phase tone burst signals which effectively doubles the single pin output voltage. The second is a lounder off-board piezo siren which has its own internal oscillator. LED1 shows the alarm status. siliconchip.com.au Australia’s electronics magazine August 2018  77 The two “sirens” applicable, shown here not far off life size. On the left is the louder of the two which must be mounted off the PCB. At right, the piezo transducer, can be mounted directly on the PCB. inserted incorrectly, the diode will conduct and safely limit the reverse voltage to IC1 at around -0.6V. Admittedly, the cell holder we use makes it rather difficult (if not impossible!) to allow the cell to be inserted incorrectly, so that is an added prevention. IC1 uses an internal 8MHz oscillator and this is divided by four to provide a clock signal of 2MHz at pin 3, CLKOUT. Most of the time IC1 is in sleep mode and its internal 8MHz oscillator is stopped. It is woken once a second by its internal watchdog timer (yep, that’s where we got the name from!) to flash the red indicator LED and check if the doorknob is being touched. Sleep mode reduces the current consumption of IC1 down to a very low level in order to maximise the life of the cell. In more detail, the internal watchdog timer runs continuously and once a second it wakes up IC1. The 8MHz oscillator then starts, the program in IC1 runs and the CLKOUT output at pin 3 then produces the 2MHz signal. This signal is applied via an adjustable trimmer capacitor, VC1, to the T0CKI (pin 5) via 470Ω resistors. The wire loop for the doorknob sensing is attached to the trimmer capacitor at the opposite side to CLKOUT. Fig.3: the complementary (anti-phase) drive signals applied to the piezo transducer, from pins 6 & 7. The two signals are at 4.05kHz and have an amplitude very close to 2.06V and 2.44V peak-to-peak, not allowing for the overshoot spikes. Therefore the total signal applied to the transducer will be close to the sum of those voltages 4.5V peak-to-peak, as shown in the purple mathematical trace. 78 Silicon Chip If the doorknob is not touched, the input to IC1 at T0CKI will receive the 2MHz signal passing through the trimmer capacitor. The block diagram of Fig.1 depicts what happens inside IC1. The 2MHz signal is applied to a divide-by-four prescaler and then to an 8-bit counter (TIMER0). At 2MHz, the output from the prescaler is 500kHz (2MHz/4). TIMER0 counts at the 500kHz rate and the overflow output (T0IF) goes high after 256 counts – the full count of the 8-bit counter. TIMER 0 reaches the full count in 512µs. If its output does not go high after this period, then the software assumes there is no signal. Lack of signal would mean that the 2MHz signal is being diverted to ground by flow through the doorknob by body capacitance. Several extra parts are used between the VC1 output and the T0CKI input. This includes the 470Ω resistors and diodes D2 & D3 which are included to protect the pin 5 input. Should the person have a static charge before touching the doorknob, diodes D2 or D3 will clamp the voltage to the positive or 0V supply depending on the static voltage polarity. The 470Ω resistor to D2 and D3 limits current. The next 470Ω resistor to pin 5 protects the internal protection diodes of IC1. The 1MΩ resistor is there to pull pin 5 to 0V so the input does not float at a voltage between 0V and the supply. Additionally, during sleep, the pin 5 input is changed from an input to a low output, further ensuring the input is not floating. A floating input will cause IC1 to draw extra current. Piezo drive The GP3 input, pin 4, connects to the 3-pin header JP1. The position of a 2-pin jumper on this header selects whether you use the lower-cost on-board piezo transducer or the louder off-board piezo siren. The selector is necessary because each is driven differently. The piezo transducer is driven by a burst signal generated by the micro, while the piezo siren has its own internal constant tone generator and is turned on when pin 7 goes high, feeding it with 3V DC. When set in the piezo position, GP3 is tied low and if Fig.4 shows the same complementary drive signals fed to the transducer as in Fig. 3 but at a slower sweep speed of 5ms/div. This shows how the signal bursts are rapidly chopped to give it a burbling sound, which is more attention-getting. Note that we are running the piezo transducer at close to its resonant frequency to maximise its audible effect. Australia’s electronics magazine siliconchip.com.au Specifications Supply voltage: 3V lithium cell which operates down to 2V Current drain: 5.4µA at 2V; 7.5µA at 3V,with LED flashing once per second (Piezo siren; when sounding add an extra 250µA) Expected cell life: ~1 year continuous use Indicator flash: 3.2ms once each 1.152s, constantly lit during detection Testing period indication: LED flashes 3.5 times per second during first 10 seconds after power up. Fully lit during detection Alarm response time: 0.5s (285ms during 10 second testing period) Piezo Transducer: 200ms bursts of 4.05kHz warbled at between 400Hz and 600Hz at a 1.55Hz rate Piezo siren alarm: Uses intermittent siren or siren burst Fig.6: the PCB component overlay, with matching photo at left. It is shown here without the piezo siren mounted to reveal the PIC and other components underneath. the doorknob is touched to trigger the alarm, the piezo transducer sounds, as pins 6 & 7 (GP1 & GP0) alternately go high and low, to deliver bursts of 4kHz signal. In a small room and at close quarters, this can be quite loud. Certainly, there is