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Mini Projects #009 – by Tim Blythman SILICON CHIP Stroboscope and Tachometer Stroboscopes and Tachometers are handy tools for measuring how fast an object like a flywheel is spinning. This Stroboscope/Tachometer is easy to build from a few Arduino modules and other parts. Warning: flashing lights, particularly in the lower frequency range from about 5Hz (300RPM) upward can induce seizures in people subject to photosensitive epilepsy. Flashing lights can also trigger a migraine attack. We recommend that people prone to these effects avoid stroboscopic lights. S troboscopes are devices that use a rapidly flashing light source to help observe a rotating object. If a light is flashed at the same rate as the object is rotating, the object is lit at the same location on each rotation. In this case, human persistence of vision means that the object appears stationary. This lets you observe something spinning too fast to see. Also, if you know the flash rate when the object appears stationary, you can estimate the rotation rate. Another way to measure rotation speed is with a fixed light source and a light sensor. The light sensor detects the light changes as the object rotates; a reflective sticker is often applied to assist this detection. Measuring the time between rotations allows the rotational speed to be calculated. Such a device is called a Tachometer. This project combines a Stroboscope and a Tachometer into one simple device. As it is based on an Arduino Uno, it is easy to modify and experiment with. We published a more advanced version of this device in the August and September 2008 issues of Silicon Chip (siliconchip.au/Series/52). We also produced a Strobe to check the speed of record turntables in December 2015 (siliconchip.au/Article/9640). This simpler design could perform many of the same jobs. Hardware We built our prototype using an Arduino Uno mainboard and a Jaycar XC4454 LCD Shield. Since the shield has pads to break out unused I/O pins, we simply soldered the required components to those pads on the shield. Parts List – Stroboscope (JMP009) 1 Arduino Uno R3 main board [Jaycar XC4410] 1 Uno-compatible LCD Keypad shield [Jaycar XC4454] 1 5mm white LED [Jaycar ZD0290] 1 3mm infrared (IR) LED [Jaycar ZD1946] 1 IR photodiode [Jaycar ZD1948] 1 100kW ½W 1% metal film axial resistor [Jaycar RR0620] 2 220W ½W 1% metal film axial resistor [Jaycar RR0556] 5cm length of 5mm diameter heatshrink tubing [Jaycar WH5553] 1 USB cable to suit the Arduino Uno [Jaycar WC7701] 58 Silicon Chip Australia's electronics magazine The LCD Shield also includes several tactile pushbuttons, so we have everything we need for a complete user interface. The character (alphanumeric) LCD on the shield is driven in four-bit mode by pins D4-D7 of the Uno, with D8 and D9 providing the RS and E signals, respectively. The pushbuttons are connected to a resistor chain that sends a different voltage to the A0 analog input, depending on which buttons are pressed. Fig.1 shows how to wire up the external components. At the top, the white LED connects between D12 and D11 with a 220W resistor in series. This makes up the Stroboscope, with the processor driving D12 to control the flash rate. D11 is permanently held low to create a convenient alternative to a ground connection. The IR LED is powered by the 5V and GND pins, so it is always on. Its job is to provide an IR light source for the IR photodiode to detect. With the arrangement we are using, the photodiode behaves somewhat like a solar cell, generating a voltage on its anode relative to the cathode. Since the photodiode behaves more like a current source than a voltage source, a parallel resistor is siliconchip.com.au provided to turn the current into a voltage that the ADC peripheral of the Uno can measure. We use a photodiode as they can respond faster than devices like LDRs. For this project, we have used an Arduino Uno R3 as other processor boards like the Arduino Leonardo use their processor to handle their USB interface. Since the Uno has a separate USB interface chip, it has fewer interruptions, making it better at managing the precise timing needed in this project. Fig.1: practically all the wiring is done by soldering components directly to the LCD Shield. You can also see the connections we’ve made in the photos. Software The software consists of an Arduino sketch and two libraries. The library to drive the LCD panel is included with the Arduino IDE, while an external ‘TimerOne’ library is used to manage the strobe timing. The sketch sets up a timer interrupt to drive the white LED with a duty cycle of 10% (ie, off for nine times longer than it’s on) at a rate