post office letter scales

7-80 — elektor july/august 1981 K. Hense post office letter scales voltage regulator, IC1. For this reason, the regis- tered tariffs are reasonably independent of changing voltage supplies (a falling battery voltage). Similar to the mechanical version, the electronic scale will indicate five different rates (see table) by means of LEDs. The relevant LED will light up depending on the weight of the letter placed on the scales. The LEDs are activated by opamps Al ... A4, which are P connected as comparators. Potentiometers he non- are used to preset the "target" voltage at t inverting inputs of the comparators. The actual voltage, which is a measure of the weight of the letter is fed to the inverting inputs via the light dependent resistor (LDR), R3. Depending on the amount of light radiated by the yellow LED, D7, actually falling on the LDR, a certain voltage will appear at all four inverting inputs of the comparators. When this voltage is identical to, or greater than, that pres...