no mistaking that the miscreant has been “pinged”. The alarm will sound while ever the doorknob is touched. As soon as the doorknob is released, the alarm will stop. The scope screen grabs of Fig. 3 & 4 show the complementary drive signals applied to the piezo transducer, from pins 6 & 7. In Fig.4, the two signals are at 4.05kHz and have an amplitude very close to 2.06V and 2.44V peak-to-peak, not allowing for the overshoot spikes. Therefore the total signal applied to the transducer will be close to the sum of those voltages÷ 4.5V peak-to-peak, as shown in the purple mathematical trace of Fig.3. By the way, the reason the signals on both sides are the transducer are not identical is explained by the presence of the series 100Ω current-limiting resistor from pin 6. Note that we are running the piezo transducer at close to its resonant frequency to maximise its audible effect. Fig.4 shows the same complementary drive signals but at a slower sweep speed of 5ms/div. This shows how the signal bursts are rapidly chopped which gives it a burbling sound which is more attention-getting. Finally, Fig.5 shows the same signal at the very low sweep speed of 500ms/div and this shows the 260ms duration and 1.55Hz frequency of burbled tone bursts from the transducer. When the JP1 jumper is in the siren position, the siren is powered by a high level (ie, the Vcc supply voltage) at the GP0 output with its other terminal connected permanently to ground. Construction Fig.5: the piezo transducer signal from pin 7 of IC1,at the very low sweep speed of 500ms/div and this illustrates the 260ms duration and 1.55Hz frequency of the burbled tone bursts from the microcontroller. The burbling of the tone bursts makes the sound seem much louder. siliconchip.com.au The Watchdog Alarm is constructed on a PCB coded 03107181, and measuring 42 x 93mm. It is presented as a bare PCB that can be hung on the doorknob. Fig.6 shows the PCB overlay. Begin construction by installing the resistors. There are only three values and of these, the only ones you could mix up are the 100Ω (brown black brown brown) and the 1MΩ (brown black green brown) in four-band code. Use a multimeter to check the value of each before inserting into the PCB. The three 470Ω resistors have yellow, purple, brown, brown coding. Diodes D1 to D3 can now be installed taking care to orient correctly and noting that D1 is the 1N4004 and the remaining diodes are 1N4148s. The 100nF capacitor can be fitted now, followed by the IC socket (for IC1). It must Australia’s electronics magazine August 2018  79 You’re likely to see this warning when programming the PIC12F617-I/P On the PICkit 3 it can be safely ignored, but other programmers may not support this programming. Parts list – Watchdog Door Alarm 1 double-sided PCB, coded 03107181, 42 x 93mm 1 SPDT PCB toggle switch (S1) [Altronics S1421] 1 20mm PCB button cell holder [Jaycar PH-9238, Altronics S5056] 1 CR2032 lithium cell 1 8-pin DIL IC socket 1 piezo transducer [Jaycar AB-3440, Altronics S6140] OR 11-13V pulsating piezo siren [Jaycar AB3456, Altronics S6117] 1 3-pin header with 2.54mm spacing (JP1) 1 jumper shunt (JP1) 2 PC stakes (optional) 2 M3 tapped x 9mm spacers 4 M3 x 6mm screws (at least two polycarbonate or Nylon) 1 4m length of multistrand insulated wire (eg 24 x 0.2mm) 1 150mm length of 6mm diameter heatshrink tubing 4 10mm diameter self-adhesive surface savers (stick-on feet) Semiconductors 1 PIC12F617-I/P microcontroller programmed with 0310718A.hex (IC1) 1 1N4004 1A diode (D1) 2 1N4148 diodes (D2,D3) 1 3mm red high brightness LED (LED1) Capacitors 1 100nF 63V or 100V MKT polyester (Code 104 or 100n) 1 9.8-60pF trimmer capacitor (VC1) Resistors (0.25W 1%) 1 1MΩ (Code brown black green brown or brown black black yellow brown) 3 470Ω (Code yellow purple brown brown or yellow purple black black brown) 1 100Ω (Code brown black brown brown or brown black black black brown) be oriented with the notch facing the 100nF capacitor. The 3-way pin header for JP1 is next. Optional PC stakes are installed at the wiring points for the piezo transducer or for the siren (the wires could instead be directly soldered to the relevant pads). Make sure the plus terminal of the button cell holder is oriented toward IC1 on the PCB. LED1 is mounted raised off the PCB (we made ours about 10mm high but it can be mounted higher). Take care with orientation – the longer (anode) lead goes to the hole marked with the ‘A’. VC1 can be installed either way around on the PCB. Switch S1 is inserted into position and soldered in place. The switch applies power when the toggle is up. The toggle Here’s how the on-board piezo transducer mounts on stand-offs above the PIC. The larger, more powerful siren can be mounted some distance away. It would connect to the “siren” pads, not the “piezo” pads as seen here. 80 Silicon Chip is protected inside the PCB cut out so is less likely to be inadvertently moved. Leave the siren or piezo transducer off for the moment. Programming the microcontroller If you purchase your PIC12F617-I/P microcontroller from the SILICON CHIP Online Shop (and tell us which project it’s for!) it will come already programmed (there is no extra charge for programming). However, if you want to program the PIC yourself, the file 0310718A.hex can be downloaded from the SILICON CHIP website. There is one caveat: we are not using pin 4 of IC1 as the master clear (MCLR) input but as an input for JP1. For master clear we use the internal MCLR instead. Some programmers will not support programming when the internal