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Mini Projects #014 – by Tim Blythman SILICON CHIP Analog Pace Clock & Stopwatch Despite the commonality of digital clocks, analog Pace Clocks are still prevalent, being used for purposes like timing swimming laps. This version is driven by a microcontroller, so it can be started and stopped and even used as a stopwatch. As well as the mounting and wiring arrangements, you can also see the jumper wire connected between D8 and ground to force the Pace Clock to operate when it is powered on. If wiring up the switches, they connect to D8-D12 and GND. P ace Clocks are used by swimmers for training and practice. It is claimed they were invented on Sydney’s Northern Beaches, just up the road from the Silicon Chip office. A pace clock has a solitary hand, and the face is marked out in seconds, making it easy and quick to check lap times. It is usually marked with a 60 (or zero) at the top and a 30 at the bottom. The hand sweeps one revolution per minute, allowing times to be measured by simply glancing at the clock at the end of each lap. It would be easy to make a Pace Clock using just the second hand of a standard quartz analog wall clock, but using a stepper motor and a microcontroller gives us several advantages. Firstly, the stepper motor is much more powerful than the motor in a quartz clock, so the Pace Clock can be built with a bigger face and longer hand, making it more visible. Secondly, adding a microcontroller means it is possible to turn the Pace Clock into a stopwatch. Our Pace Clock can be started, stopped and reset. Since this can be done using digital inputs on the microcontroller, you could even Parts List – Pace Clock (JMP014) 1 5V Stepper Motor with Controller [Jaycar XC4458] 1 Arduino Leonardo main board [Jaycar XC4430] 6 male-female jumper wires [Jaycar WC6028] 1 male-male jumper wire [Jaycar WC6024] power supply for the Leonardo (eg, a USB power supply & micro-USB cable) 2 M4 × 15mm panhead machine screws [Jaycar HP0453] 4 M4 flat washers [Jaycar HP0465] 2 M4 hex nuts [Jaycar HP0462] Nylon M3 screws, nuts and washers to mount the Leonardo & stepper driver 5 SPST momentary pushbutton switches (optional) wire to connect switches to Leonardo board (optional) 1 sheet of cardboard, Corflute or thin ply 1 printed clock face (see text) 1 clock hand (eg, cut from cardboard or 3D-printed) space-filling glue such as epoxy, hot melt glue or neutral-cure silicone sealant siliconchip.com.au Australia's electronics magazine use it for automated race timing with the right accessory hardware. You can see a short video of the Clock in operation at siliconchip.au/ Videos/Pace+Clock The Leonardo board we are using (like many Arduino-compatible boards) has a crystal oscillator, so it will be pretty accurate, typically within 50ppm; that’s certainly accurate enough for an analog clock that is meant to be read by eye. We’ll detail the construction of our prototype, which is based on a clock face around 20cm across (allowing it to be printed on A4 paper). But you should have no trouble scaling up your version to be larger if needed. Circuit details Fig.1 shows the circuit. The stepper motor and its controller are on the right and are connected by a harness terminated with a polarised plug, so it can only plug in one way. The boxed area shows the parts on the stepper motor control module. Note how they connect to the motor and the Arduino Leonardo microcontroller board. This stepper motor has four windings, each of which is positioned in conjunction with an arrangement of fixed metal teeth. When energised November 2024  59 ALL DIMENSIONS ARE IN MILLIMETRES sequentially, they attract the corresponding teeth on the rotor. Because the teeth are positioned at intervals, the motor’s position can be set quite accurately. The motor we are using has 32 teeth and is also connected to a 1:64 reduction gearbox. That is equivalent to a simple stepper motor with 2048 teeth, which is more than enough to count out seconds with precision. The driver IC is a ULN2003 chip with open-collector Darlington transistor outputs; only four of its seven channels are used. The ULN2003 pulls its outputs to ground when the corresponding input is driven high. In addition to the connections to the motor windings, there are four LEDs with series resistors. Their anodes are connected to 5V and the cathodes to the outputs, so the LEDs light up as each winding is activated. Since the stepper motor is a unipolar type, this simple control system works well. A bipolar stepper motor type would require a more complex circuit, such as an H-bridge, that can drive positive and negative voltages. 60 Silicon Chip Six jumper wires provide power and control signals from the Leonardo to the stepper motor controller. For more information on stepper motors, see our primer article (January 2019 issue; siliconchip.au/Article/11370). To build a functioning circuit, you need only the first five items in the parts list. You might like to test and assemble them first to get a feel for the stepper motor’s operation. Assembly We tested our prototype with an Arduino Leonardo, so we know that it works, but just about any Arduino board with enough pins should also work. The functions of the pins are set by #defines in the sketch so that they can be changed if needed. The motor draws around 100mA, which any Arduino board can supply from its 5V pin. Connect the six jumper wires between the Leonardo and driver board as shown in Fig.1. For testing, connect an extra jumper wire between D8 and ground; that will start the clock when it is powered on (you can see this Australia's electronics magazine in our lead photo showing the back of the Clock). Software You’ll need