7-Segment LED Display Hookup

[KROLRADIO]

I am building the Digital Tachometer in May 2006 issue of N&V.I never hooked up these 7 segment displays before.Pins 3,5,11,6 are listed as common anodes on data-sheet.

Do ALL of these pins (3,5,11,6) go to the 220ohm resistor & 5 volts in schematic?
What are pin 6-cathode LHDP & pin 9-cathode RHDP used for?

Thanks,
KLR

[dyarker]

"What are pin 6-cathode LHDP & pin 9-cathode RHDP used for?"

LHDP = Left Hand Decimal Point
RHDP = Right Hand Decimal Point

[k7elp60]

To insure proper operation of the display connect all the common anode pins together and to the 220 ohm resistor.

I have found on some displays all the common pins are connected internally and don't require the external jumpers. You could test one by
connecting one of the common anode pins thru the 220 ohm resistor to
+5v and the individually connecting each of the segements a thru g to ground to see if they light up.

[KamPutty]

To insure proper operation of the display connect all the common anode pins together and to the 220 ohm resistor.

I thougth that sharing a single res for all the segments could exceed the res value if the segments where not multiplexed.

Meaning, if I turn on only 1 segment at a time, the 220res would work. But if I where to turn on 2 segments at the same time, now the res value needs to be different.

I have always put each segment with its own res (rightly or wrongly!)

~Kam (^8*

[k7elp60]

I agree with KamPutty in that there should be a current limiting resistor for each segment of the displays, however the author of the article chose not to do so and so stated. He chose to save space.

If I were to build the circuit I would eliminate the 220 ohm resistor connected to the common anode and put individual resistors from the segments of the displays to the 74LS47 output pins.

[dyarker]

My May issue has disappeared already.

If there are four digits in one unit, then the the four common anode connections are for each digit. Don't tie them together, they are for multiplexing.

One resistor for each cathode.

To multiplex. Turn on anode for first digit, turn on cathodes of segments needed to show first digit number, turn off cathodes, turn off first digit anode; turn on second digit anode, turn on cathodes to show number in second digit, etc, etc, etc. Anything higher than like 100 digits per second, and flicker won't be visable.