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Constructional Project PROJECT BY PHIL PROSSER Tool Safety Timer Have you ever accidentally left your soldering iron switched on? This project will help allay your fears. It’s also a great idea for fitting to a hot air rework system or other mains-powered tools. » Switches a mains-powered device off after a period of inactivity » Activity of user movement sensed using a PIR (passive infrared) sensor » Selectable timeout of 10, 20, 30 or 60 minutes » Pushbutton to switch tool back on again after timeout » Indicator LED shows when PIR is picking up motion » Power supply: 12-20V DC, 9-15V AC <at> 200mA maximum with the device handling up to 10A/2.3kW T he hot air gun we use is one of those ubiquitous ones sold under many different brands. These tools are quite low in cost but function surprisingly well. To operate them, you lift the handpiece and the heater and fan start automatically. They run until you place the handpiece back in the cradle, which is sensed by a magnet in the cradle and everything cools down. The hot air gun is often set well in excess of 300°C and is therefore fully capable of igniting combustible material. Unfortunately, this author is really forgetful and often comes back hours later to find the hot air gun ready to go. If, for example, a mischievous cat brushes against it, or a child decides to play with it, the hot air gun may fall from its cradle with nobody around to avert disaster. This has the real potential to set the house on fire, which is the one thing about this tool that we really dislike. It should shut itself down after a period of being idle. On a somewhat less dramatic note, our soldering iron is left on more often than not. The one in our lab does not have an automatic shutdown, and we have found it running at full temperature after many days. This is much less likely to cause a fire, but we have replaced many more tips than necessary (and wasted expensive electricity too). My modified hot air gun. We mounted the PCB to the inside of the lid, a transformer to the base and the sensor and start switch to the side. This has proven to be very effective. Yes, our hot air gun has seen some use. 60 This project is to provide a very simple and convenient approach to automatically switching these tools off. You run your tool from a switched mains source with a timer you can set. If you are present and moving the tool around, or moving around yourself, the timer will be reset. If you leave the room, when the timer expires, your tool is switched off. When you come back later, you just need to press a button on the box to restart it. You might be sitting there for an hour or two and the tool might run for this time, but our theory is that if you are present, you would catch any significant hazard. Once you leave, the system will ‘time out’ and protect you and your tools. Our tests have shown that with the sensor on the desk near the user and tool, it will easily detect the user’s hand, and especially a hot soldering iron or hot air gun. We recommended the PIR is pointed in the direction of your tool stand, ideally away from yourself and definitely away from thoroughfares. This way, it can detect when you stop using the tool or when you leave the desk. In practice, this works as long as there is a line-of-sight from the sensor to you or the tool when you place it in its stand. We considered two ways to use the Tool Timer. The first is to mount it in a UB1 ABS plastic case and power it from an external 12V DC plugpack. Practical Electronics | May | 2026 Tool Safety Timer 230V AC MAINS INPUT CON4 +3.3V 100nF 3× 4.7kW JP1 LED1 4.7kW 2 2 3 2 IC1 PIC16F15214 RA4/AN4 RA1/ICSPCLK 1 4 JP2 1 2 MCLR RA3/MCLR/Vpp AN2/RA2 7 4 PGD 5 PGC S1 L RLY1 N C 4.7kW B E 4.7kW 5 Q1 BC338 +~~– D2–D4: 1N4148 K 4.7kW 4 1 MOD1 CON6 V+ 1 8 E A IC 1 W04 8 230V AC MAINS OUTPUT 2 PIR 2 3 +V O UT GN D MINI PIR MOTION SENSOR MODULE BC338 CON5 ICSP START D2 A 6 3 E CON2 1 K 4.7kW A N R1 1 D3 V ss +3.3V l K RA0/ICSPDAT RA5/AN5 1 2 K 1 V dd L CON8 A 2 F1 1 1A CON3 LM317T IN V+ REG1 LM317T CON7 START CON1 O UT V+ 1 BR1 W0 4 + IN 470mF ~ ~ – JP1 IN IN OUT OUT B ADJ 2 100nF O UT ADJ E C +3.3V JP2 MINUTES IN 10 O UT 20 30 IN 60 O UT K 330W D4 10mF 100nF A 560W Fig.1: the circuit is based around microcontroller IC1 and the PIR sensor. When the sensor’s output goes high, meaning it has detected movement, the timer is reset. Once the time period has elapsed, the relay is de-energised, switching off the tool. It can be switched on again by pressing S1. The second is to embed this in a tool, like our hot air gun. We will describe how to package it in the UB1 case in detail. Because we do not know the specifics of your tools, we can’t go into great detail on how to fit it inside a tool. However, we will provide notes and advice on this. If you are