Electronic Design

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.
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