A self-hosted service that pings webhooks or other URLs on a user-defined schedule. Works a little bit like cron. Can even do things every X minutes or hours, like cron.
Written in PHP, uses SQLite.
If you don't want to set it up yourself: https://hookless.co/
A website that takes a parrticular frequency (in KHz) and a duty cycle (at least 50%, or on half the time, off half the time) and calculates the values of the two resistors and the capacitor required to generate that frequency using a 555 IC.
3743 links, including 197 private