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Totally new to board design and need help.

Sparkey

Jan 24, 2010
3
Joined
Jan 24, 2010
Messages
3
Hi everyone.
I'm the new kid on the block here and would like some help please.
I've never designed a ckt bd before and have a need for something
like the LED light chasers you can find as kits but with a twist.

(Also I'd just rather try to build it myself, vs buy a kit, for fun.)

I'm looking for a plan to build a low voltage board that allows you
to run a sequence of (8-16) delay or fade LEDs from on to off.
The device should allow you to set the time interval from one LED to
the next and is to be activated by a dry contact switch closure.
Also, the LEDs will not be on the board, they will be attached to wires
so I could make like a run-way type effect directing people along the floor.

Basically:
Activation (turn on),
L1 fades on (adjustable time for fade on) then
L2 fades on (adjustable time for fade on is same for L1) then
L1 fades off (adjustable time for fade off is same for L1 fade on)
when L1 is off L3 fades on...
when the last LED fades off the device stops until re-activated.

I would like to buy one of those pre-hole punched boards from Radio
Shack and just be able to solder the wires together with jumper wire
on the back. The device should run off a battery or wall-wart

If this can be done by an novice any help would be GREATLY appreciated.

Thanks!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
This is one of those things that sounds harder than it is!

The simple solution is to create a ramped voltage source, add some noise, and pass the output to a dot/bar generator.

First the ramped voltage -- you need a capacitor in series with a current source.

A suitable (and simple) current source is a jfet with the gate connected to the source see http://en.wikipedia.org/wiki/Current_source (note that you can purchase 2 terminal devices that are the equivalent to this such as http://www.rockby.com.au/searchres.cfm?select=23&subcode=55&offset=121&stock_no=28794 (a 0.22 mA constant current source).

Placed in series with a capacitor, the capacitor will charge linearly (at least up to the point that the "diode" loses regulation). A switch across the capacitor (to discharge it) will reset the circuit.

A dot/bar display driver such as an LM3914 will change this increasing ramp to a dot that moves along a series of 10 LEDs. http://www.national.com/mpf/LM/LM3914.html#Overview

The time that it takes to go from the first to the last LED will depend on the constant current source, the capacitor value, and the range of voltages you set for the LM3914.

The amount of overlap between the LEDs will be quite small. To increase this you could add "noise" by capacitively coupling the output of an oscillator to the voltage from the capacitor (sounds harder than it is).

I'd buy a solderless breadboard to set this up. Far quicker than soldering, and far easier to fix mistakes or to try something a little different. You'd be looking for something like this http://www.rockby.com.au/searchres....&imageField.x=0&imageField.y=0&stock_no=30753 although you would not need one so large.

You need to decide on a capacitor, lets assume you're using that 0.22 mA source, and you intend the full range of the dots to be displayed over 0 to 3V.

The simple relationship is that 1A constant current will place 1V across a 1 Farad capacitor in 1 second, so how many farads do we require if we want 3 volts in 15 seconds at 0.22 mA

C = I V T
= 0.22 x 10^-3 * 3 * 15
= 10,000 uF (approx)

That's a pretty large capacitor, and it would be better to have a lower current source.

If you look at the circuits for creating constant current sources, you'll see that it's not that hard to do it with a jfet, and you could construct one that has (say) 0.05 ma (50 uA) which would reduce the capacitor to around 2200 uF.

The problem with lower and lower current sources is that leakage becomes an issue. Even measuring the voltage across the capacitor would affect things. A variable current source (see the jfet model) would allow you to trim the timing to balance out the possibly large effects of loading and capacitor tolerance.

p.s. Someone might like to check my math -- the results sound pretty reasonable, but I don't want to lead this person astray by making silly mistakes.
 

Sparkey

Jan 24, 2010
3
Joined
Jan 24, 2010
Messages
3
Thank you for writing back. I get most of this but I need to seriously don't understand how to figure out what I need for components, like you did the math for farads. I'll re-read this a few times. I do have a no solder breadboard that I have been trying to learn with and did intend for that to be the very first step. One more question if I may. You selected a 10 bar LED driver, can these be connected in series so I can get more than 10 to sequence? Thanks again for responding!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Initially you need to get the constant current source working. It may be as simple as a jfet with the gate connected to the source. Find one with a Idss around 0.1 to 0.5 mA.

The next thing is to connect it to a capacitor and look at how it charges it. This is best done with an analogue voltmeter across the capacitor. Short the capacitor out (perhaps via a 10 ohm resistor) to reset it to start again.

Initially use a capacitor approximately the same as I suggested. You can alter either the current source, or the capacitor later if it is wildly wrong in timing.

Yes the LM3914 can be cascaded for 20 LEDs. You should try to get hold of the datasheet for any chip you're intending to use. They often have sample circuits (like page 9 of the LM3914 datasheet which directly answers your question). In addition manufacturers also often produce Application Notes which contain even more circuit and design ideas, some verging on the esoteric.

For the LM3914, the datasheet is here: http://www.national.com/ds/LM/LM3914.pdf and a page containing application notes is here: http://www.national.com/apnotes/LEDDrivers.html

In both cases I simply googled. There are often different datasheets from different manufacturers and it may pay to look at all of them. In many cases you don't know whose component you'll get, and in any case some datasheets are more informative than others.
 
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