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Mini Projects #018 – by Tim Blythman SILICON CHIP Gesture-Controlled USB Lamp We designed this circuit to work with a lamp, but it could control just about any USB-powered device (rated at 5V). You could add an IR receiver for IR remote control, or an LDR to make it an automatic night light. By waving your hand over the small purple module, you can switch power to the USB socket; perfect for controlling a USB lamp. B ack in March 2022, Jim Rowe wrote about gesture recognition modules such as Jaycar’s XC3742. These nifty little modules are capable of recognising about 10 different hand gestures using an integrated IR pixel array (siliconchip.au/Article/15247). Now, we’re using this module to control a USB lamp. Since this project switches power to a USB socket, it could be used to switch any number of devices that run from USB power. You can see a video of it working at siliconchip.au/Videos/Gesture+Lamp We built it on a prototyping shield that sits above a Leonardo main board. That makes it easy to tweak the circuit if you wanted to make modifications. Circuit Fig.1 shows the circuit. Apart from the Leonardo board, all the parts shown there are fitted to a prototyping shield. There are two sections; on the right is the gesture recognition module, while on the left we have the USB power switching circuit. The wiring to the gesture recognition module is simple enough. It just needs connections from 5V, GND, SDA and SCL to the module, which incorporates the I2C pullup resistors and a voltage regulator to power the onboard chip. We’re using a pair of transistors to switch power to the USB socket’s positive (5V) pin. The ground pin is permanently connected. Pin A0 (which can be used as an analog input) is configured as a digital output. This keeps the wiring on the prototyping shield neat. When A0 is pulled high, about Parts List – Gesture-based USB Lamp (JMP018) 1 Arduino Leonardo [Jaycar XC4430] 1 Arduino prototyping shield [Jaycar XC4482] 1 Hand Gesture Recognition Module [Jaycar XC3742] 1 1kW 5% (or better) axial ¼W (or more) resistor [Jaycar RR0572] 1 120W 5% (or better) axial ¼W (or more) resistor [Jaycar RR0550] 1 PCB-mounting USB Type-A socket [Jaycar PS0916] 1 TIP32 40V 3A PNP transistor, TO-220 [Jaycar ZT2290] 1 BC546, BC547, BC548 or BC549 100mA NPN transistor, TO-92 [Jaycar ZT2154] 1 30cm length of insulated wire in various colours [Jaycar WH3032] 1 USB-A to micro-USB cable to suit the Leonardo board 1 USB-powered light or similar device to control 48 Silicon Chip Australia's electronics magazine 4mA flows through the 1kW resistor and base-emitter junction of the NPN transistor, so it switches on and allows current to flow through its collector to its emitter. This, in turn, causes about 40mA to flow through the PNP transistor’s base via the 120W resistor, which switches it on as well. That means that 5V is available at the USB socket to power a connected device. When A0 is held low, both transistors are off and there is no voltage at the USB socket. It might seem unnecessary to have two transistors, but this arrangement provides enough drive to the PNP transistor to ensure it is switched fully on and does not drop any significant voltage. It also means that our input at A0 is intuitive; a high level swiches the output on and a low level swiches it off. The circuit operation depends on firmware loaded on the Leonardo, which we will discuss later. Construction We built everything on a prototyping shield to create something reasonably robust. You should be able to see how everything is wired up from the photos. We’ve used yellow wires for 5V connections, since red could be difficult to see against the red shield PCB. There is no significant wiring under the shield, so everything is visible from siliconchip.com.au Fig.1: all parts of this circuit apart from the Leonardo board are fitted to a prototyping shield. It supplies 5V to the USB socket on the left when the A0 pin is brought high. above. The only thing to note is that the four wires connecting to the gesture recognition module do so underneath the shield, connecting to the immediately adjacent wire in each case. We’ve positioned the USB socket to make use of the IC breakout pads on the shield. It also means that the USB input (to the Leonardo) and the output (on our shield) are at the same end, making external connections tidier. That puts the gesture sensor at the other end, where it can be accessed easily. Start by soldering the header onto the gesture module and then solder it to the shield. We lifted ours up slightly so it sits just below the top of the shield header sockets. Run the four connecting wires next. There is a yellow wire from 5V on the shield to Vcc on the module, as well as a single black