MCLR and internal oscillator are selected. If you are using a PICkit 3, the warning can be ignored and programming continued. Make sure IC1 is oriented correctly before inserting into its socket (the notch on the IC matches the notch on the socket). Now install the CR2032 cell in its holder and place a jumper link onto the 3-way header at JP1. Switch on S1 and if all is well, the LED will light or flash rapidly to acknowledge power has been connected. All that’s left now is to fit the piezo transducer or the off-board siren. If you choose the piezo transducer, it is mounted to the PCB on 9mm spacers using 15mm M3 screws. It sits up 10mm above the PCB surface as there are other components (including IC1) underneath. The two flanges on the transducer housing will need the holes drilled out to 3mm. There’s a little wrinkle here: the piezo housing flanges do not quite allow for M3 screw heads, as the heads foul the circular side of the transducer. With our prototype, the sides of the heads were filed down for each screw that secures the piezo transducer. Plastic polycarbonate or Nylon screws are easier to file down than steel. To secure the two screws, the standoff is rotated onto the screw thread instead of rotating the screw. Then the Piezo and standoffs can be secured to the PCB with the screws on the underside of the PCB. If you choose the significantly louder off-board siren, note that it is polarised – the negative (usually black) wire goes to the – siren terminal while the “pulse” wire (usually yellow) goes to the + siren terminal. The red wire is not used for three-wire sirens. By extending the siren’s black Australia’s electronics magazine The lip on the piezo transducer doesn’t quite allow the screw heads to fit, so we filed off one edge before mounting. We used Nylon screws because they’re a lot easier to file than normal screws! siliconchip.com.au Here’s the door handle loop before heatshrinking and soldering in place. It consists of four turns of hookup wire, 90mm in diameter. The heatshrink helps hold its shape. and yellow wires with suitable hookup or thin figure-8 wire, you can locate it some distance away from the PCB – even a few metres or so, if you wish. Finally, don’t forget to install the jumper shunt at JP1 in the correct position for the piezo transducer or siren whichever is used (the PCB is clearly marked). Wiring The loops for the door handle are made up using a 1.2m length of insulated wire to make four turns at 90mm in diameter. We fed our loops through lengths of 6mm diameter heatshrink tubing so that the loops would stay in place without unravelling. Strip back the two wire ends a few millimetres and solder the ends into the doorknob holes on the PCB. For the counterpoise, cut three 900mm lengths of insulated wire, strip insulation from one end of each and solder to the counterpoise holes located at the bottom of the PCB. In use, these are spread out over the door and fixed using Blu-Tack or tape. It makes sense, if possible, to use wires the same colour (or close) as the door to make them unobtrusive. If you are placing this on a door that is not your own, then check to see if the mounting method does not stain or leave a mark on the door. In some cases, just having the three wires loosely dangling straight down will be sufficient. And here it is shrunk and soldered. The loop simply drops over the door handle – no electrical connection is required as it detects capacitance – in this case, the capacitance of the person touching the doorknob on the other side of the door. When they do so . . . GOTCHA! Place the wire loop over a doorknob and switch on. Adjust VC1 so that the LED flashes with the door handle untouched but lights up when touched. This is a trial and error adjustment, so try various settings of VC1. Once you find a good position where the hand is detected readily, the adjustment should not need changing again. Note that for the first 10s after power is switched on, the LED will flash at a fast rate before flashing about once per second. That is if it is not detecting a touched door knob and the adjustment of VC1 is correct. The period when the LED is flashing at the faster rate indicates that the piezo or siren, when connected, will not sound when the doorknob is touched until the 10 seconds has expired. This is to allow the testing of the Personal Door Alarm when first switched on without causing a lot of noise from the alarm. If you wish, stick some self-adhesive surface savers (hemi-spherical adhesive buttons) to the corners of the PCB to protect against scratching the door. SC Testing Note that the Watchdog Alarm will not work if the door is metal-sheathed or if the door jamb is metal. It works best with timber-framed and timber doors with metal doorknobs. There is no need for an electrical connection from the doorknob to the wire loop, so the doorknob can be lacquered (such as coated gold or brass finishes) or exposed metal (such as brushed aluminium). The three counterpoise wires can, like the doorknob loop, be made from any surplus hookup wire. They should be about 900mm long each – but can be a little shorter if your door handle is lower than standard. They solder to the PCB but don’t connect to anything else. The short length of heatshrink tubing provides strain relief to the solder joints on the PCB. siliconchip.com.au Australia’s electronics magazine Fig.7: you need to secure the counterpoise wires to the door to ensure consistent operation. Blu-Tack is good because it doesn’t usually leave marks when removed. August 2018  81