Strobing LEDs that are being chased by 4017

Thread Starter

mxabeles

Joined Apr 25, 2009
266
Correction. You're using integrated Darlingtons designed to be driven from 3.3V or 5V logic without external resistors.
Right. But when it comes to the strobing transistor, I'm using a standalone transistor. I haven't had issue with the 2803, like I said it was working with the 4017. It's when I introduced the stand alone transistor to switch the LEDs on or off, current leakage became an issue. Again, I'll be posing a sketch :) Thanks!
 

Thread Starter

mxabeles

Joined Apr 25, 2009
266
Dennis, here are the LED specs: 1.9v, 30mA. I'm doing two in series for each 4017 / transistor output. So I have a 40 ohm resistor in series with each LED pair.

I've got it the strobe effect working with a relay shorting out the 5v+ line. I'm just going to go with this and call it a day for now. This is a "one off" project - and whatever gets the job done is my motto at the moment. I'm also doing painting, sculpture, musical composition, and MaxMSP coding for this as well as all soldering. So, I can't stay too stuck in one arena. Thanks for your help - I may cycle back and test stuff out later.
 

dl324

Joined Mar 30, 2015
16,846
1.9v, 30mA. I'm doing two in series for each 4017 / transistor output. So I have a 40 ohm resistor in series with each LED pair.
Your calculation isn't quite right.

To get a 30mA LED current, you should be using:
\( R = \frac{V}{I}=\frac{V_{CC} - (2*V_{LED}+V_{SAT})}{30mA} = \frac{5V-(3.8V+0.7V)}{30mA}=\frac{0.5V}{30mA}=17\Omega \)

40 ohms will give you more like 12.5mA.

EDIT: correct typo for 2*LED voltage. Should be 3.8V, not 3.9.
 
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Thread Starter

mxabeles

Joined Apr 25, 2009
266
Your calculation isn't quite right.

To get a 30mA LED current, you should be using:
\( R = \frac{V}{I}=\frac{V_{CC} - (2*V_{LED}+V_{SAT})}{30mA} = \frac{5V-(3.9V+0.7V)}{30mA}=\frac{0.5V}{30mA}=17\Omega \)

40 ohms will give you more like 12.5mA.
 

Thread Starter

mxabeles

Joined Apr 25, 2009
266
Thanks Dennis. I feel like a real noob, but whats the additional VSat (0.7v added to the sum of LEDs forward voltage)?
 

dl324

Joined Mar 30, 2015
16,846
Thanks Dennis. I feel like a real noob, but whats the additional VSat (0.7v added to the sum of LEDs forward voltage)?
A disadvantage of Darlington transistors is that the saturation voltage is one diode drop higher than with a single (non-Darlington) transistor:
clipimage.jpg

The datasheet gave this:
clipimage.jpg
clipimage.jpg

But that's a worst case number, so I chose to use the graphs for typical devices to get a more typical saturation voltage:
clipimage.jpg

Since you're operating at 30mA, I used 0.7V.

EDIT: circuit diagram and specs taken from a TI datasheet.
 
Last edited:

Thread Starter

mxabeles

Joined Apr 25, 2009
266
A disadvantage of Darlington transistors is that the saturation voltage is one diode drop higher than with a single (non-Darlington) transistor:
View attachment 214393

The datasheet gave this:
View attachment 214389
View attachment 214390

But that's a worst case number, so I chose to use the graphs for typical devices to get a more typical saturation voltage:
View attachment 214391

Since you're operating at 30mA, I used 0.7V.
I can't lie, I'm in over my head now! :) Once I start seeing small numbers and letters my brain starts to bleed haha. I will keep trying to decipher this over the years.
 

soyez

Joined Aug 17, 2020
51
Hi there -
The title is a little misleading, the 4017 is not chasing my LEDS around the workshop. I wish it was THAT exciting.

So, I have an arduino clocking my 4017. It works to chase my jumbo LEDS (I'm using a ULN2803 to increase current to LEDs).

What I would like to do, is have a second arduino pin turn the LEDS on or off whenever I desire, while the 4017 cycles through the row of LEDs independently. I *thought* I could take the common emitter from the ULN2803 and tie that to a second transistor. Unfortunately, if the 2803's emitter is detached from ground the LEDS are still faintly visible.

I now realize I need to have a switch between 5v+ and my LEDs. Any suggestions for a good positive rail switcher? At first I thought to use a reed relay, but that will probably need replacing and slower than some other non electro-magnetic options. Would a P channel MOSFET be a good idea? It's the best thing I could find lounging around the interwebs.

/// As always, thank you for reading and checking up :)
IC1 (NE555) is the well known clock IC wired in the Astable Multivibrator mode. Resistors R1, VR1 and capacitor C1 go about as the circumstance parts and the yield beats are accessible from the yield pin 3 of IC1. These heartbeats are given to the information pin 14 of the Johnson decade counter IC CD4017. Out of the 10 yields of IC2, eight yields are utilized to drive the LEDs. The ninth yield pin 9 is associated with the reset pin 15 to stop the tallying. With the goal that the cycle rehashes. With the estimation of C1, each LED stays on for 1second. At the point when one LED kills, the second on turns on. This cycle continues giving the running light appearance. Resistor R3 keeps the info pin 14 of IC2 low after each pulse.VR1 changes the speed of LED pursuing.
 

soyez

Joined Aug 17, 2020
51
Hi there -
The title is a little misleading, the 4017 is not chasing my LEDS around the workshop. I wish it was THAT exciting.

So, I have an arduino clocking my 4017. It works to chase my jumbo LEDS (I'm using a ULN2803 to increase current to LEDs).

What I would like to do, is have a second arduino pin turn the LEDS on or off whenever I desire, while the 4017 cycles through the row of LEDs independently. I *thought* I could take the common emitter from the ULN2803 and tie that to a second transistor. Unfortunately, if the 2803's emitter is detached from ground the LEDS are still faintly visible.

I now realize I need to have a switch between 5v+ and my LEDs. Any suggestions for a good positive rail switcher? At first I thought to use a reed relay, but that will probably need replacing and slower than some other non electro-magnetic options. Would a P channel MOSFET be a good idea? It's the best thing I could find lounging around the interwebs.

/// As always, thank you for reading and checking up :)
The strobing oscillator and the timing oscillator can be fluctuated discretely through the separate pots for making client characterized charming LED strobe impacts.

The LEDs regular cathode end isn't associated with its typical position, for example to the ground; rather it's associated with the yield of the cradle NOT entryways.

The oscillator from the 4049 IC sends; fast high and low rationale heartbeats to the cushions which convey forward the reaction to the LED cathode.
 

AnalogKid

Joined Aug 1, 2013
10,987
COM is for suppression diode if switching motors/relays. connect to v+ but not needed to..
It an application like this one, The COM pin can be used as a "lamp test" pin: Ground it and all lights come on. Be careful not to exceed the maximum-current-for-one-pin rating.

ak
 
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