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Workshop/Shed Alarm Simple Electronic Projects with Julian Edgar This remote-control alarm uses a two-stage siren and can optionally switch on inside and outside lights when triggered. The design uses commonly available prebuilt modules and relays. M Photo 1: I built my alarm into a plastic utility box. Two terminal strips provide the external connections. y new home workshop was recently completed. It’s built on the block of land next to where we currently live (one day, we will build a house on the new block as well) and is a few hundred metres away from our existing house. I decided to install an alarm in the workshop – but then the fun started. Or didn’t, actually. I thought what I wanted was very simple. I wanted an alarm that could be armed/disarmed by a keyfob remote control that I’d carry on my workshop keys. When the alarm was armed, I wanted LEDs flashing at each door. If the alarm was triggered by unauthorised entry through the opening of any door, I wanted a siren to sound, quietly at first (in case I forget to deactivate the alarm), then subsequently at full volume for a set period. When the alarm was triggered, I also wanted interior and exterior LED floodlights to switch on. Finally, I wanted the LED lights, system controller and siren to run off 12V provided by a rechargeable battery. I couldn’t find anything even close to these specifications! Instead, I found very complex systems that would send me emails or text messages, ones that used single motion sensors that could never cover the interior area of the workshop, or others that were so expensive I just couldn’t believe it. Cheap car alarms came closest, but they tended to have very poor instructions that would take hours to sort out (I know, I bought one) – and that’s before adapting the system to these unique requirements. So I decided to build my own alarm. If you break the above requirements down, all that is needed to achieve the above list is: • An off-the-shelf remote control module and fob. • Door switches. • A latching system so that the alarm continues to sound even if a door is shut again. • Two timers – one for giving the ‘quiet siren’ period and the other the ‘total siren’ period. • Flashing door LEDs. • Switched power for the floodlights. • A siren, battery etc. Photo 2: note how I extended the curly antenna of the remote-control module. Behind the remote-control module is the latching relay that keeps the alarm sounding even if an opened door is later closed. Photo 3: the two red boards are the timers. One switches off the siren after a pre-set time and the other causes the siren to switch to full loudness after a short period. The relay at the back switches on 12V floodlights if the alarm is triggered. Photo 4: any 12V-powered siren can be used. This one was originally supplied in a car alarm kit and cycles between different sounds – very attention-getting! 72 Silicon Chip Australia's electronics magazine siliconchip.com.au Rather than taking an Arduino or similar approach, I decided that the controller would primarily consist of relays – yes, old-fashioned relays! One relay could drive the 12V lighting, while another could provide the latching function. The timers could be provided by some low-cost eBay modules, again with relay outputs. The system could be activated when the remote control module’s output relay closed, feeding power to the rest of the system. That left only the flashing LEDs – easily sourced, complete with dropping resistors for the 12V supply – and a battery and plugpack charger. Fig.1: when the alarm is armed via the remote control, power is fed to the flashing door LEDs and a latching relay. The latter stays dormant until a door is opened. Opening a door sends power to Timer 1, which feeds Timer 2, resulting in a quiet sound from the siren, followed by a loud one if the unit is not quickly disarmed. Design Fig.1 shows a block diagram of the system, while Photo 1 shows the completed unit. When the alarm is armed via the remote control, power is fed to the flashing door LEDs. Power is also then available to the latching relay, but it stays dormant until a door is opened. Door opening causes the relay’s coil to be powered, its contacts to close and then stay latched via one of its two sets of contacts. This feeds power to Timer 1, which starts counting. The timer output that is used is the Normally Closed one – so when this timer’s relay activates after about a minute, the output is switched off, silencing the siren. Timer 1 feeds Timer 2, which supplies only a low voltage to the siren for the first seven seconds before switching to full voltage. The latching relay also switches on the lighting relay, activating interior and exterior LED floodlights. 12V floodlights (eg, those sold for ancillary car lighting) are suitable and, these days, are quite cheap. These lights stay on until the system is reset by the keyfob (or by removing battery power). Alternatively, you could feed the LED floodlight relay following Timer 1, so the lights would go off when the siren stops. If you use high-power lights, taking this latter approach will help to stop the battery from going flat. Fig.2 shows the circuit. There are a few things to note: 1. Both external relays are double-­ pole, single-throw designs (DPST). Only an SPST relay is needed for the lights, but for the sake of convenience, I used the same type of relay for both latching and lighting functions. 