LED clock too dimm

[prenavin]

I'm building a digital clock using LED's in a 7 segment pattern. Each segment has around 35 LEDs and the height of a complete display is around 12 inches. i'm using two displays each, for hours, minutes and seconds. A PIC 16f877 sets the number to be displayed and a 74HC238 demultiplexer selects each display. Only one display is on at any given time, to save power. Problem is the LEDs are not bright enough. I tried using transistors to increase the current but it doesn't help, because its switching between each display so fast. Please help!!!

 

[Nigel Goodwin]

I'm building a digital clock using LED's in a 7 segment pattern. Each segment has around 35 LEDs and the height of a complete display is around 12 inches. i'm using two displays each, for hours, minutes and seconds. A PIC 16f877 sets the number to be displayed and a 74HC238 demultiplexer selects each display. Only one display is on at any given time, to save power. Problem is the LEDs are not bright enough. I tried using transistors to increase the current but it doesn't help, because its switching between each display so fast. Please help!!!

When you're multiplexing displays you have to drive them MUCH harder than you normally would - so if you would normally drive it at 50mA, for four multiplexed together you would need 200mA to give the same brightness (1/4 of 200mA is 50mA).

In your case, with six displays, you need six times the current to give a similar brightness.

 

[prenavin]

More help please!!!

Firstly, thanks Nigel for the great PIC software, it rocks. I agree with you. Thats why i've tried to amplify the current using transistors. Please note: i'm using 5mm red LED's, each segment has around 35 LEDs. Thats 35 LEDs x 7 segments x 6 displays = 1470 LEDs.
To test the circuit I built it on a bread board with conventional 7 segment displays and it works fine, nice and bright with no flicker. When I add just one LED to the circuit its too dimm. i even tried removing the 330 ohm current limiting resistor, not much difference. Could it be that i'm biasing the transistors incorrectly. For test purposes, I'm using two BC547's on one LED . Its still not working though.

 

[bonxer]

What kind of power supply are you using? One that can handle at least 2.5 amps I would hope. :shock: You are strobing the displays, so that's good to save the power. But still, even for powering a single one of your segments, think about it. If you display the number 8, all seven segments will be on. 7 segments * 35 LEDs / segment = 245 LEDs.

Assuming you are using a 5 volt supply, and also assuming all of the LEDs in a single segment are running in parallel, have a 1.8 volt drop, and each have their own 330 ohm resistor, the current in each LED is (5 - 1.8 ) / 330 = 9.7mA per LED. Multiply that by 245 LEDs, and you get 2.38 amps. If the supply you are using is rated below that, it's going to eat itself to death. Plus, if it can't even supply the current with the limiting resistors in place, it definitely won't be able to supply enough current without them! You definitely have to use transistors, as each segment would draw 340 mA.

If you've got all that taken care of, have you tried to decrease the resistance feeding into the bases of your transistors?

 

[Roff]

Re: More help please!!!

Firstly, thanks Nigel for the great PIC software, it rocks. I agree with you. Thats why i've tried to amplify the current using transistors. Please note: i'm using 5mm red LED's, each segment has around 35 LEDs. Thats 35 LEDs x 7 segments x 6 displays = 1470 LEDs.
To test the circuit I built it on a bread board with conventional 7 segment displays and it works fine, nice and bright with no flicker. When I add just one LED to the circuit its too dimm. i even tried removing the 330 ohm current limiting resistor, not much difference. Could it be that i'm biasing the transistors incorrectly. For test purposes, I'm using two BC547's on one LED . Its still not working though.

Post a schematic of your transistor driver.

 

[Nigel Goodwin]

Re: More help please!!!

Post a schematic of your transistor driver.

Yes, I agree, you need to show us what you are currently trying to do!.

 

[audioguru]

Yeah, I want to see how a 74HC238 with a 25mA maximum output current rating and a little BC547 or two with a 100mA maximum output current rating can drive 245 LEDs requiring a total of 2.38A!

 

[checkmate]

If you want each LED to be driven 100% of the time, you either have separate inputs for each and every of them, or you multiplex them but with latches in between, like 74573s.

 

[Roff]

The LEDs in each segment could be in series. Of course, you would need a 70-80 volt supply...
But the current requirement would be relatively low.

 

[audioguru]

My Christmas tree LEDs are all in series (120VAC) with a rectifier but no filter cap and no current-limiting resistor. Needless to say, they flicker.
They use only about 1/50 the current of incandescent ones so the electric utility gave them away for free to save power. Before they gave them away just before Christmas, my city's power consumption was at a record high. If everbody used the same ones the utility's sine-wave would be flat on one side!
The manufacturer of those LED strings should alternate the polarity each day of production.

Maybe the LEDs in Prenavin's giant display are dim because he can't change its little 9V battery fast enough! :lol:
I'm still waiting to see its schematic.

 

[prenavin]

Its still not working

Ok guys, I took your advice. I brought some new transistors, BD 679 to drive the LEDs anode and BD678 to drive the cathode. They can handle up to 4A with a gain of 750. I'm using a 1K resistor on the their bases. This should fully saturate them. A 10 ohm resistor limits current to about 0.5A.

Forget 35 LEDs x 7 segments x 6 displays = 1470, I just want to get ONE LED to work correctly for now. I powered up the new circuit with the BD's, no difference. It seems like these transistors don't make a difference. I'm using a 7805 regulator (1A type) for power. 1A should be enough to drive ONE multiplexed LED. Could it be a software problem?? Highly unlikely, cause my normal 0.5 inch 7 segments work perfectly.