not completely confident and comfortable analysing and understanding how your specific tool operates, we recommend you stick to the UB1-packaged version. Circuit details The Tool Timer circuit is shown in Fig.1. Switching of the tool power is via a 10A mains-rated relay. The recommended relay can actually carry and break 16A, which is more than enough. We have heavy tracks on both sides of the PCB so it can handle 10A continuously. The Tool Timer switches the Live line only. Never assume that a device that is disabled by it is isolated from the mains. We use an IEC plug/socket combo for the mains input and output. This has the Earth link integrated. We link Practical Electronics | May | 2026 Neutral to Neutral directly on this part, and wire the Live input through our PCB to the Live output. It does not matter which pins of CON2 are used as the input and output. For the PIR sensor, we chose one that can operate from a wide range of voltages, up to 20V DC, and provides a 3.3V logic high level on the middle pin when an object is detected. We have included a test point on the PCB to which you could connect another LED with a series resistor to indicate when an object is being detected by the PIR. This is labelled “PIR”. This PIR signal is fed to the PIC and, if movement is detected before the Tool Timer has switched the load off, it will reset the timer. This means that if you use a soldering iron or heat gun, taking the tool from its holder and replacing it, the PIR will sense this and reset the timer. So it will never time out while you are actively working. The PIC has two jumpers, JP1 and JP2, that allow you to program a timeout period of 10, 20, 30 or 60 minutes. The timer starts when the PIC is powered up, and the relay is switched on while the timer is counting down. In practice, it won’t expire if you are using the tool; there is even time to get a cup of coffee, even from the shop, if you are using 30 or 60 minute timeout! Once the timer expires, the relay switches off, and will only switch on again if you press the START button on the front panel (S1). That is really all there is to this project. Other uses of the Tool Timer are quite varied. It could be used as a night light that times out after 60 minutes and after people stop moving in the room. Another potential use is as a timer for kitchen equipment, such as coffee makers, toaster ovens etc. You can start the device by pressing the button and it will continue to run while people are around and for a set period thereafter. This will help avoid that burned coffee smell when the pot is left running too long, or that toasty maker sitting there running forever, waiting to burn unsuspecting fingers. Power supply The power supply includes provision to mount a bridge rectifier, which might seem odd. The intent is that if 61 Constructional Project you are integrating this into an existing piece of equipment, it may be easier to power it from AC. Our heat gun, for example, has 9V AC accessible. With the bridge rectifier, this works just fine. Of course, you can run DC via the bridge rectifier, provided you have at least 12V available for driving the relay. We use an LM317 regulator to generate 3.3V for the PIC, but we operate the PIR from the raw DC. The recommended PIR has its own regulator and can accept 5-20V DC. This same PIR sensor module is widely available from various suppliers on the internet, so it should not be difficult to get. Table 1 – R1 values for a DC supply (BR1 linked out) Table 2 – R1 values for an AC supply (BR1 loaded) Supply R1 value Supply R1 value 12V 0W (wire link) 9V 0W (wire link) 14V 33W ¼W 11V 39W ¼W 16V 68W ½W 13V 82W ½W 18V 100W 1W 15V 150W 1W 20V 150W 1W OUT D4 4148 START CON7 330W PIR 560W REG1 LM317T Q1 BC338 GND A K CON8 LED 4.7kW 4.7kW 4.7kW 4.7kW R1 Tool Timer v12 Nov 2024 62 CAUTION 230V AC 4.7kW 4148 + PIR CON6 10mF RLY1 250V AC 10A 12V DC COIL 4.7kW CON5 ICSP ~ – + ~ 12-20V DC/AC +3.3V 4.7kW 4.7kW PCB and in Fig.2. In our testing, using a 12V DC plugpack, the relay operates just fine with this bridge in place, but it is not necessary. Double-check that you are inserting it the right way around (pay attention PCB design to the markings on the device and the We have added slots in the PCB to board), as these are really fiddly to ensure isolation between the mains remove from the PCB if installed inand low-voltage sections of the board. correctly. They will make certain that whatever Follow by fitting the fuse clips (with load you connect won’t cause arcing the retention tabs on the outside) and across the relay tracks. all the capacitors, watching the polarity of the electrolytics. Next, mount the Construction headers and screw terminal. The screw The Tool Timer is built on a double-­ terminal specified is rated to 300V sided board coded 10104251 that meas- AC and 16A. This part has