IntermediateDisplays & Logic
3-to-8 Logic Decoder LED Panel
Decode a three-bit switch setting into one of eight active-low outputs and visualize the selected channel.
Parts from the database
- SN74HC138N × 1Three-to-eight decoder
Other supplies
- breadboardCommon build supply
- 5 V supplyCommon build supply
- three switchesCommon build supply
- three 10 kΩ pull-downsCommon build supply
- eight LEDsCommon build supply
- eight 1 kΩ resistorsCommon build supply
- 100 nF bypass capacitorCommon build supply
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Connections
- DIP-16 pin 16 +5 V, pin 8 ground; bypass. Address A0/A1/A2 are pins 1/2/3; each switch goes to +5 V with a 10 kΩ pull-down.
- Enable with G1 pin 6 high and /G2A pin 4 and /G2B pin 5 low.
- Outputs Y0–Y7 are pins 15,14,13,12,11,10,9,7. Connect each output to an LED and 1 kΩ resistor in series from +5 V; the selected low output sinks current.
Build steps
- All address switches low should select Y0.
- Change each switch and record the selected output.
- Test all eight binary patterns against a 3-to-8 truth table.
- Disable one enable input and verify no output remains selected.
Software and firmware setup
This is a hardware-only circuit; no firmware is required.
Test your build
Exactly one output goes low for each binary address when all three enable conditions are met.
Troubleshooting
- No LED: verify G1 high and both /G2 inputs low.
- Wrong channel: check A0/A1/A2 order and output pin numbering.
- Random address: repair input pull-downs.
Safety and limitations
Use 5 V and a separate resistor for every LED; keep current within the HC138 output rating.
Manufacturer and platform documentation
Community Notes for This Project
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