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LED chaser mod help

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Space Ace

New Member
Newbie here. I am working on a project that uses a pre-assembled PIC based LED chaser. I have a bunch of questions about it.

The chaser that I am going to use currently has 8 white LEDs, one per channel, however I want to change that. For my project, I will remove the white LEDs and each channel will have a different number of yellow LEDs. The number of LEDs of all the channels combined will be around 250. If you need to know the specific number for each channel, I will list them. I figure the best way to wire the LEDs to each channel is in parallel, yes...no? The specs are: 5mm yellow, forward voltage 1.8 ~ 2.2, max continuous current 30 mA. Now how do I figure out what resistor value I need to use per LED? Would I go by the total amount of LEDs I use (around 250), or would the resistor value be based on how many LEDs are in each channel? This will be run on 12 volt battery power, although that could change if needed.

Also, the chaser unit that I have uses a small tactile switch to change patterns. I don't like the switch very well because it seems like it won't last very long, and the push button needs to be longer anyway. Could I replace this with a different sturdier switch to select the patterns? If so, what kind of switch, SPST momentary? N.C. or N.O.? I'm not familiar with the electronical makeup of tactile switches. :eek:

One last thing, the chaser unit has a pot to change the speed of the pattern. I want to change this to a larger pot. Will any value pot be ok, or will I have to use the same value as what is currently on there? I'm not sure of the value, but there is a number "103" on it.

Thank you in advance. :)
 

audioguru

Well-Known Member
Most Helpful Member
Most ordinary LEDs have a max allowed current of 30mA. A PIC has a max allowed output current of 25mA.

So you need a transistor circuit to drive many LEDs.

Each LED should have a current calculated at 20mA so an LED that is at the minimum forward voltage of 1.8V and with a 5V supply that is a little high will still have less than 30mA.
Calculating a resistor for each LED is simple, use Ohm's Law.
If the yellow LED has a forward voltage of 2.0V and the supply is 5V then the current-limiting resistor will have (5V - 2.0V=) 3.0V across it. Ohm's Law says that the resistor value is (3.0V/20mA=) 150 ohms.

You can use any kind of switch. Most momentary switches are N.O. but yours might be the opposite. Measure it with an ohm-meter.

The "103" pot is 10k ohms. It is 10 followed by three zeros. A 100k pot is marked 104.
 

Space-Ace

New Member
Most ordinary LEDs have a max allowed current of 30mA. A PIC has a max allowed output current of 25mA.

So you need a transistor circuit to drive many LEDs.

Would this work as a driver for the LEDs?:

FK090 Output Driver, 8 outputs

If not, do you know where I can get one? Something small.


Each LED should have a current calculated at 20mA so an LED that is at the minimum forward voltage of 1.8V and with a 5V supply that is a little high will still have less than 30mA.
Calculating a resistor for each LED is simple, use Ohm's Law.
If the yellow LED has a forward voltage of 2.0V and the supply is 5V then the current-limiting resistor will have (5V - 2.0V=) 3.0V across it. Ohm's Law says that the resistor value is (3.0V/20mA=) 150 ohms.


Just curious, where did you come up with the 5V rating? Do I need to use 5V to power this, or can I use 12V?

About the resistors....would I need to replace the resistors that are currently on my chaser unit to the same value I will be attaching to each LED, or doesn't that matter?

Finally about the pot....what would happen if I used a 100k pot instead of a 10k pot? Would it affect the speed at all?

Thank you. :)
 

Space-Ace

New Member
So you need a transistor circuit to drive many LEDs.

Would this work?

FK090 Output Driver, 8 outputs

If not, do you know where I can get one that is not too large?

Each LED should have a current calculated at 20mA so an LED that is at the minimum forward voltage of 1.8V and with a 5V supply that is a little high will still have less than 30mA.
Calculating a resistor for each LED is simple, use Ohm's Law.
If the yellow LED has a forward voltage of 2.0V and the supply is 5V then the current-limiting resistor will have (5V - 2.0V=) 3.0V across it. Ohm's Law says that the resistor value is (3.0V/20mA=) 150 ohms.

Just wondering, where did you get the 5V value for the supply? I was planning on running this on a 12V battery supply. Do I need to use 5V or will 12V work?

About the pot...what would the effect be if I used a 100k pot instead of a 10k pot?

One last thing, do I need to change the resistors in the chaser circuit to the same value that I will be attaching to each LED, or can I keep the ones that are there already?

Thank you.
 

audioguru

Well-Known Member
Most Helpful Member
The regulated voltage for the PIC is 5V so I assumed you will use the same 5V supply for the LED driver circuit. Use 12V for groups of LEDs in series and use a 5V regulator to supply the PIC.

