Exploring Phenomena with Stroboscopes:
Physics teachers often depend on testing equipment to render certain phenomena visible. Stroboscopes prove valuable in examining vibrating strings and rotating motor components up close. Nevertheless, conventional stroboscopes may not always flash rapidly enough to effectively ‘freeze’ motion for detailed observation. The author firmly believes that a practical hands-on demonstration is more instructive than hours of traditional chalkboard explanations. When it comes to strings vibrating at audible frequencies, these vibrations occur in the realm of a few hundred Hertz. Achieving such a flash rate with a xenon flash tube can be challenging. Flash tubes also tend to be delicate and necessitate high DC voltage applications to make the flash frequency audible.
Transistor T2 is only indispensable when employing the CMOS version of the NE556, a fact worth noting. The second half of the dual timer chip is configured as a monostable multivibrator to fulfill a pulse shaping function. A differentiating RC network, composed of C4 and R1, generates a 10 µs pulse during the positive and negative edges of the output signal from IC1B. The negative-going pulse is transmitted to the trigger input of IC1.A via diode D1. R4 and C6 are selected to ensure that IC1’s operation remains unaffected.
Pulse Generation and Current Control Circuit Description
A trigger pulse input generates a 50 µs positive output pulse, activating MOSFET T3 via resistor R5. The LEDs are powered by a constant current source, where the voltage drop across shunt resistors R7, R8, and R9, along with the forward conduction voltage VF of D2, regulates transistor T1. When the shunt resistor voltage drop reaches 300 to 350 mV, T1 conducts, diverting the drive voltage of T3 to ground, thus limiting LED current to approximately 1 A. C1 effectively reduces the power supply’s source impedance, enabling high current pulses to reach the LEDs, even when powered by four AA size dry batteries.
LED and Current Management Strategy
The author utilized an ‘OSLON SSL LCWCQ7P’ LED from OSRAM, specified to withstand 2 A current pulses lasting 50 ms, without requiring extra cooling when operated with 1 A, 50 µs pulses. This specific LED features an electrically isolated ‘thermal pad’ between its power connections, serving as a potential heat sink contact point. The addition of a heat sink allows the LED to handle higher currents. IC1.A produces constant pulse widths, enabling increased power dissipation by reducing the value of C5 to raise the maximum strobe flash frequency. Experimentation with more powerful LEDs and adjustments to the shunt resistor sizes can also enhance LED current.
Current Handling Capacity and Brightness Considerations
T3 maintains a safety margin, even when dealing with 3 A pulse currents in this application. To achieve maximum brightness, it’s advisable to steer clear of warm white LED variants, as higher color temperatures are perceived as brighter. Using the recommended components and a CMOS version of the NE556, the LED current averages around 20 mA when operated at 650 Hz. With the speaker activated, this figure rises to roughly 40 mA. Opting for the standard non-CMOS NE556 increases the current draw by approximately 5 mA. The author has thoughtfully provided a straightforward PCB layout for this design, utilizing standard non-SMD components for easy assembly. The PCB layout files can be obtained from the project’s website  for download.
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