A simple scheme based on the operational amplifier and two LEDs allows you to build a device that imitates the hourglass, which operate as part of a three-minute timer.
The schema contains two LEDs triangular configuration of Panasonic, two operational amplifiers and a few add-on components. To start the timer opens the switch S1. As a result, the led D2 glows brighter and dim D1; this process continues until, while D2 does not open fully, a D1 is locked, noting the expiration of time interval. When this logic output goes from low to a state with a high level.
In addition to hours, this device can be used to display other analog quantities such as voltage or frequency detuning.
Author: Dan Baker; Publication: N. Bolshakov, rf.atnn.ru