Freeze Scanned Seven-Segment LED

By using an optically coupled signal-averaging low-pass filter, a continuously scanned seven-segment LED can be easily frozen into a continuous seven-segment output (see the figure).

A seven-segment driver drives the internal LED of the optocoupler, which indirectly makes the transistor switch discharge the state hold capacitor C₁. When the segment LED is off, the optoswitch also remains in the off state. C₁ stores the charge through R₂, and the output S1 goes high, representing the segment LED status.

On the other hand, when the segment is on, glowing at the rate of a segment scan clock, the optoswitch discharges the capacitor. R₁ may have to be adjusted to optimize the capacitor charging rate with respect to the scan clock. At slower scan-clock rates, a larger value of R₂ may be needed.

Three advantages can be derived from this design scheme:

1. The output is floating with respect to the segment input due to the optoswitch. Hence, this circuit can handle a range of input signal levels (R₁ should be increased when the input is from a larger voltage source).
2. The output is readable by ordinary devices because the circuit is isolated from the scan clock. It can also handle a large variation in the scan clock, therefore the circuit remains virtually independent of the clock.
3. No synchronization is needed to retrieve the information.