the Arduino IDE software to upload the sketch to the board. It can be downloaded from: siliconchip.au/link/aatq Once it is installed and running, choose the Leonardo board and its corresponding serial port in its menus. Download the PACE_CLOCK sketch from our website (siliconchip. au/Shop/6/486), open it in the IDE and upload it to the Leonardo board (Ctrl-U). The default sketch scans all the switches shown in Fig.1; they are simply ignored if they are unconnected. You should see the LEDs on the driver board start to move in a quick sequence. Connect the motor and it should start to rotate at 1 RPM. If the motor buzzes or hums without turning, check that all the wires are connected and in the correct sequence. You can test the other switch functions by disconnecting the jumper wire from D8 and touching it to each siliconchip.com.au Fig.1: the driver module and pluggable wiring harness make this a very easy project to build, at least electronically. Take care with the wiring between the Leonardo and the driver module and you should have no trouble getting this circuit up and running. Fig.2: the dimensions for mounting the stepper motor. The larger 9mm hole accommodates the boss that protrudes from the body of the motor; the shaft is only 5mm in diameter and 3mm across the flats. The centre of the 9mm hole also marks the centre of the clock face. of D9-D12. The function pins are set out in the sketch. When the Start switch is pressed, the clock begins delivering the sequence to the control board needed to advance the clock hand. When the Stop switch is closed, the sequence is paused. Pressing Reset causes a faster sequence to be generated in reverse order, which rewinds the hand to the zero position. All we need to do this accurately is to keep count of the steps that the hand has moved. The Trim+ and Trim- buttons only work when the hand is stopped. They move the hand forwards and backwards at a moderate pace, to allow it to move to a new zero position, such as when first powered on. This also resets the step number. Clock face An online image search for “pace clock face” gave us many samples that could be printed out for use on the Pace Clock. We simply printed ours on a sheet of A4 copy paper and glued it to a piece of cardboard. siliconchip.com.au Our clock face was printed on a sheet of A4 paper and glued to a piece of cardboard. The metal screws retain the stepper motor while the plastic screw heads are for the Leonardo and driver module; the positions of the latter are not critical. If you want something a bit more polished, Bunnings has sheets of white Corflute (corrugated plastic sheet) that would also work quite well as a baseboard. We used Fig.2 to mark out the holes we would need and carefully cut them out with a sharp hobby knife. Wad punches would work quite well if you have them. Use M4 machine screws to mount the stepper motor to the board, with just the shaft poking out the front. The driver board and Leonardo have 3mm mounting holes, so they can be mounted with the Nylon M3 hardware. The positions are not critical; we recommend placing the Leonardo near the bottom of the clock so its power lead can hang down. We also designed a 3D-printed hand and a bracket to help mount a custom-­ built hand to the stepper motor’s shaft. These are available as part of the software download. We printed ours on a resin printer, and you can see them in our finished clock in the photos. A simpler approach would be to use a cardboard cutout for the hand. When Australia's electronics magazine The hand on the right has a socket on its underside, like the bracket at lower left, that makes it a friction fit to the stepper motor shaft. You might like to add some glue to help secure it. November 2024  61 gluing the parts, apply the glue and then rest the clock face-down. That will prevent the glue from running back up the shaft and into the workings of the motor. Customisation Our Pace Clock is not waterproof at all, so you will need to install it in a waterproof enclosure for use around the pool or at the beach. For simplicity, we left the control switches off our prototype. You could mount the Trim switches on the back since they won’t be used often. You could mount the Stop, Start and Reset buttons remotely, so that they can be controlled from a convenient location. If you want to make your Pace Clock more robust, something like the XC4482 prototyping shield could be used to mount the wires and switches. A few #defines in the sketch can be used to customise the Pace Clock. The PERIOD #define sets the time for one revolution and could be changed if you wanted a different period (eg, 30 seconds or two minutes). We have seen some Pace Clocks at swimming pools with dual opposing hands of different colours, so you The Palm Beach Scientific Training Group poses with the world’s first swimming pace clock at the Palm Beach rock pool north of Sydney, Australia. Source: https://swimswam.com/history-swimming-pace-clock/ Songbird don’t have to wait as long for one to reach the zero. If you need the clock to operate anti-clockwise, use the ANTICLOCKWISE #define in the code. An easy-to-build project Unfortunately, this will not make time run backwards. As noted earlier, the nine I/O pins that are used are also set by #defines, so you can change them SC too if you wish. that is perfect as a gift. SC6633 ($30 plus postage): Songbird Kit Choose from one of four colours for the PCB (purple, green, yellow or red). The kit includes nearly all parts, plus the piezo buzzer, 3D-printed piezo mount and switched battery box (base/stand not included). See the May 2023 issue for details: siliconchip.au/Article/15785 62 Silicon Chip Australia's electronics magazine siliconchip.com.au