2. The door switches carry only the current needed to operate the latching siliconchip.com.au Fig.2: each of the five main parts – the remote-control module, latching relay, lighting relay and two timers – can be wired and then tested before proceeding to the next stage. Timer 1 switches its output off when the timed period is activated, while Timer 2 bypasses the series resistor feeding the siren when the timed period has elapsed. relay’s coil, which is very little. The switches, in the circuit configuration shown here, need to close when the door is opened. 3. I used 6-core cable to connect the door switches and also to power the flashing LED at each door. Only 4-core cable is needed, but I had a large roll of 6-core cable that I’d acquired cheaply. The workshop has six doors (five roller doors and one personal access door) and the cable runs are long. However, there are no problems with voltage drops as the currents are so low. Components Here’s what you will need to build this alarm (also see the Parts List). • A 12V remote control module with relay output. Almost any 12V relay output remote module that has a latching function will be suitable. Australia's electronics magazine Latching means that the output relay stays engaged after you have taken your finger off the fob’s button. Some remotes require you the press the button again to unlatch, and others have separate ‘on’ and ‘off’ buttons – either approach is suitable. • A DPST (or DPDT) 12V-coil relay with 5A-rated contacts. This relay acts as the latch and also supplies all current to the rest of the circuit. • An SPST 12V-coil relay rated to drive the LED floodlights. This relay drives the lighting circuit. You could also use a DPST or DPDT relay. • Two variable delay modules (Photo 3). Almost any cheap delay module that has a relay output will work. However, the modules must operate from 12V, and they also need to have at least a single pole, double throw (SPDT) relay output. This means they will have Common, Normally March 2025  73 Open and Normally Closed relay connections. • A 12V siren (Photo 4). I used the one from the car alarm I bought. It draws about 800mA at 12.5V and is quite loud. It also cycles through different sounds, which is attention-­ getting. A variety of 12V sirens is available from about $12. • A resistor to reduce the siren’s output for the quiet period. I found an appropriate value resistor through some quick testing. In my case, with the siren being fed 12V, 180W gave the required reduction in siren volume, and the ½W resistor did not get warm. Different sirens will require different values. Start with values around 200W Photo 5: the alarm is triggered by door switches that must close when the door is opened. Here, an industrial roller switch has been used, activated by the folded aluminium bracket screwed to the door frame. Smaller, less expensive door switches are available. and increase it if the siren is still too loud. Ensure the resistor does not get warm – if it does, increase its wattage. Going too high in wattage is no problem. • Flashing LEDs, pre-wired for 12V use. These are cheap and commonly available. Choose whatever colour you want! (See Photo 7.) • A 12V battery. See the discussion below on options. • A means of charging the battery (eg, a solar panel or plugpack charger). • 12V LED floodlights. Using car accessory lights is cheapest, but ensure you do not select very powerful lights. Otherwise, you’ll need to upgrade the relay and battery. Photo 6: the opening of roller doors can be tricky to detect, but this is achieved here using another industrial roller switch, with this one equipped with a long lever. The switch has been protected by being mounted inside galvanised brackets. • Door switches. A wide variety of switches is suitable, including microswitches and reed switches. I used industrial roller switches (Photos 5 & 6). These are normally quite expensive, but I found a supplier that had them on sale for about $5 each. They are splashproof and durable over many cycles. Their large rollers are also easy to trigger from door movement. You can use as many switches as you like – just wire them in parallel. Remember, the switch needs to close when the door is opened. • A box to house the alarm, terminal blocks, standoffs, screws and nuts, cable etc. Battery choice Literally any 12V rechargeable battery can be used. If you charge the battery from a float charger, the battery needs to supply power to the system only during a mains power failure. Thus, the battery doesn’t need to do a lot, and it’s likely a salvaged ex-car lead acid battery will be fine. Your local car mechanic is likely to have half a dozen waiting to go to the recycler. They’ll be free or only at nominal cost. If you are using a solar panel to charge the battery, the battery will need to power the system for perhaps up to a week in rainy weather. Current consumption will depend on the specific remote module, relays and LEDs you use. As a guide, my system had a current consumption of 12mA (unarmed) and 41mA (armed), plus an average consumption of each flashing LED of 13mA. When activated (relays engaged, siren running) the current consumption was about 1A. Building it Photo 7: the flashing LED (circled in green) is inconspicuous in the daytime but very obvious at night. It is bright and flashes at 1Hz. It is mounted in an aluminium bezel and sealed with silicone. 