# I Like The Circuit, Just Too Many LEDs

#### Michael Knight

Joined Nov 5, 2015
90
I found this circuit I like for a Random LED Flasher for a project I'm working on, I did a little experimentation changing the capacitor value to a 0.22 uF
in order to change the flash rate, The circuit originally calls for a 47K Variable resistor, I only had a 50K Variable resistor, so it's a little higher but still is able to adjust the speed a little.

Like I say I like this circuit but it just has too many LEDs for what I want. I tied taking out the middle LEDs in each LED group only the circuit no longer worked after that. Does anyone with a little more electronics experience know how I can do that?

I have attached the Circuit Diagram I found.

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#### bertus

Joined Apr 5, 2008
20,454
Hello,

What kind of leds do you use?
When you want to have less leds in a string, you must add a resistor in series with the remaining leds to reduce the current.

Bertus

#### Michael Knight

Joined Nov 5, 2015
90
Hello,

What kind of leds do you use?
When you want to have less leds in a string, you must add a resistor in series with the remaining leds to reduce the current.

Bertus
Thanks for the very quick response. I've using red LEDs

#### bertus

Joined Apr 5, 2008
20,454
Hello,

Red leds will have a voltage drop between 1.8 and 2.2 volts, depending on brand and age (the newer types have higher voltages).
https://www.oksolar.com/led/led_color_chart.htm
So, when you use 4 leds in stead of 6 leds, the full voltage of the battery is accross the leds and may blow them.
I would use a resistor in each branch of leds.
The resistor value should be (V_supply - V_leds) / I_leds

Bertus

#### Michael Knight

Joined Nov 5, 2015
90
Hello,

Red leds will have a voltage drop between 1.8 and 2.2 volts, depending on brand and age (the newer types have higher voltages).
https://www.oksolar.com/led/led_color_chart.htm
So, when you use 4 leds in stead of 6 leds, the full voltage of the battery is accross the leds and may blow them.
I would use a resistor in each branch of leds.
The resistor value should be (V_supply - V_leds) / I_leds

Bertus
Thanks,
Yeah I've been experimenting with using using 3 resistors in each brach on the negative side down at the very bottom, so far I tried 150K's and 100K's... so far just the bottom three rows I can get to work.... maybe 3 10K's might work?

#### bertus

Joined Apr 5, 2008
20,454
Hello,

When the supply voltage is 9 Volts and the led voltage about 2 volts each (times 2).
The voltage accross the resistor should be 9 - (2 X 2) = 5 Volts.
If you want a led current of 10 mA, the resistor should be 5 Volts / 0.01 A = 500 Ohms.
So you will need 6 resistors of 500 Ohms, a 470 Ohms resistor should also work, 1 in each branch of leds.

Bertus

#### Michael Knight

Joined Nov 5, 2015
90
Hello,

When the supply voltage is 9 Volts and the led voltage about 2 volts each (times 2).
The voltage accross the resistor should be 9 - (2 X 2) = 5 Volts.
If you want a led current of 10 mA, the resistor should be 5 Volts / 0.01 A = 500 Ohms.
So you will need 6 resistors of 500 Ohms, a 470 Ohms resistor should also work, 1 in each branch of leds.

Bertus
Thanks
Yup, that worked. I put 3 470 ohms resistors across the top 3 rows on the positive inputs and 3 470 ohms resistors across the bottom 3 rows on the negative side. They all light up great and I still have the flash rate I'm looking for.
Thanks

#### -live wire-

Joined Dec 22, 2017
911
When you want to have less leds in a string, you must add a resistor in series with the remaining leds to reduce the current.

Bertus
Ahh... once I was too lazy to do that and so my LED exploded. It split clean in half. It was a standard 5mm one. I don't even know why it did that. Maybe it was not too happy about greatly exceeding the voltage ratings and completely neglecting the resistor. Oh well.

#### Sensacell

Joined Jun 19, 2012
2,567
The original circuit is a really bad design, don't blame yourself for that.

The moral of the story is that many of the circuits floating around on the internet are poor examples of electronic design.
Some might work, but would stress parts, making failure highly likely.

Some internet circuits look really simple, which is always attractive to beginners, but the simplicity is achieved by removing the parts that would make the design stable, reliable and repeatable.

#### Michael Knight

Joined Nov 5, 2015
90
The original circuit is a really bad design, don't blame yourself for that.

The moral of the story is that many of the circuits floating around on the internet are poor examples of electronic design.
Some might work, but would stress parts, making failure highly likely.

Some internet circuits look really simple, which is always attractive to beginners, but the simplicity is achieved by removing the parts that would make the design stable, reliable and repeatable.
Thanks @Sensacell,
This is what I ended up doing to make it work, would you say that this is OK?
See attached.

#### Michael Knight

Joined Nov 5, 2015
90
OK this is what I've been playing trying to come up with.... you'll have to forgive my messy layout.... it's messy because I'm trying to make the circuit look a certain way. If you look at the first attached image you can see the effect and look I'm trying to duplicate.

In the TV Show Knight Rider episode was called "Soul Survivor" I think
Anyways, KITT's Brain / CPU was like this.
Basically I'm working on a mini PC with a mockup circuit board that would fit into the project box above the mini PC under the Lexan clear plastic cover.

This is my somewhat messy photoshop layout I've been working on to try and arrange the parts as close as possible to match the circuit in the image above.

The fat red components are just home made jumpers I'm planning to make just using some hollow plastic tubing cut into short pieces and then covered in heat shrink tubing to try and mimic the fat red ones you see in the original image. They will just be jumpers.

I think on my bread board I swapped pin's 11 & 3 around because I liked the effect better.
See video for the work in progress video:

I guess one of my concerns is not only trying to get the circuit to look as close as i can to the original but also as @Sensacell had mentioned "Stability" That is something I have kind of taken for granted with a lot of these circuit diagrams one finds on the Internet.
I have run it for a long time and it seems to be OK, but I'm no electronics expert, moderately skilled hobbyist at best I'd say.
So if there is any way to improve it or suggestions you might have that would be great guys