a standard ures 71 × 88mm. Assembly is fairly easy 5.08mm lead spacing; if you substias all the parts are through-hole types tute it, make sure the part is rated for and the board is not tightly packed. at least 250V AC and the current you Start with the resistors; fit all the 4.7kW are switching. parts, then the others. Next, mount the Now mount the transistor, relay and three 1N4148 diodes, making sure they regulator. The relay has a very standard face as shown in the PCB overlay dia- footprint and you will find many opgram, Fig.2. tions. If you substitute this part, again We have specified a W02/W04 (or you need to pay attention first to the similar) diode bridge. If you are run- voltage and current ratings, as well as ning this from DC, you could install the coil voltage rating. wire links in its place, as shown on the We have wired the relay to operate from the input supply. If JP1 IN, JP2 IN → 10min FIT RED WIRE this is a 12V DC plugpack, we JP1 IN, JP2 OUT → 20min LINKS INSTEAD JP2 JP1 OUT, JP2 IN → 30min want this to power the relay OF BR1 FOR DC JP1 OUT, JP2 OUT → 60min JP1 directly. In this case, R1 is a F1 1A wire link (ie, 0W). If your DC BR1 supply is more than 12V, we W02/W04(M) CON2 want to drop this back to 12V IC1 TO LOAD + PIC16F15214 with a resistor. This dropper does not need 470mF to be very accurate. If the relay 100nF D3 gets a supply within a volt CON1 100nF Power 100nF or two of 12V, it will be OK. COIL 4148 D2 Fig.2: there is nothing terribly difficult about assembling this PCB. We don’t recommend using a socket for IC1, as it could fall out and cause a safety hazard. Watch the orientation of the bridge rectifier, electrolytic capacitors, regulator, diode and IC and note the three safety clearance slots. Table 1 provides values for R1 if your DC supply us higher than 12V, while Table 2 provides various R1 values for AC supplies. If using an AC supply, it must not exceed 15V. With all the parts aside from the PIC loaded, apply power to your board using a plugpack or bench supply and measure the voltage between pins 1 and 8 of IC1’s pads. The reading should be 3.3V ±0.2V. If this is not right, check that the bridge is in the right way around and that the regulator is mounted correctly. Also check the 330W and 560W resistors and diodes. With the power supply operating OK, disconnect the power supply and fit the PIC. Double-check its orientation before soldering it. If yours is preprogrammed, you can move onto the Packaging section. There is a programming header on the PCB (CON5); this uses the standard Microchip pinout so a SNAP or PIC kit can be plugged straight in. You can download the required firmware from siliconchip.au/ Shop/6/1825 Packaging A UB1 Jiffy box is a good match for this board and associated parts. Figs.3 & 4 show the holes and cutouts that are required. Mark and drill the round holes first. We used a stepped drill for the PIR hole. If you have never used one of these, we reckon you should try one – they are awesome for making larger holes in plastic and aluminium. There are a couple of locations where you will need to trim back the PCB guide rails on the inside of the case using a sharp knife or chisel. For the holes in the base for the PCB, a quick cheat is to drop the board into the case and mark through the mounting holes. We wanted the PCB on the opposite side of the box from the PIR to allow room for connectors, and we suggest you do the same. We used an oscillating multi-tool for the rectangular cutout, which made a Practical Electronics | May | 2026 Tool Safety Timer Figs.3 & 4: just four holes are required in the base – you can use the PCB as a template to mark their positions before drilling. Make the holes as shown here (and in Fig.6 overleaf) to verify how they relate to each other. You can use a multitool or rotary tool to cut the rectangular opening, or drill a series of small holes inside its perimeter and then file it to shape. Fig.5: the label for the Tool Safety Timer is shown here at 50% actual size. You can download it from siliconchip.au/Shop/11/1827 63 Constructional Project Fig.6: here is how to run the wiring. While you could solder most of the wires to the PCB, the headers and plugs make it much easier to disassemble it should you need to. Don’t omit the cable ties and make sure the mains wires are correctly rated, the right colours and routed to the appropriate terminals (they are usually marked L (or A), N & E on the connector). somewhat fiddly job easy. So if you have one, crack it out for this part of the job. The recommended case is made of ABS plastic, which is not hard, so you can easily use a small hand saw for this. Make sure this hole starts smaller than needed and file it to the final size. Lastly, you should add a 1.6mm thickness (or more) piece of insulating material like fibreglass between the IEC