Engagement of students in traditional engineering tutorials can be low, especially where the level of preparation varies widely across the student population. Online tutorials are a way of addressing this problem, as they offer the chance for students to work at their own pace, at their own preferred times, while staff can add and update questions, links, and hints in almost-real time. We created such a set of tutorials in an introductory electronics course, incorporating a strong Threshold-Concept focus. The tutorials were coded by one of us (Balsom) in PHP, and this allowed us to extensively and flexibly control reporting to examine student usage. We benchmarked students from year to year, introduced the eTutorials, and measured their impact. We employed surveys and interviews for additional feedback. We quantitatively and qualitatively address the question of how effective the eTutorials were in comparison with well-staffed, well-attended, conventional tutorials addressing the exact same material in the previous year. We also search for correlations between student usage and eventual grade with the aim of early detection of students requiring intervention. The cost is compared with use of commercially-available eTutorials in Physics that are used by a parallel introductory Physics course in the same school.