wire for ground (GND) and two blue wires for SCL and SDA. Make sure those are routed as shown. Refer to the photo of the USB sockets that show how we’ve bent the two large tabs outwards. That allows them to be soldered to the top of the shield. The four smaller pins should slot into the pads with a bit of wiggling; the pad spacing is not quite the same as the pin spacing on the socket, but it is close enough. We have left a row of pads behind the USB socket so that wires can be attached there. Add a generous amount siliconchip.com.au of solder to the larger tabs to give them some mechanical strength, then solder the four smaller pins. Next, fit the two transistors, being careful with their orientation. In our photos, from left-to-right, the pins are (for the PNP transistor) emitter, collector, then base, followed by (for the NPN transistor) collector, base and emitter. The photo overleaf shows the wiring most clearly, also check that your wiring matches the circuit. Next, add the two resistors, with the 1kW resistor going from A0 to the NPN transistor’s base and the 120W resistor going between the PNP transistor’s base and the NPN transistor’s collector. Follow with the remaining wires as shown in the photos. Note that some 5V and GND connections are made on pads near the USB socket. They should be marked, but you can carefully follow the copper traces on the prototyping shield to be certain. Finally, plug the completed shield into the Leonardo and connect it to a computer for programming. Software The sketch is quite simple and just needs one external library for the gesture sensing; we used one of the same libraries Jim used back in 2022. It can be installed by searching for “RevEng_PAJ7620” in the library Bend the tabs on the USB socket as shown here (on the right) so they can be soldered to the top of the prototyping shield. This will provide the needed mechanical strength for devices being plugged and unplugged. Australia's electronics magazine January 2025  49 Silicon Chip PDFs on USB ¯ A treasure trove of Silicon Chip magazines on a 32GB custom-made USB. ¯ Each USB is filled with a set of issues as PDFs – fully searchable and with a separate index – you just need a PDF viewer. ¯ Ordering the USB also provides you with download access for the relevant PDFs, once your order has been processed ¯ 10% off your order (not including postage cost) if you are currently subscribed to the magazine. ¯ Receive an extra discount If you already own digital copies of the magazine (in the block you are ordering). EACH BLOCK OF ISSUES COSTS $100 NOVEMBER 1987 – DECEMBER 1994 JANUARY 1995 – DECEMBER 1999 JANUARY 2000 – DECEMBER 2004 JANUARY 2005 – DECEMBER 2009 JANUARY 2010 – DECEMBER 2014 JANUARY 2015 – DECEMBER 2019 OR PAY $500 FOR ALL SIX (+ POST) WWW.SILICONCHIP.COM. AU/SHOP/DIGITAL_PDFS 50 Silicon Chip Follow the wiring to match our circuit (Fig.1). The wires for the gesture module connect to their adjacent pads under the PCB. That is the only wiring on the underside of the shield PCB. You can also see the generous solder blobs that we have added to the USB socket to secure it to the board. manager. Alternatively, you can install the zipped copy we are including with the software package, which can all be downloaded from siliconchip.au/ Shop/6/526 The sketch initialises the sensor. If that fails, the Leonardo’s onboard LED flashes. Otherwise, the software monitors the sensor, switching on the USB output with an ‘up’ gesture and off with a ‘down’ gesture. The Leonardo’s onboard LED also indicates the on/off state, while other debugging information is available on the serial terminal. Some of the values that correspond to other gestures are listed if you wish to change the default behaviour. To program the Leonardo board, open the sketch in the Arduino IDE, choose the correct board in the dropdown menus along with its serial port and then upload the sketch. The sketch and circuit should work without changes with the Uno Australia's electronics magazine R3 board instead of the Leonardo, although we have not tested that. Testing While connected to your computer, wave your hand above the module and verify that the Serial Monitor reports the gesture correctly. We found that moving our hand about 10cm above the sensor worked well. You will also see the Leonardo’s onboard LED switch on and off. Plug a USB device into the socket and confirm that it operates as expected. If all is well, you can plug the Leonardo into a USB power supply to untether it from your computer. The transistor and thus USB socket can deliver about 1A at most; of course, that will depend on your power supply being able to provide enough power. In our testing with a 500mA lamp, there was no noticeable temperature rise in any of the components. SC siliconchip.com.au