Attendance counter

  
The display shows each increment. By using mode 2, external debouncing of the gate switch is unnecessary, provided the switch bounce is less than 35ms. The 3 V lithium battery can be replaced without disturbing operation if a suitable capacitor is connected in parallel with it. The display should be disconnected, if possible, during the procedure to minimize current drain. The capacitor should be large enough to store charge for the amount of time needed to physically replace the battery (t = VC/1). A 100 ^F capacitor initially charged to 3 V will supply a current of 1.0 µ A for 50 seconds before its voltage drops to 2.5 V, which is the minimum operating voltage for the ICM7249.
Attendance counter - schematic

Before the battery is removed, the capacitor should be placed in parallel, across the VDD and GND terminals. After the battery is replaced, the capacitor can be removed and the display reconnected.




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