OpenPTU: a low-cost, open-source programmable timing unit with sub-microsecond precision for PIV
Abstract
Programmable timing units (PTUs) play a critical role in synchronizing camera and laser hardware for optical measurement of flow, such as particle image velocimetry (PIV). While high-end timing architectures provide exceptional resolution for specialized ultra-high-speed regimes, a distinct need exists for accessible, low-cost timing alternatives tailored specifically for standard laboratory research, educational demonstrations, and classroom deployment. To address this requirement, this paper introduces OpenPTU, an open-source, deterministic TTL triggering environment constructed with widely available components at a total hardware cost of approximately $30 USD. The architecture leverages the ubiquitous 8-bit ATmega328P (Arduino UNO) microcontroller, utilizing its native 5 V TTL capability to interface directly with standard laboratory hardware without external signal conditioning. By bypassing high-level software abstraction layers and executing bare-metal register manipulation of the 16-bit hardware timer (Timer1), the system achieves a deterministic 62.5 ns quantization interval. Oscilloscope characterizations confirm excellent performance linearity ( R2=1.000000) and tightly bounded microscopic random jitter for cycle-to-cycle repeatability of the measured electrical edge separation ( σ=0.8 ns and 3.2 ns for 50μs and 500μs pulse separation intervals, respectively). Uncertainty propagation analysis indicates that the microscopic temporal uncertainty contributes a negligible fraction ( 0.00335%) to the total variance in the representative instantaneous velocity error budget. Furthermore, peak-to-peak macroscopic sampling variations ( 0.3%) restrict temporal phase displacement to just 0.15% relative to the fastest resolvable fluid timescales at the Nyquist limit. Ultimately, OpenPTU provides a reproducible, low-cost trigger-output timing platform for standard laboratory PIV, with electrically measured sub-microsecond precision over the tested operating range.