you can control. The strobe can also be switched off. Professional strobes use a much lower duty cycle at a higher power level to more accurately ‘freeze’ the view. The sketch also samples the photodiode voltage at 10ms intervals (100 times per second), then calculates and displays a rate based on the time between detected pulses. The display can be set to RPM (revolutions per minute) or Hz (revolutions per second) for both the Strobe and Tachometer. The Stroboscope/ Tachometer uses a white LED for the Stroboscope; the strobe rate can be set by pushbuttons. The Tachometer consists of an IR LED and photodiode to sense changing light reflection due to rotation. Assembly Start by soldering the LEDs to their 220W resistors by cutting each anode (longer) lead short. Cut down one lead of each 220W resistor to a similar length. Solder the resistor to the LED and use a few centimetres of heatshrink tubing to cover the resistor. You can then solder the LED assemblies to the LCD shield as shown. The white LED connects between the second and third pads at the top of the shield, with the cathode on the third pad. The IR LED (which is blue) is wired between 5V and GND, with its cathode to GND. Trim any excess lead length from these components. Solder the 100kW resistor between the other GND pin and A1; it should be a comfortable fit. The longer anode lead of the photodiode is also soldered siliconchip.com.au Australia's electronics magazine August 2024  59 to A1, with the cathode going to GND. The active area of the photodiode is the curved lens, so bend its leads to point the lens in the same direction as the IR LED. Finally, plug the LCD shield into the top of the Uno and hook it up to your computer for programming. You should see the power LED on the LCD shield light up. Programming You can download the Arduino sketch for this project (siliconchip. au/Shop/6/448). We have included a copy of the Timer­One library with the sketch download, but it can also be installed by searching for “timerone” in the Arduino Library Manager. Use the Arduino IDE (download from www.arduino.cc/en/software) to upload the sketch to the Uno, being sure to select the correct port and use the Uno board profile. Screens 1, 2 & 3 show some of the typical displays. Using it Screen 1: this help screen can be seen when the SEL button is held down. The SEL button also toggles between the RPM and Hz displays, shown in Screen 2 and Screen 3, respectively. Screen 2: pressing the LEFT button should cause the white LED to start flickering. You can change the rate with the UP and DOWN buttons. Screen 3: the RIGHT button will change the steps by which the rate is changed. This is always shown in RPM, even if Hz is selected as the unit. To use it as a Stroboscope, shine the white LED at a rotating object and adjust the rate until the object appears stationary. Remember that the object will also appear stationary if the rate is a fraction (eg, 1/2 or 1/3) of the rotation speed. The correct rate is the highest rate at which the object appears stationary. When using the Stroboscope, remember that the object that appears stationary might not be! This can be dangerous if that object is machinery, as you might be tempted to touch it. So take great care when using the Stroboscope near running machinery. The Tachometer is used by aiming the IR LED and photodiode at a rotating object and reading out the value in the lower-right corner of the LCD screen. You should be able to get a reading of a few Hz or a few hundred RPM by waving your hand a few centimetres in front of the LED/photodiode. If you don’t get a good reading, check that the IR LED is emitting by pointing a mobile phone camera at it. The camera should show a red or purple glow that isn’t visible to human eyes. Other IR emitting sources (eg, remote controls) might cause interference, so keep the unit away from them. Remember that objects like fans with multiple blades will cause multiple events per revolution, so you may have to account for this in your calculations. One way around this is to place a piece of reflective tape on the object so that you can easily pick up one event per revolution. Summary Assembly of the Stroboscope involves plugging the modified LCD Keypad shield into an Arduino Uno (shown above). 60 Silicon Chip Australia's electronics magazine The Stroboscope/Tachometer is a simple and handy tool for checking the speed of rotating objects. It may not be the best tool for calibrating heavy machinery, but we think it would be a convenient way to check if your record turntable is spinning at the correct rate, for example. To check a turntable rotation speed, you also need a separate strobe disc with markings. SC siliconchip.com.au