# Need help with another kit.

#### elec_mech

Joined Nov 12, 2008
1,500
Unless you want the PCB, you can save some money and build it yourself on a protoboard. They don't provide a schematic, but this appears to be nothing more than a 555 IC in astable mode providing a clock input to a CD4017. Check out this: http://www.delabs-circuits.com/cirdir/theory/gates/doc00020.html

And yes, you can easily use blue LEDs. Forward voltage of red LEDs is about 1.8VDC, blue can vary between 3.2V and 3.7V, but you can safely use 3.5VDC to calculate the current-limiting resistor value. Since only one LED will be on at a time, I assume they are only using one current-limiting resistor for all the LEDs.

They are using a 9VDC battery. If you want to stick with that, then you current-limiting resistor value for blue LEDs will be 9-3.5/0.02 = 275Ω. 330Ω is a common value that you could use. In reference to the link I provided, this would be the value of R1.

The circuit in the link above also uses a potentiometer which will allow you to change the speed on the fly. Based on the picture of the kit, it appears you are stuck with one speed.

Lastly, note the LEDs in the kit are diffused, meaning you can look directly at them and they won't blind you. Most commonly available blue LEDs are much, much brighter. If you'd like to look at lights, then look for diffused blue LEDs with an mcd rating of 500 or less (100 or less would probably be more ideal). I can provide you with a couple of part numbers for diffused blue LEDs I've used in the past if you'd like, though I'll need about a week as I'm away from home right now.

Good luck!

#### Adjuster

Joined Dec 26, 2010
2,148
If the LEDs seem too bright, try increasing the resistor. Some modern LEDs might be bright enough at just a few mA, so you may find that it would work with 1kΩ or more. As well as moderating the brightness, this would make the battery last a lot longer.

You can't rely on this making the LEDs dim enough for safety if they are non-diffused though.

#### Wendy

Joined Mar 24, 2008
22,647
Guys, I don't think this kit uses resistors, at least I don't see any. My digital clock I'm designing doesn't either, it uses the natural current limiting ability of CMOS. I'm still puzzling how that works, but it does seem to work.

In either case, blue LEDs are OK.

There are other things you can do...

LEDs, 555s, Flashers, and Light Chasers

#### iONic

Joined Nov 16, 2007
1,654
You can't rely on this making the LEDs dim enough for safety if they are non-diffused though.
Joeyvenkman,

If all you have or can find are clear, non-diffused blue LED's you can make them
diffuse by either sanding the dome section of the LED with sand paper. I grind them flat with a grinding wheel and they are no longer an issue with your eyes as they become quite soft in appearance.

#### SgtWookie

Joined Jul 17, 2007
22,221
Guys, I don't think this kit uses resistors, at least I don't see any. <snip>
Bill, I looked closely at the board; there are two resistors and a cap right around the 555 timer, and then there is another resistor below the 4th LED from the right.

There also appears to be a wire jumper from near where the battery positive lead connects, across several traces on the other side of the board.

#### thatoneguy

Joined Feb 19, 2009
6,359
Bill, I looked closely at the board; there are two resistors and a cap right around the 555 timer, and then there is another resistor below the 4th LED from the right.

There also appears to be a wire jumper from near where the battery positive lead connects, across several traces on the other side of the board.
I agree, Black wire from battery has it's own trace on the edge of the board from the left edge to in between the 5th and 7th LEDs from the left. At that point, it goes through a resistor to the common cathode trace of all the LEDs, while a jumper carries the ground trace over the LED traces to the ground of the ICs. Resistor appears to be 220 Ohm (Red-Red-Brown)

It would be this resistor that would need a new value based on the new LED color used.

#### Wendy

Joined Mar 24, 2008
22,647
Good enough. While you could adjust this resistor for max brightness they are using the 4017 chip as the LED driver, which is not optimal (works, I've done it). If the OP uses the old resistor value it will work, it will just be a little dimmer. The max current a 4017 can provide is 16ma, less is better. It is the real current limiter.

It is a lot cheaper to build this from scratch than to buy though. I like PCBs, but I tend to make them myself nowdays.

#### SgtWookie

Joined Jul 17, 2007
22,221
Well, it's cheaper if you already have all the stuff you need to make PCB's!

Eight bucks really isn't that bad, especially considering what it would cost to have a custom PCB made for a project like this. Granted, the layout used on the PCB in the kit isn't pretty, but it works.

They could've saved one more resistor had they used pin 3 for the RC time...

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