socket and PCB. The insulation material should be cut to fit the UB1 case (89 × 48mm), and you will need to use a sharp knife to cut a small hole to allow wiring to pass through. Setup and wiring With the box ready, we can now start pulling it all together. Use the wiring diagram, Fig.6, and photo opposite as guides while you read the following steps. Headers JP1 and JP2 set the timeout period to 10, 20, 30 or 60 minutes. We are using jumpers since our experience is that once you settle on a workable period, you don’t need to change it. It is possible to wire these to a switch or switches if you have an application in which this is necessary. Refer to the circuit diagram (Fig.1) to determine whether to place shunts on JP1 and/or JP2 for your preferred timeout and set that up now. Next, com- We have used a 33W resistor for the relay, as our DC supply is 14V. You need to select the right value for your application. 64 plete the wiring to the panel-­mounting LED, start button, PIR sensor and power input connector. You need these installed in the case and secured with the appropriate washers and nuts before you connect the headers to their connecting wires, as the headers will not fit through the holes in the case. Assuming your DC supply is from a plugpack and you’ll be using a barrel socket, wire it to the board using two 90mm lengths of light-duty hookup wire. We used red and black, with the red wire going to the positive middle pin on our 2.1mm inner diameter barrel socket. For a DC supply, make sure that your positive wire goes to the pin of CON1 marked +. If you don’t have the crimping tool for the plug, you can use a pair of side cutters or needle-­ nose pliers to crimp the wire in. This crimp will not be great, so solder over your temporary crimp, and it will be secure. Just don’t add a lot of solder or you won’t be able to insert the pin into the plastic block. For the PIR, use 70mm of light-duty wire. We used a ribbon cable offcut as this helps keep the wiring tidy. You need to ensure that the headers are wired correctly, ie, the + pin on the PCB goes to the positive supply on the Practical Electronics | May | 2026 Tool Safety Timer module, the – pin goes to the negative supply and the centre pin goes to the PIR output. For the LED, use two 90mm lengths of light-duty hookup wire or another ribbon cable offcut. You need to make sure you get the anode and cathode to the right pins. The cathode has a chamfer on the side of the LED and a shorter pin (it goes to the pad marked K). For the start button, use another two 90mm lengths of light-duty hookup wire or a ribbon cable offcut. Any colour will do. The button is not polarised, so connect the button to the two-way plug however you want. Testing With the timer set to 10 minutes (JP1 & JP2 in), apply power. Put something over the PIR sensor so it cannot detect your presence; a sheet of paper will do the trick. The LED should light and stay lit for pretty close to 10 minutes, then it should switch off. You should hear the relay switching along with the LED. If the LED stays on permanently, check that you have the PIR covered and that it is not detecting you move around, as it will reset the timer and the LED won’t go off. Also check that the voltages on the PIR are right. You should be able to monitor the PIR output using a DVM on the PCB test point, and see the PIR detecting your hand if you wave it in front of the PIR. If it still isn’t working, check for short circuits on the PCB around the PIC and the PIR header, and verify that the transistor is in the right way around and that you have used an NPN type (BC337 or BC338). Once it has timed out, press the start button. The relay should switch and the LED should come on again for a further 10 minutes. Finally, run the same test with the PIR pointing in your general direction and wait the 10 minutes. Unless you sit statue still, the PIR will sense you and the LED and relay should stay on. Final assembly We can now assemble this lot into the enclosure. Install the standoffs using shakeproof washers and machine screws, then mount the PCB into the enclosure. If you haven’t already, plug the power, start, PIR and LED headers onto the board. Now you can zip tie both the PIR Practical Electronics | May | 2026 The wiring for the Tool Safety Timer in the recommended UB1 enclosure. Use 10A mains-rated wire for the connections to the IEC connector. and power wires to the PCB using the two holes provided next to the power connectors. Similarly, zip tie the start button and LED wires using the lower set of 3mm holes provided on the PCB. We recommend applying a drop of Loctite or similar glue to the connectors to secure them to the board. Now install the IEC connector into the enclosure. Ensure it is secure and the mounting snap-in tabs hold it in place. If it is not totally secure, fix that before proceeding. 