Its ok. Resistors of almost every single value are available for me. Will they really last that short? I mean even a digital clock with all those fancy alarm and features works for quite long (a year or more). Finding a suitable type of battery will be the last decision. After that the soldering can begin.

 

Are you talking about parallel LEDs? Digital clocks use something called multiplexing, which isn't even close to what you are doing (the extra circuitry is prohibitive, unless it is on a chip). A µC would use it, but not a project like this.

The number of LEDs you would have on is a variable, a major one. This alone killes the idea.

First things first, figure out the displays, then start designing around them. I don't think it will be as bad as you think. BTW, I am assuming ¼W resistors.

 

Yes 1/4 resistors, and I will draw out each display and which lines to wire.

Oh I thought digital clocks use a timer IC like 555 and then a 7490 counter and something like 7447 BCD to 7 segment display driver which then drives each LED in a parallel circuit. Guess I am wrong.

PIC controllers seem to be a lot more convenient but the start up costs and initial learning process are major obstacles. I will definitely look into that as I pursue my hobby further and start building more programmable circuits.

 

I would like to clarify, if one display is running with all 14 LEDs lighted up, the current drawn would be about 30mA x 14, which is 420mA? Man that's quite a lot. I would like to believe that the whole display is drawing just 30mA.

 

Nope, you don't have to use the full 30ma, but that is a valid number. 10ma would be bright enough though. Again, you are not using every display segment, and LED current draw is not calculated for the entire display, just the individual segments (which happen to be single LEDs).

 

Yea so altogether its about 30-40 segments on average. Anyway i was wondering what is the difference between using 4 AA size batteries and 3 of them? Other than the fact that one supplies 4.5V and one supplies 6V? I will be using 330Ohm resistors.

Obviously using 4AA batteries will make it slightly brighter since it supplies higher current with same resistor. But is it really necessary?

Also, due to the displays being common anode in nature. I will have to connect them like in circuit A rather than circuit B which was in my previous schematic. No difference in both circuits right? Basically only the transistors change places. Refer to attachment. Assume I need all 6 LEDs to light up to show the first word and 3 LEDs to light up to show second word.

sorry switch the first and second input in circuit B. so the word with 3 LEDs will show up on second input.

 

You can use fewer batteries, the resistors will need to be adjusted (or not). You will need the steering diodes I mentioned (and you did not include), and they will absorb some voltage (typically 0.7VDC). Basically you will be very close to the edge, and if the battery voltage drops even a little the LEDs will not have enough juice to light well. It is up to you, and it will work for a while, but I would stick with the 6.0VDC.

You can buy small diodes in bulk reasonably cheaply, even from Radio Shack.

 

So I just need a diode in front of each transistor right? And shucks I just realized that the maximum rating for the BC547 is 100mA for collector current, which is clearly not enough for my 30-40 LEDs. At least 800mA or even 1A is needed. Do you have any recomendations for an alternative to that transistor?

 

Pick up a TI LaunchPad. They're full microcontroller kits for $4.30 and include a lot of stuff.

http://processors.wiki.ti.com/index...G2)?DCMP=launchpad&HQS=Other+OT+launchpadwiki

 

No, you need the diodes where I showed them. I forgot to add the resistors, which is a little embarrassing, but it shows how two transistors will light the same LED in a common anode configuration while not lighting another.

I have said this several times, you need to sit down and map out the display. This will determine everything else. Until it is done, you are spitting into the wind. It will determine the schematics and the numbers of components needed.

Since you only want 3 words there will only be three transistors, which are much bulkier than diodes.

To marshall3: Unless someone is willing to mentor the OP a µC is not exactly a practical possibility for a beginner. The hardware is extremely cheap, but coding is hard, for someone with no background outrageously so. It is probably the hardest learning curve the OP could face, unless he has previous experience coding. This is a fundamental most members can't seem to wrap their heads around. If you check the early posts this was recommended, along with buying an off the shelf unit that also used a µC that was already programmed. Off the shelf is $20-25, DIY is slightly more expensive, but you can't come close to the miniaturization.

Back to the OP:

This schematic simply will not do what you want. That is, to light an LED in two steps of a sequence while staying dark on a third step. There is no individual control of LEDs, therefore when you light one it will light every step. The individual control is missing.

Going back to your display's, they control everything. You control the concept, but once you come up with the names you then design around the display, not the other way around. I could help if I knew the first name, but I have noticed you don't want to give it out. This is OK, but it does mean you are going to have to work at the concepts to DIY. You are also going to have to put the resistors on the other side of the LED, although it doesn't make any real difference except accuracy.

Quick side note about graphics, I use M/S Paint. There are many graphics programs out there though. The important thing is file type. The two best are .png and .gif, among the worst are .jpg (which fuzz images badly) and .bmp (which are uncompressed, and huge).

My blog has lots of little tidbits how to do a post up right and pretty.

Bill's Index

How to Display Attachments Full Size

 

Oh I just had the displays completely mapped out, every single cathode in it. And also which LEDs will be shared among the words etc. Can I ask about the function of the steering diodes? Your circuit seems to be able to work without them as well. Since current flows from Vcc to ground with or without the diode.

I might need two transistors for one word because the transistors don't seem to amplify enough current to light up all the LEDs. So the output from the first transistor will be further amplified by the second transistor.

 

Follow the currents. A lot of people new to electronics have problems with diodes, but all they are is electrical check valves. In this case they prevent the ground from one transistor when it is on from translating to another transistor. The way the transistors are shown they are simple switches, either open or closed.

Since you have only 3 states you only need 3 transistors switching to ground. In cases where a LED is used all three times they are not really needed, but they probably ought to be used.

What part of the world are you? Helps when recommending parts.

 

I'm from Singapore. Basically a lot of things are available here and I can always get them from online too. So getting components is ok, including diodes.

What I mean by needing more transistors is that I believe they do not amplify enough current to power 30-40 LEDs. So I will need something like below (the left transistor) to amplify the current twice.

 

They don''t need to be expensive transistors either, components like 2N3904 or 2n (or PN) 2222 would be fine. There are actually tens of thousands that would work and all of these are available for next to nothing, especially in quantity.

I usually stock at least 100 of the 2N3904 since they're only 0.02 each from Mouser even in single quantities.

 

This is a modified darlington. It would probably work as you've drawn it. Don't forget the base resistor.

The real problem you have is current. If you used 10ma per LED at 30 LEDs it will be 300ma, almost 1/3 of a amp! You could drop this down to 5ma, and reduce the current that way.

 

Yes 300mA is really a lot! Even the 2N3904 can barely handle it. I can't really really think of a way to drop it without severely affecting the lighting of the LEDs. However I think a BC337 NPN transistor would work fine since its max collector current rating is 800mA.

I will connect 2 of those BC337 transistor that way (with base resistor).

 

1A NPNs for 0.04 each:
KSD1616
2N4953

 

Yea thats great! thanks!

 

A 2N2222A can handle 600ma. Small transistors are better than you think sometimes.

 

Thank you. I just ordered 20 of those transistors. Anyway do you have any recommendations for the diodes? There are so many types of them. Omg.