# 7 segment LED circut

Discussion in 'LEDs and Optoelectronics' started by Rick Whidden, Jul 24, 2015.

1. ### Rick Whidden

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Jul 24, 2015
I have 5 banks of 9 switches each. What I would like to do is if switch 3 on bank one is close then the corresponding LED would read 3, if switch 7 is closed on the same bank (switch 3 open) then the LED would read 7. Only one switch on each bank would be closed at one time. End result would be LED output (example) 38572. I am a beginner but competent.

2. ### Le Sy Hau

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Jul 20, 2015
how many pins of each LED? Your LEDs are BCD type or normal type?

3. ### Rick Whidden

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Jul 24, 2015
I have not purchased any parts yet, looking for advise?

4. ### Le Sy Hau

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Jul 20, 2015
yeah, its different between BCD type and normal type, but I think you should use normal type because it is cheaper and more popular, and the circuit becomes more simple. I assumed that we use COMMON CATHODE LED. First, you need to connect left side of all switch together and connect them to VCC (5VDC or 12VDC). I made a table for you, follow the logic signal in this table, you can build your circuit ease. example: if logic signal is "not(1+2+3)", that's mean you have to OR signals from switch 1, 2 and 3 together, and then NOT them.

5. ### Rick Whidden

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Jul 24, 2015
Very clear, I will be using 4 AA batteries for power with a diode before the circuit to lower the voltage to 5V. Sound easy enough. Thanks so much!

6. ### Le Sy Hau

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Jul 20, 2015
I think you shouldn't use AA battery for your circuit because the resistance of AA battery is quite high, and it's can not supply for long time. You can use Li-on battery, this is best choice for circuits which running on battery, or use 5V DC from USB port. And your 7 segment LED will need a strong power supply, so you can not use battery in this case, 5V DC from USB port is the best choice.

7. ### Rick Whidden

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Jul 24, 2015
USB power is the best choice. Did not think of that option.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
7 segment displays use very low current. AA batteries would be fine. Most operate at just a couple of mA per segment.

What you need to do is wire up diodes from each switch to just light up the segments required. If you're clever, you can wire other contacts (if you have them, so that only one switch is active at any one time).

Fran describes the technique here. In this case, she uses a 4017 which has only one output active at a time.

You could simply wire up your switches as if they were the 10 outputs of the 4017, but turning on 2 or more would likely result in something undesirable. There are ways around this, but you assert that it can't (or won't) happen.

9. ### eetech00

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Nov 17, 2014
Does it have to have 5 banks of switches?

How about one bank of switches (1-9), a set and reset button, and five LED displays?

You would set five switches, then press the set(or reset) button.