65 Constructional Project Parts List – Tool Safety Timer 1 double-sided PCB coded 10104251, 71 × 88mm 1 UB1 Jiffy box, 158 × 95 × 53mm 1 1.6mm-thick insulation material cut to 89 × 48mm (fibreglass, acrylic, Presspahn or similar) 1 12V DC 200mA+ power supply (eg, plugpack) 1 12V DC coil 250VAC/10A+ SPST PCB-mount relay (RLY1) [eg, SMI-S-112LM (AliExpress 1005004502346634] 1 PIR motion sensor module (MOD1) [HC-SR501, AliExpress 1005006824167570] 2 M205 PCB-mounting fuse clips (F1) 1 M205 1A fast-blow fuse (F1) 1 SPST red panel-mount pushbutton switch (S1) 2 2-way headers, 2.54mm pitch (JP1, JP2) 2 jumper shunts (JP1, JP2) Connectors 3 2-pin polarised headers with matching plugs and pins (CON1, CON7 & CON8) 1 2-way mini terminal block, 5/5.08mm pitch (CON2) 1 chassis-mounting barrel socket (to suit power supply) (CON3) 1 10A 250V IEC mains power input (C13) & output (C14) socket combination (CON4) 1 5-pin header, 2.54mm pitch (CON5; optional – for ICSP) 1 3-pin polarised header with matching plug and pins (CON6) Hardware, cable & wire 8 M3 × 6mm panhead machine screws 8 M3 shakeproof washers 4 M3 × 10mm tapped spacers 7 100 × 2.5mm Nylon cable ties 1 100mm length of 5mm diameter black heatshrink tubing 1 100mm length of light blue 10A mains-rated wire 1 300mm length of brown 10A mains-rated wire 1 IEC C13-C14 mains extension cable 4 stick-on rubber feet Semiconductors 1 PIC16F15214-I/P programmed with 1010425A.HEX, DIP-8 (IC1) 1 LM317T 1A adjustable regulator, TO-220 (REG1) 1 BC337 or BC338 25V 800mA NPN transistor, TO-92 (Q1) 1 red 5mm LED with bezel (LED1) 1 W02(M) or W04(M) 1.2A bridge rectifier (BR1) 3 1N4148 75V 200mA signal diodes (D2-D4) Capacitors 1 470μF 35V electrolytic 1 10μF 50V electrolytic 3 100nF 63V MKT Resistors (all ¼W 1% axial unless noted) 8 4.7kW 1 330W 1 560W 1 0-150W 0.25-1W resistor (R1; see Table 1 & text) Check that the Earth link is good (that the two Earth pins in the IEC socket have a very low resistance between them). It is part of the connector and it’s critical for safety, so we want to make sure it is OK. You will need to solder the Earth link on the back of the IEC connector as its fixing assumes a wire will be soldered to it. Now connect a short length of 10A light blue mains-rated wire between 66 the terminals labelled N on the rear of the connector. Use two pieces of 5mm diameter heatshrink tubing to insulate the connections of this wire to the IEC connector tabs. Note that this wire will cross over the Earth strap. Now take two 150mm lengths of brown 10A mains-rated wire and connect them to the Live terminals on the IEC connector. Insulate these with 5mm heatshrink tubing too. Run a 2.5mm cable tie around the Live and Neutral wires, under the ground strap, and secure them together. With the PCB in the case, trim the length of the two brown Live wires so they neatly present to the load switch connector, CON2, on the PCB. Once wired to the terminal, secure it to the PCB using a cable tie. Stick on four rubber feet onto the case so you don’t scratch your desk, and the Tool Timer is complete. Finally, you can attach a label to the lid. This is a simple and cheap way of making this utilitarian project that little bit neater. The label can be downloaded from siliconchip.au/Shop/11/1827 and you can find instructions for printing and attaching it at siliconchip.com.au/ Help/FrontPanels We used a 0.75m IEC mains extension cable to connect our soldering station to the tool timer. In operation, simply press “Start” on the tool timer and, while you are around and moving, your tool will stay on. When you wander off, it will switch itself off. Simple! We will not go into detail regarding how the tool timer can be installed inside a heat gun as there are too many inconsistencies in how these are built for instructions to be safe. Only attempt to do this if you thoroughly understand what you are doing. After reverse engineering our hot air gun, we concluded the safest approach would be to shut down the entire controller after the timeout. We chose to install a small 12.6V transformer in the case alongside the main transformer to power the Timer. We then used the Timer’s onboard relay to disable the main transformer that ran the hot air gun controller. Now I am less worried about my thoughtless cat setting my house on fire! Modifications If you want a timeout period different from the four options we have provided, the source code for the PIC firmware is available to download from siliconchip.com.au/Shop/6/1825 The definitions for the timeout period are in the header file “util.h”, defined in four lines (starting with #define Time_10Min_Runtime). The timer counts in tenth of a second intervals, so 10 minutes is 6000 counts. Hence, each value is the required number of seconds multiplied by ten. PE Practical Electronics | May | 2026