Needing help to bridge the gap on my LED flasher

Thread Starter

Kevinscouch

Joined Jun 23, 2020
14
I have attached a couple of drawings of my project. My son and daughter in law just made me a grandpaw a few weeks ago. I have taken a 555 timer and configured it as a astable multivibrator. I am not to worried with the output frequency at this time and I will refine that later. I have taken the output of the 555 and ran it into the clock input of a cd4017 counter. I will be using the first five outputs of the 4017 to light up some LED arrays. So the first output will light the blue LEDs then the next output will light the green LEDs until the sixth output. The sixth output will be tied to the reset pin to restart the counter. The red, orange, and yellow LEDs will be wired like the top LED array and will draw 40 mA from the source. The blue LEDs will be wired like the middle array and will draw 100 mA from the source. The green LEDs will be wired like the bottom array and will draw 100 mA from the source. Each LED array will make one letter of my granddaughters name. If your still with me and understand my ramblings here is my question. I don't think the CD4017 outputs can source the current needed for the LEDs. I thought about using 5 2N2222 transistors to drive the LED arrays. I came upon a IC I have never used. The ULN 2804 has eight Darlington transistor arrays. I am looking at getting a 12VDC 1A wall wort to supply the project. Would I be better with the 5 2N2222 or the UNL2804? Thanks for all you help and I really enjoy this site.
 

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AlbertHall

Joined Jun 4, 2014
9,986
Either will do the job. The '2804 has the resistors needed for the base of the transistors built in so that makes it easier to build. For either you will still need resistors to limit the LED current.
 

Thread Starter

Kevinscouch

Joined Jun 23, 2020
14
Thank you for your answer Albert. So the resistors I have at the end of the LED arrays will not limit the current? Should I put the resistors at the front of the LED strings?
 

jpanhalt

Joined Jan 18, 2008
9,648
Whether you put the resistors before or after the LED's won't make a difference.

As for fonts, there are several sources with various maximum number of dots, e.g., 5x8. They are balanced and a little easier than using paper. Although, designing your own can be fun.

Congratulations on #1.
 

Thread Starter

Kevinscouch

Joined Jun 23, 2020
14
Thank you for your answer jpanhalt. So the way I have the arrays drawn I should not need another resistor to limit the current? Also I guess I should have stated that I will be taking the graph paper and laying over the enclosure I and drilling a 5mm hole where each square is colored. I know its a lot of work but I think it will be neat for my granddaughter to learn colors and letters when she gets old enough.
 

ci139

Joined Jul 11, 2016
1,528
depending the particular LED type the 5x in series (1.6 to 2.3 V or more - drop on each) might be just about fitting your 12V margin ... or not

for the saturated 2N2222 at 40mA the CE voltage drop of 100mV is achievable at least by Fig.4 on https://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF

the darlington may be more limited for it's CE forward drop . . . Fig.11 shows about 0.8 ... 1V https://www.st.com/resource/en/datasheet/uln2801a.pdf
so it's again - defining the max number of LED-s you can set in series for the collector load
 

ElectricSpidey

Joined Dec 2, 2017
1,174
The resistors mentioned in post #4 would have been in between the 4017 outputs and the 2N2222 bases. The Darlington array has these pre-installed.
 

dl324

Joined Mar 30, 2015
10,911
Brightened images:
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If you're using connection dots, you shouldn't use "humps" to indicate crossovers with no connection. Humps have long been out of favor.

CD4017 can't source enough current to drive 2N2222, so you should use the darlingtons and drive the LEDs low side. Since you don't need 8 of them, use the 7 darlington variant (they're cheaper). Or you could use N channel MOSFETs.
 

Thread Starter

Kevinscouch

Joined Jun 23, 2020
14
Thanks to everyone who has made a reply. I think I will go with the Darlington array and see how it goes. If it does not work then I will got with some N channel MOFETS.
 

Thread Starter

Kevinscouch

Joined Jun 23, 2020
14
Hello everyone. I have bridged the gap in my project with a ULN2803. I ran the first 5 outputs to the first 5 base inputs of the 2803. Then I ran the collector outputs to my LED arrays. The project is working like I want it to. I have tried to read the total amount of current in the whole circuit. I am using a SMPS its rated for 12 volts at 1.2 Amps max. My meter is a Fluke 789 process meter. I am breaking the positive lead of the power supply and placing the meter in series to read the current. When power is applied the circuit acts abnormally in that it wont progress through the LEDS in order. The meter reading is not stable as well. Could I get some help please? Thanks.
 

AlbertHall

Joined Jun 4, 2014
9,986
I wouldn't expect a stable reading as the current will vary depending on which group of LEDs are lit.

One possibility for the meter making a difference is a lack of decoupling capacitors. You should have two 100nF across the '555 and '4017 power pins with the shortest leads possible and a 100uF across the power supply somewhere.
 

Thread Starter

Kevinscouch

Joined Jun 23, 2020
14
Thank you Albert the caps fixed the problem of the circuit not working properly when I installed my amp meter. I knew the current would change with each different array being lit. The circuit just would not advance the leds in the correct order until I put the caps in. Once again thanks.
 

eetech00

Joined Jun 8, 2013
1,931
I have attached a couple of drawings of my project. My son and daughter in law just made me a grandpaw a few weeks ago. I have taken a 555 timer and configured it as a astable multivibrator. I am not to worried with the output frequency at this time and I will refine that later. I have taken the output of the 555 and ran it into the clock input of a cd4017 counter. I will be using the first five outputs of the 4017 to light up some LED arrays. So the first output will light the blue LEDs then the next output will light the green LEDs until the sixth output. The sixth output will be tied to the reset pin to restart the counter. The red, orange, and yellow LEDs will be wired like the top LED array and will draw 40 mA from the source. The blue LEDs will be wired like the middle array and will draw 100 mA from the source. The green LEDs will be wired like the bottom array and will draw 100 mA from the source. Each LED array will make one letter of my granddaughters name. If your still with me and understand my ramblings here is my question. I don't think the CD4017 outputs can source the current needed for the LEDs. I thought about using 5 2N2222 transistors to drive the LED arrays. I came upon a IC I have never used. The ULN 2804 has eight Darlington transistor arrays. I am looking at getting a 12VDC 1A wall wort to supply the project. Would I be better with the 5 2N2222 or the UNL2804? Thanks for all you help and I really enjoy this site.
use a Mosfet array TBD2003A. There will be less of a drop vs a BJT array.
 
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