A project that includes a stage with three selectable gains (0.1, 1.0, and 10) requires that the rotary-gain switch also function to illuminate one of three LEDs. These panel-mounted LEDs serve to indicate the gain in use. This arrangement requires two switch sections—one for selecting the op amp’s gain-set resistor and one to light the corresponding indicator LED. The circuit in Figure 1 requires only one more switch section to provide a true, two-digit, seven-section numeric LED display. The circuit needs no active driver-decoder circuitry.
The two switch sections for the numeric readout and the section for choosing the op amp’s gain-set resistor are combined in one common three-position, three-pole rotary switch. This configuration uses a common-anode pair of seven-segment display digits. Panasonic LN513RA (one-digit) or LN524RA (two-digit) displays are examples of appropriate devices. Once you understand that the segments are lit in only two groups, the operation of the circuit in Figure 1 is easy to grasp. The group that forms "1" (segments b and c) is continuously lit in both digits.
To change a displayed "1" to "0," you need only power the remaining four segments in either the left- or right-hand digit, according to the gain-setting switch position. You use the decimal point only in the 0.1 and 1.0 positions. The center section (g) never receives power in either digit. Permanently grounded resistors provide a current path to keep segments b and c ("1") always lit. The grounded rotor in switch section A provides a current path for segments a, d, e, and f to form a "0" in either display. Section B lights the decimal point. Resistor values depend on the supply voltage. Typical values are 820 ohms for 15V, 680 ohms for 12V, and 180 ohms for 5V. (DI #1799)