74 Silicon Chip Australia's electronics magazine I built my alarm into a plastic box that measured 190 × 110 × 80mm. This is a little bigger than required, but it gives room for the remote module’s normally coiled antenna to be stretched upwards – something that gives noticeably better range (see Photo 2). I suggest you build the Alarm stepby-step on the bench, testing it at each step. Start by connecting power to the remote control module. Check that the output relay clicks appropriately when the remote fob button is pressed (Photo 8). The relay should switch on and stay pulled in, then with another button press, switch off. siliconchip.com.au Next, add one of the flashing LEDs. Check that the LED flashes when the alarm is armed via the remote and turns off when the alarm is disarmed. As with the switches, you can use as many LEDs as required, again wired in parallel. Wire in the latching relay next. Do this in two steps. The first step is to ensure that when the alarm is armed via the remote and a door switch is closed, this relay pulls in. Then add the relay’s ‘latching’ wiring and repeat the test. This time, the relay should stay pulled in, even when the ‘door’ is again closed (ie, the door switch is opened). Disarming via the remote should cause this relay to unlatch and the flashing LED to switch off. Wire in the lighting relay next and check it operates when the alarm is triggered. The two delay modules are next. Note that the Normally Closed relay output connection is used for the main timer – that is, the output is energised until the timed period elapses, whereupon the output is switched off as the relay contacts are pulled in. Wire in this module and check its relay activates at the end of the period that you want the siren to sound for. These timers typically have an onboard pot that allows the period to be adjusted. In the case of the timers shown here, the maximum period was a bit short (10 seconds). I extended it by soldering a 470μF 16V capacitor in parallel with the main timing capacitor, giving a one-minute maximum period. This sounds like a short time for the siren to sound, but in the quiet location where I live, it’s plenty. The second timer, that allows the siren to sound only quietly at first, uses both relay outputs. The Normally Closed output (that is energised when the timed period has not yet elapsed) feeds the siren through the resistor. When the timed period has finished, the relay switches and the Normally Open contact is energised. This feeds the siren directly, so bypassing the resistor and causing the siren to sound at full loudness. Wire this relay in next. The complete system can now be bench-tested, with the siren suitably muffled with a towel or similar. Check that: 1. The alarm can be armed and disarmed by the remote, with the flashing LED indicating the status. siliconchip.com.au Parts List – USB Solar Charging System 1 12V remote control module with relay output and latching function [eBay 155694654180] 2 DPST (or DPDT) 12V DC coil relays [Jaycar SY4065] 2 variable delay modules [eBay 235710400707] 1 12V siren [Jaycar LA8908] 1 12V rechargeable battery [Jaycar SB2484] 1 12V battery charger [Jaycar MB3619] flashing LEDs, pre-wired for 12V use [Jaycar LA5082] door switches [Jaycar LE8777] 12V LED floodlights [eBay 235086391538] 1 plastic case, large enough to house the parts (I used 190 × 110 × 80mm) 1 chassis-mount fuse holder & fuse rated to suit maximum total draw cabling and wire to suit installation various machine screws, nuts, standoffs and terminal strips as required 2. When the alarm is armed, closing a door switch (opening a door) causes the lighting relay to pull in and the siren to start operating, quietly at first before then switching to full volume. 3. The quiet siren period is as you have set it (eg, seven seconds) and the full siren period is also as set (eg, one minute). 4. You can switch the operating siren and lights off by deactivating the system via the remote. Installation How you install the system is largely up to your individual requirements. As my main workshop wiring was being done simultaneously with the alarm installation, I used the same approach for the alarm wiring as for the normal mains wiring – that is, placing the cables in plastic conduit. This protects and conceals the alarm wiring. I placed the siren high in the workshop (out of reach!). The door switches are triggered by small aluminium brackets that I bent to the required shape. The alarm controller and the sealed lead-acid (SLA) battery are concealed in a timber enclosure within shelves – it’s not obvious where they are. In my application, the battery is charged by a solar panel working through a small solar charge regulator. Conclusion There’s something to be said for working with electronics where you can see components (like relays and switches) actually working. Also, apart from the door switches and siren, every other component was already SC in my parts drawers! Photo 8: the alarm is activated and deactivated with this remote control. Australia's electronics magazine March 2025  75