How could I go about building a circuit that can do this?

Question

All right, so I plan to build an LED lightsaber made from just bout 6 strings of 10 LEDs in parallel, stacked one on top of the other. When I hit a push-button latching switch, I'd like each of these strings to light up from the bottom to the top, then turn off from top to bottom when the switch is turned off. I've been told that this can be done with a bunch of transistors and capacitors.

Answer 1

The LEDs will take appreciable power, so any analogue system will not work without considerable complexity (it sounds like you have not done this sort of thing before).

So you will need a timer circuit and separate 'power drivers' to switch the current on to the LEDs.  The simplest way of doing that is not to have six strings of ten but ten rings of six... the common anode of each ring is connected to your supply and the common cathode is switched by a common-emitter configured medium-power bipolar switching transistor.

Each switching transistor can be switched on by the output from a shift register constructed out of standard CMOS logic chips.  These can be difficult to handle and so TTL, although it takes more current may be a more reliable approach for you.

Drop me an email address if you want to know more.  I'd avoid doing this with repeated R-C and transistor blocks because it will be just too tricky to make it all work if you don't have prior experience of this sort of thing.

-EDIT-

I knew I had used one of these things before! :)

Take a look at this: http://www.national.com/mpf/LM/LM3914.ht...

It does almost everything you need, in one chip.  It is a complex analogue solution but as most of it is inside the chip, you don't need to worry about it.

You will need to give it an input ramp voltage derived from a constant current source.  You will also need to buffer the LED switching outputs with a transistor and one resistor for each output as it only drives a maximum of 30mA and that will not be enough for your needs (I'd guess you will need at least 150mA for each ring of six LEDs). You will not need base resistors for your buffers (assuming you use bipolar transistors) because the chip outputs are constant-current-limited. As I said before, drop me your email address and I'll be able to give you more help.

Answer 2

the transistor and capacitor sting is the best solution. there is a configuration that is a little simpler but it would take the same nuber of components and alot more math. this is just an LED in parellel with an RC timer. doing it this way asures that one led must come on before tne next. the time between lights will be aprox R*C. leave the transitor off on the last LED. the resistor in series with the LED must be present. use about a 3.3k ohm resitor to start with. adust up or down for briteness. you will want one that barely dims the LED. too bright and you will damage the transitor and the LED. too dim and it would not be fun. this will alow you to use the cheapest smallest transitor.

see if this link works:

http://i61.photobucket.com/albums/h68/ra...

EDIT: darn! that will only work for turning it on... let me think

i cant come up with a dicrest circuit to power up and poer down the LEDs. it can be done, but it gets complicated fast. you will probably need to use digital to simplify it. it would require a minium of a 1-6 demux with input forming logic (state machine).  let me think some more.