Electronic Baby Scale That Holds a Steady Reading: Hino & Itani, 1981

Couple the article placing pan, such as a basket for a baby, mechanically to a strain-gauge load cell in the casing. The load cell gives an analog voltage that follows the weight on the pan; an amplifier, its gain set by a variable resistor, raises it.

An analog-to-digital converter turns the amplified voltage into digital data at each pulse from a timing pulse generator.

Pass the data through a digital multiplier. Set the variable resistor so the span is slightly too large, then set a multiplier below 0.9999 on the numerical value setting switch (its two upper digits are fixed at 9, the two lower ones are set) until a known rated weight on the pan reads correctly. The resistor gives the coarse adjustment, the switch the fine one; digital multiplication is not upset by temperature or humidity.

Feed the data into three registers in cascade that shift it on at each timing pulse, so they hold the three previous readings. A coincidence detecting circuit (exclusive-NOR and AND gates) gives a high output when the newest reading equals all three: the weight is steady. (A second version subtracts the previous reading from the newest and calls it steady when the difference is within a set value, which may be zero.)

Load the display buffer with each new reading through an inhibit gate. When the detector reports a steady weight, it sets a J-K flip-flop; the flip-flop's output, through an OR gate, closes that inhibit gate, so the buffer keeps the steady weight even if the baby moves. A digital comparator checks each new reading against a reference value, for example 2 grams: when the baby is lifted off and the reading falls to it, the flip-flop resets and the display follows the new readings again. No button is needed to release the hold.

Drive a four-digit display of seven-segment numerals from the buffer through a segment decoder. Fit two switches outside the casing: one keeps the flip-flop reset, so the newest weight is always shown; the other starts a timer that freezes the display for a set time, say eight seconds, so the weight can be read and written down without a mistake.