I have attached the schmatics, sorry i'm not very good with MS paint. Should i revamp the entire circuit ? More importantly would this solve the problem ? thanks guys I really appreciate your advice

 

[Roff]

Your Darlingtons are running as emitter followers, and won't saturate. They will each have about 1.4 to 1.5V from collector to emitter. Add 2v or so for the LED, and calculating a current limiting resistor is impossible. The uncertainty in those voltage drops is too high. I ran a sim on your circuit, and got 17ma through the LED. Not too good for a multiplexed LED.
You need to run your transistors in common emitter mode. You can invert the PIC outputs in software. Use a 74CH138 (inverting) in place of the HC238. Even then, Vce will be on the order of 0.7V, because they are Darlingtons. MOSFETs might be better choices. Use N channel for the cathode drivers and P channel for the anode drivers. Or, use the BD678s for the anode drivers, and just replace the cathode drivers with N channel MOSFETs.

 

[Nigel Goodwin]

For a start change the transistors over, you want the NPN's at the bottom (emitters to gnd) and the PNP's at the top (emitters to 5V), and rewrite your software accordingly.

As it is you hardly have any voltage left to light an LED!.

As you're wanting to drive large quantities of LED's I would also suggest using a higher supply voltage to do so, putting the LED's in series/parallel (with each series chain having it's own current limiting resistor). In order to do that, the top transistors (now PNP!) will need an NPN driver from the PIC.

 

[audioguru]

Hey Prenavin,
Before you didn't have enough current, now you don't have enough voltage to light your LEDs.
Just use your fingers to count all the base-emitter voltage drops of your darlingtons, 2.6V or more. With only a 5V supply, it leaves only 2.4V or less for your LEDs and current-limiting resistor. If the LEDs need 2V, their current is 40mA, which drops to an average of only 6.7mA when multiplexed 6 ways. The current will be much lower when driving 35 LEDs because the base-emitter voltage drops increase with current.
Power Mosfets would be a better choice as drivers, used common source instead of source-follower. Their input should be spec'd for logic-level.

How are you going to connect all 35 LEDs? If simply in parallel, the one with the lowest voltage requirement (they won't all be exactly the same) will hog most or all the current.

 

[prenavin]

Yipeeeeeeeeeeee, it works

GUESS WHAT ??? Its working ! When those LEDs illuminated, I couldn't stop smiling. You guys are my mentors, thanks.

I swapped the NPN's and PNP's around. I added BC 547's from the 74HC238 to drive the bases of the PNP's. I've only used about 15 LEDs so far cause my power supply goes up to 30V. I'm sure if I add another 15 LEDs or so with the correct voltage, it will be bright enough. Strangely though, if one looks closely, the LEDs are illuminated slightly when they supposed to be off. I'm sure its a software problem, an error i probably made when i swapped the BD's.

I initially wanted to wire the LED's in parallel and use a PC power supply ( 30A on the 5V line). However, I now can see the benefits of wiring it in series. Which brings me to the problem of power supply. I need around 70V DC with 4A at least. I have 220V A.C. from the wall and i haven't worked with A.C. alot, so PWM seems challenging. I was thinking of stepping up a DC voltage, but then current becomes a problem. Is this a good idea ? YAY !!!! it works. he he he.

 

[Roff]

Did you invert all the cathode bits (in software)?
Your schematic doesn't show any current limiting resistors. Is this an oversight in the schematic?
Regarding current: With series LEDs, if you had all 42 segments on (un-multiplexed) at 20ma each, you would only need 840ma. If you mux and drive each segment with 120ma at 1/6 duty cycle, you still only need 840ma max. Why do you need 4 amps? are you planning to run 100ma average current per segment (600ma instantaneous)?

 

[audioguru]

Hi Prenavin,
Congratulations, it works!
Now you have enough voltage to light the LEDs but you certainly don't need 4A for the segments with the LEDs in series. The very slow speed of the darlingtons is probably causing them to dimly light when they should be off. With the reduced current requirement for the segments, ordinary fast PNP transistors (with suitable base input resistors) would eliminate the ghosting.
You are taking a gamble without current-limiting resistors. The voltage across LEDs changes with temperature so you could have a melt-down (or a cool burn-out).
With extra voltage to allow for current-limiting resistors in series with 35 LEDs that need 2V each, an unregulated supply of about 80V would be fine. You would get about 80VDC from a 58 or 60VAC transformer, full-wave bridge rectifier and filter cap.

 

[prenavin]

JUst curious

Hi there! i have a quick question. I connected 12 red LED's in series with a 100 ohm resistor to a 30V supply, no transistors. Its looks really bright. But according to my calculations, 30v - (12*1.7) = 20.4V. therefore 30v -20.4v=9.6V. 9.6v/100ohm= 0.096A. Thats the current that should flow throuh the LEDs, however i'm only getting 20mA. Where is the extra resistance coming from?? I thought when foward bias resistance is negiable.

 

[Nigel Goodwin]

Re: JUst curious

Hi there! i have a quick question. I connected 12 red LED's in series with a 100 ohm resistor to a 30V supply, no transistors. Its looks really bright. But according to my calculations, 30v - (12*1.7) = 20.4V. therefore 30v -20.4v=9.6V. 9.6v/100ohm= 0.096A. Thats the current that should flow throuh the LEDs, however i'm only getting 20mA. Where is the extra resistance coming from?? I thought when foward bias resistance is negiable.

Try measuring the voltage drop across the LED's, I presume you're only guessing at 1.7V per LED?.

You could also measure the voltage drop across the resistor, and calculate the current.

 

[audioguru]

Hi Prenavin,
The extra resistance was probably provided by your current meter and you are lucky it was there.
The maximum continuous current for most LEDs is only 30 to 50mA. 96mA would fry them unless it is in short pulses. Why did you use such a low resistance or such a high voltage?