The afterglow time of this phosphorescent material varies with ambient light conditions. Best performance is achieved under normal room light.
The original design of the sand clock was modified to fit a laser module running on 3-5 V. A switch allows starting the time writing sequence at will rather than at a preset interval.
By the way, if you already have your sand clock and if you install the most recent software for this device, you can choose your own refresh lapse and add a button, so your sand clock writes the time just when you want.
This upgrade-kit with all parts required to transform your sand clock into a laser writer is available in your Elektor Store, together with a detailed construction manual.