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I have a 7-segment decoder (SN74LS47N) wired to a 7 segment display.

I'm looking for a way for the display to gradually fade out whilst still maintaining its reading for a little while (I don't want it to switch to 0 the moment the power goes out, and slowly fade out).

Here is what I have so far:

circuit diagram

I've tried:

  1. adding a capacitor to the power input of the 7 segment display, and wiring Gnd of the decoder through it
  2. adding a capacitor for each input, where the capacitor is directly wired to Gnd

Neither appears to work. Can I get some insight into this?

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    \$\begingroup\$ You mention one decoder and show a circuit with another. The schematic appears to be from All About Circuits, so you'll need to properly cite it or in this case you should use the schematic editor to create a new one with the decoder and circuit you're actually using. \$\endgroup\$ Commented Nov 8, 2024 at 15:09
  • \$\begingroup\$ don't want it to switch to 0 the moment the power goes out, and slowly fade out What is the desired state once the previous display is done fading? Note that most segments are lit when 0 is displayed. \$\endgroup\$ Commented Nov 9, 2024 at 6:52

1 Answer 1

Reset to default
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The CD4511 has a BJT emitter-follower with an active MOSFET pull-down.

I think to do what you want you'd need to add a diode plus a capacitor and a resistor too, for every segment output. Approximately this, but you'd have to play with the capacitor value.

schematic

simulate this circuit – Schematic created using CircuitLab

The LS47 is a common-anode open-collector display driver with NPN BJT outputs. So you could probably just add a 100Ω resistor in series with a capacitor to each output.

All bulky and messy stuff. Or slide into the future and use an MCU with a few lines of code to PWM the display and make it appear to fade out.

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    \$\begingroup\$ You don't even need to slide into the future; just slide forward from 1970s to 1980s. \$\endgroup\$ Commented Nov 8, 2024 at 15:50
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    \$\begingroup\$ yikes, looks painful to implement. Thank you very much though! \$\endgroup\$ Commented Nov 8, 2024 at 16:17

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