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7 Segment LED display

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noltex

New Member
I need a circuit that will allow me to display digits 0-9 on a 7 segment LED that it switched by a 10 position rotary switch. Each position on the rotary switch corresponds to a digit. I'm assuming I'll need a 10 line to 4 line encoder and a bcd to 7 seg/decoder/driver. I just need help with a schematic to make it happen.

Jeff
 

Pommie

Well-Known Member
Most Helpful Member
In the olde days this would have been done using diodes. You connect a diode from each contact on your rotary switch to the required segments on the display. To limit current, you tie together the segment ends of the diodes and insert a resistor from that point to the display.

Mike.
 

ronsimpson

Well-Known Member
Most Helpful Member
Get the data sheet for these parts. 74hc147 & 74ls47
You can get the data from digikey.com
See what you can do on your own.
 

yngndrw

New Member
I think what he was meaning was even if you can't design the full circit, see what you can do your self. You know what a block diagram is ? Make one. (OR Google it if you don't)

Every system has an input, process and an output. You input is the encoder, your process is what you're trying to make, and the output is a seven segment display.

First you need to decide hat input and output signals you have / need - This includes what format they are in.

You've been given the part numbers for what IC's you need, so google the datasheets for them and see what they are. Then develop a block diagram. All the system is doing is converting one data format into another.

After you have the block diagram, start converting it into a circuit by using the datasheets.

Then we can help you tie up the loose ends. :)

Hope that helps.
 

noltex

New Member
I'm fairly certain I understand the circuit for putting the encoder and the BCD decoder together. I've looked at the data sheets and I'm well beyond the block diagram stage. My specific problem is how a 10 position switch needs to be wired to the encoder to obtain the required input to obtain the desired output at the display. Essentially, converting the truth table into a circuit for a 10 position switch.
 

kchriste

New Member
Forum Supporter
Do you have the rotary switch yet? If not, then buy a BCD coded switch and use a display encoder like the 74LS47 etc. If you really like wiring, then Pommie's diode idea will be best. :D
 

noltex

New Member
kchriste said:
Do you have the rotary switch yet? If not, then buy a BCD coded switch and use a display encoder like the 74LS47 etc. If you really like wiring, then Pommie's diode idea will be best. :D
You, sir, may be a genius! I wasn't aware that BCD coded switches existed. That should work perfectly. Now, I just have to find one that isn't a DIP miniature, since my application needs a standard size that I can mount a knob on. Even if not, I'm pretty sure I can make do.

A huge thanks!
 

mike11298

New Member
kchriste, when you say BCD codes switch are you refering to one of those thumbwheel switched with the little window displaying the number? I think noltex needs a rotary switch to show the values.

Noltex, definently check out those datasheets, expecially the 74HC147. That pretty much does it all for you. 74LS47 is very much a modern 4511, if you have them instead.
 

noltex

New Member
My problem with the data sheets are that it only specifies an input, but I don't know what the input should be. I haven't found a huge variety of BCD switches yet, since most seem to be of the PCB DIP type. However, there are a few with shafts. Let me know if anyone knows a source for standard size.
 

kchriste

New Member
Forum Supporter
Cannon has some, probably soon to be discontinued:
**broken link removed**
You could also take a 4 pole 10 position switch and wire it for BCD output on the 4 common terminals. Can get expensive though:
https://rocky.digikey.com/WebLib/ElectroSwitch/Web Data/C4 Series.pdf
kchriste, when you say BCD codes switch are you refering to one of those thumbwheel switched with the little window displaying the number?
No, actual rotary switches. You don't see them used in new designs because they are relatively expensive.
 

mike11298

New Member
The chip represent a BCD number in relation to what the inputs are, in order of their priority A8 has the highest prority. EG: if there is an input (active low, remember) at A2, then the BCD will display a three (in BCD) on the outputs. If there is an input on A2 and A5, A5 has priority (it is the one that will be displayed) because it is higher up.

Just a note.. I have never used this chip before, in fact, I hadnt even HEARD of it before now! But.. This is what I am guessing. If I were you, go buy the chip (if you can even find them.. Dec 1990!!) and whck it into your breadbord and see how it works. If you need more of an explanation, ask, and I shall attemp to help.
 
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