The Bakatronics LED driver circuit can be used to drive eight sets of LEDs or the ULN2803A IC that it uses. The ULN2803A has darlington transistors and series base resistors.
LEDs should never be connected in parallel unless their forward voltages are all matched to be identical or they will burn out. Each LED needs its own series current-limiting resistor or connected in series and in series with a current-limiting resistor.
 

Space-Ace

New Member
What would the effect be if I used a 100k pot instead of a 10k pot?

Also, do I need to change the resistors in the chaser circuit to the same value that I will be attaching to each LED, or can I keep the ones that are there already?

Now about the ULN2803A IC. In the Bakatronics kit, it has a resistor hooked up to each unput 1-8, which would go to the chaser board where each LED would normally go. Since there are resistors already on the chaser board that connect to the LEDs, would I even need to hook these resistors between the ULN2803A and the chaser board? It seems like the resistors would just flow into the ones that are there. Could I just use wire to go from the IC to where the LEDs are right now? Here is the chaser board that I will be using: 8 LED Chaser PIC Microcontroller Light Show - NEW - eBay (item 230389101008 end time Nov-15-09 08:50:10 PST)

Thank you.
 

audioguru

Well-Known Member
Most Helpful Member
I don't know which 10k pot you are talking about. If it is the one on the PIC Chaser board then we don't know anything about it. We don't know it it works with a 100k pot.

There is not enough information about the PIC Chaser circuit to connect a power booster. We don't know if its output goes high or goes low to light an LED.
If its output goes low then the Bakatronics driver with the ULN2803A IC will not work.

Since you are driving many LEDs then the current-limiting resistors on the PIC Chaser board are not used.
 

MrDEB

Well-Known Member
That video is KNIGHT RIDER on steriods

For $10, you get a pretty simple display.
now your talking 250 leds??
thats 30+ leds per output.
NO do not connect 12v to this as most micro-controllers are 5volts.
at 30 LEDs you now need 600ma or a different configuration?? or poof !!
Might be easier to build from scratch and use something else instead of LEDs.
xmas light strings come to mind.
some moc3023 opti couplers, some triacs (build a SSR)
fairly easy to build. I have about 40 of the buggers with 4 channels per board.
Computer Christmas light season is coming.
THEN connect the outputs to the SSR's.
do a computer Christmas search. website has how to build as well as boards.
If you need pc boards I have couple dozen as well as most of the components.
Ebay has solid state relays as well. ALL assembled ready to go.
Xmas lights is probably least headaches.
 

audioguru

Well-Known Member
Most Helpful Member
The PIC chaser micro-controller circuit says it works with a supply from 8V to 14V so the TO-92 little "transistor" on the circuit board is a small 5V regulator.
 

MrDEB

Well-Known Member
missed that point

Problem is HEAT!
the leftover 7volts has to dissapiate somewhere??
 

audioguru

Well-Known Member
Most Helpful Member
The PIC uses a couple of mA at 5.0V and the little regulator will not get warm.
The LEDs are in series groups and are driven from 12V to 14V.
The ULN2803A driver IC saturates its outputs so it does not get too hot but the many current-limiting resistors will cause a lot of heat.
 

audioguru

Well-Known Member
Most Helpful Member
We still do not know if the output of the PIC goes high or goes low to turn on lights.
 

MrDEB

Well-Known Member
look at the LEDs

what are the flat sides of the LEDs connected to?
either all together or each connected to a resistor then to a pin on the PIC controller IC.
The flat side should tell us weather the contorller is going low or high hopfully.
about 95% sure.
 

MrDEB

Well-Known Member
looking at circuit board

I would think that looking at which side the flat on the LEDs are connected to should hopefully tell weather the PIC outputs are going low or high??
 

Space-Ace

New Member
what are the flat sides of the LEDs connected to?
either all together or each connected to a resistor then to a pin on the PIC controller IC.
The flat side should tell us weather the contorller is going low or high hopfully.
about 95% sure.

The flat sides connect together. The round side of each LED connects to an individual resistor, then the resistor goes to the IC.
 

audioguru

Well-Known Member
Most Helpful Member
The flat sides connect together. The round side of each LED connects to an individual resistor, then the resistor goes to the IC.
The flat side is the cathode of an LED that connects to a lower voltage for the LED to light. Since the flat sides of the LEDs connect together and to ground then the outputs of the micro-controller go high to light them.

Then you are lucky that the micro-controller can also turn on an input of an ULN2803 IC or turn on an NPN transistor.
 

MrDEB

Well-Known Member
still have theproblem of paralel LEDS

parallel LEDs unless they are all the same voltage drop etc.
we is talking 250 LEDs here.
the 2803 is perfect choice but then what?
a kizillion resistors and ???
I would still opt for using the ULN2803 driving several SSR's
I think the photo showed 10 LEDs?
so 10 SSR's then xmas light strings.
EASY way out
 
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