Large 7 segment display

youngbill

I want to make a large 7 segment display,( aprox 2 inch high). I plan six leds per segment. Should they be arranged series or parallel?

Any ideas on how to drive the display? Once this section is built ,I would use a stamp to display various numbers,,ie position of a pot etc,

To display a "pot, position ,could i not do away with the stamp a together?

Jessica Uelmen

Hello!

This thread is being moved from the Stamps in Class forum to the BASIC Stamp forum, as it is sure to get more responses there.

-- Jessica

Franklin

http://www.nutsvolts.com/index.php?/magazine/article/build_the_vex_two_digit_scoreboard

youngbill

Does this show the wiring of the leds?,,,Also can anyone help with the hookup wiring of either a electronic relay ( or mechanical ) or transistor, to drive the display

Irone

How Ya Doin'? If you want six LED's for a bar you can put them in series if you have enough voltage. A red or green LED takes about 1.7 volts each. with six of them you will need at least 10.2 volts. With twelve volts you have to use a resistor to take the amperage down or you will burn up the LED's. 1.8 volts divided by .010 amps leaves a resistor of 180 ohms. Wattage is voltage (1.8) times amps (.010) which equals .018 watts which is well below .125 which is an 1/8 watt resistor. You will need seven bars for the display and each will be controlled by an output. Six times .010 times seven equals .420 amps necessary to light up an 8 on your display. You must have more power which means you must use a transistor with the base connected to your output pin of a transistor and the alternate supply running through your LED's through the transistor and connected to ground. Different types of transistors vary but if you look up the datasheet of whatever transistor you are using and go to http://www.daycounter.com/Calculators/Transistor-Switch-Saturation-Calculator.phtml it will tell you the resistor needed for the base.

Phil Pilgrim (PhiPi)

There's also nothing wrong with a series-parallel connection if you don't have the voltage headroom for an entire string in series. Each series strand requires its own current-limiting resistor. Here's a sample schematic:



-Phil

youngbill

Would it be safe,to use series parallel at 5 volt supply ,,Three in series,,and no resistor? I think it would be SAFER to go to 12 V supply and use a resistor.

Has any of you seen these new strips of white led ribbions,,they have 3 leds in series..then parallel ,and come aprx 15 feet long.
But from what i have seen, they run very hot,,,I believe the heat is coming mostly from the resistor,,,why don't they just put more(say 7leds in series for 12 V sys) in series and get rid of the inline resistor all together?
Thanks 4YR.

Mike Green

You have to have a resistor in each series leg. LEDs conduct any time the voltage across them exceeds their forward voltage (Vf) and there's nothing to limit the amount of current except when the LED or its internal wiring melts. For most red LEDs, Vf is 1.7V. That means that the total Vf of a string of 3 LEDs is 5.1V. This won't light if you only apply 5V. If you apply more than 5.1V, the LEDs will take all the current your power supply can furnish. They'll get hot, then one of them will burn out.

The series resistor limits the current to whatever the LEDs are designed to handle. The resistor drops any additional voltage according to Ohm's Law.

For a string of 3 LEDs and a 6V supply, the resistor needs to drop 6V - (3 x 1.7V) = 0.9V. If the LEDs are designed to operate at 20mA maximum and you want them to work at that current, you need a resistor of 0.9V / 0.02A = 45 Ohms. The nearest standard value is 47 Ohms.

Irone

How Ya Doin'? Different colors of LEDs are made up from different materials. The ordinary ones used are red, green and yellow. These take from 1.7 thru 2.1 or so volts each to light up. There are low power LEDs and high intensity LED's. There are infrared LEDs, blue LEDs and white LEDs. If you have a meter with a diode function you can use it to find out the correct foreward voltage of an LED if it under 2 volts.
There is a chart at http://www.oksolar.com/led/led_color_chart.htm that gives you the foreward voltage for most LEDs. Who knew just lighting a crummy LED could get so complicated?

youngbill

Yes that makes so much sense,yet sometimes we skip small details,Thanks.

Now I have the load figured to apro 30 Ma. And I want to use a 74ls47 bcd to 7 seg decoder,,but it sinks( open collector op I believed its called)..So if i pick a transistor,,it must be PNP,correct?

Next qustion is what spec should i be looking at to pick a transistor. Ie I want to go with a 5 amp tip 125 or 3 amp tip30B or a 600Ma pn2222 (in a pnp equal) What spec do I use to decide Ie,,,,whats the danger of using TOO large a transistor?
Will it NOT saturate,,,will i need to try one and test it C-E voltage to see if its saturated enought?...What voltage limit ,,( 12 volt system) should I then say,,,its not in saturation?
I only have npn transistors,is there a solution to use them with a 74ls47?
I have a few p channel IRF9540 mosfets,but I have no experience with them, and would like to stay with transistors,unless someone could guide me in that direction. Thanks

youngbill

Ahh ,,,also If I use a IRF9540 which needs a negative gate will the output of the 74ls47 (ie a low ) be enought of a negative to operate the mosfet?

Should there be a resistor between the two components?

many thanks,I know this is basic stuff but I used the max chip with multiplexing and found adding the mosfets etc to add to that circuit (to drive large displays too complicated)..

Irone

How Ya Doin'? I looked up a data sheet for a 7447 IC and found out is has to be run by 5 volts. The maximum amperage it puts out is 25 milliamps. This will light only two red LEDs. I believe it was made only for seven segment displays, not enough power for six LEDs at the same time. The data sheet is kinda old, but I am also old. Are you trying to save I/O pins on your board? I am only asking because it takes four outputs to run with a 7447. With three more outputs you could run it with your Parralax board using chapter six of "What's a Microcontroler". If you need a decimal point you need another pin.

If you are only use the transistor to ground your LEDs a 2N2222 can sink 800 milliamps and would need a 5.6K resistor for the base. I was going to get a breadboard out and see if I made a mistake calculating the resistor but my wife says it is getting late and I have to quit playing and come to bed. If you want to learn more about basic electronics try going here: http://www.play-hookey.com/

Also let me know what voltage you are using and how your LEDs are lined up.

youngbill

I will use a 12 volt system with a 7805 to run the 7447,
The leds will be 3 in series with 330 ohm R..paralleled to another like wise. Thanks

youngbill

I was planning on using a counter such as a 74ls190,,,and then switch to a stamp

Irone

How Ya Doin'? I got out a breadboard and put six LEDs on it. I then hooked them up as two series of three with a 330 ohm resistor for each series and ran 12 volts to them. I then added a 2n2222 transistor which grounds both series and used a 10K ohm resistor for the base through a five volt supply using the same ground. I do not know if your 74ls47 sources or sinks voltage but if it sources you may be able to use it. I am still wondering what you plan to do with your circuit but what I did lit up all six LEDs.

youngbill

What I want is a counter ,to drive a large display. Then as my knowledge builds,i will modify it to a stamp driven display.

Does anyone know if the max 7219 chip has been discontinued,,,and/or if there is a better chip available now?