Electronic Sand-Glass

EE JULY/AUGUST 1988 05 This electronic version of the reversible 5V sand-glass uses a set of LEDs to simulate the passing of sand grains from the up- per to the lower bulb. The simple to build circuit is accurate enough for most domestic timing applications. The circuit diagram appears in Fig. 1. On power-up, shift registers IC3 and IC4 are reset by the low pulse from network R35-C7. A few seconds later, the sand- glass is started. The oscillator in IC2 generates a clock signal for the shift registers. The clock frequency is adjust- able with Pl. Switch S2 enables selecting one of the three timing periods stated in the circuit diagram. Si is a small mer- cury or ball changeover switch mounted inside the sand-glass. When this is reversed, the switch toggles and so selects the odd or even numbered LEDs. Assuming that Si is set as shown in the circuit diagram, every clock pulse causes a logic high level to be shifted into IC3 , for as long as pin 13 of IC4 re- mains logic low. The MS...