Science Forums

# Circuitry help!

## Recommended Posts

This isn't exactly homework, it's a thing I want to build to help my sister with. But the last time I played around with circuits was in the late 80's.

I need someone to help me with the following:

I need two LED's to alternately light up and die down. Here's the idea. The first LED starts up, not full brightness, but building up towards it. It should take about three seconds to reach full brightness. This means I need some sort of variable resistor. It should be at full brightness for another three seconds. Then it should go dead, abruptly - not slow like it started up with. When it's dead, the other one should repeat the exact same process whilst the first one stays dead.

The idea behind this is to help my sister with an art project for 'varsity. She's building a rastaman draggin' on a spliff. The first LED is built into the coal part of the joint. The slow lighting up is supposed to simulate the drawing on it. Then, when it's dead, the second one lights up behind the rastaman's eyeballs - diffusely, so as not to be a bright single light source. Ever seen a rastaman's red eyes from too much mary jane?

Why am I doing this? Why is she doing this? I have no idea. Something to do with adding humour to a social commentary sculpture. Or something. But I promised to help her. So, now I need some sort of circuitry boffin here to help me.

Any takers?

##### Share on other sites

I need someone to help me with the following:

I need two LED's to alternately light up and die down. Here's the idea. The first LED starts up, not full brightness, but building up towards it. It should take about three seconds to reach full brightness. This means I need some sort of variable resistor. It should be at full brightness for another three seconds. Then it should go dead, abruptly - not slow like it started up with. When it's dead, the other one should repeat the exact same process whilst the first one stays dead.

...

Any takers?

Sounds like a cascade array of capacitors might fill the bill. I'm not so sure the variable resistor is needed.

##### Share on other sites

Interesting...

You don't maybe know of a freeware 'curcuit simulator'?

I had one years ago, where you can connect different resistors, capacitors, etc., and see what happens on your computer screen, without going to the trouble of building it. I can't remember its name though...

I'm off searching for one of 'em suckers.

##### Share on other sites

Get Crocodile clips - great program, not sure if its free though..

Like turtle said, Variable resistor not needed, but capacitors should do the job if wired correctly with some diodes.

I might have a play around if I get a chance, but I have a MATLAB assignment due in the morning..

##### Share on other sites

Could be done with 4 BJTs, 4 diodes, 4 capacitors and a few resistors (as well as the LEDs of course). Simple enough?

##### Share on other sites

Could be done with 4 BJTs, 4 diodes, 4 capacitors and a few resistors (as well as the LEDs of course). Simple enough?

No! Not simple! I need the diagram, a pair of gloves, a helmet, a set of Wellingtons, a cup of tea and a vice grip!

(But I'll settle for the diagram, though...)

:cat:

##### Share on other sites

I'll try to get at least the pair of gloves through the wire, untill I can get something done with Paint. I don't exactly have a CAD-CAM installed here.

An even simpler solution could be just an astable multivibrator with 2 medium power BJTs, altered with 2 LEDs and 2 capacitors, but that might take a lot more fiddling so I was less sure of suggesting it. To be more reliable you could make one with low power BJTs, each driving a medium power one of opposite polarity and in constant current confinguration. If you know the classic diagrams for these things you should be able to get it. Of course you could always use TTL gates, whichever you find more convenient.

##### Share on other sites

This isnt an easy problem.. I played around with crocodile clips for a while but couldnt get what you specified, the best I did was getting them to alternate on and off..

##### Share on other sites

That took a bit of doing, I hope I've managed to upload the pair of wellies for you. Now it's so obvious which diodes are the light emitting ones that I ain't gonna put a white bullet through them.

The simplest is the bottom right but I'm not sure if it would be easily workable and reliable in practice, as the LEDs are direct loads of the AMs transistors which therefore need to handle enough current, but it might be harder to have the oscillation work reliably.

The most guaranteed model (statutory rights not affected) is the upper left. Two possible simplifications are shown, of course these would give four combinations but you can work out the fourth 'cause it ain't included in the price. One is that the diodes for discharging the LED's caps might not be all that necessary, you'd have to try the difference in how gradually they light up and extinguish. The other does away with those double diodes that give a constant current config, using a normal common emmitter instead; this would need to be designed for a more exact supply voltage which could however be a bit lower if necessary.

Edit: The diodes in the constant current config need to be ordinary silicon ones while the dischargers can preferrably be Schottky barrier.

##### Share on other sites

i built the lower right one, not sure on the ratings of all the componants so I will just play around a little and see it I can get them to flash

EDIT - ok didnt work.. trying the top right, but not much action happening there either!

##### Share on other sites

Great replies, guys!

I looked up Crocodile Clips, Jay - for the cheapest version they wanna rip me \$400!!! ...seems like I'm gonna be skipping on that one for a while.

If you can get Q's circuit to work on Croc Clips, I'll build it!

This is why I just looooove Hypo!

##### Share on other sites

Which \$ are you talking about? Japanese?

Well it suddenly hit me what a dumb thing I had done, putting the caps directly on the LEDs even with the extra transistors! Caps can go on the base of these! Attachment for both common emitter and constant current.

JQ, it'll take a bit of careful calculation to work out the right values for resistors and capacitors, according to transistor ratings and supply voltage etc.

##### Share on other sites

Might be late in the game now, but because of size limitations, I'm look at using those flat 3V lithium batteries - like those on a motherboard. So I suppose any multiple of 3V would do as input voltage. If you need more, I can supply 6V, 9V, etc. - I'll just keep adding batteries.

##### Share on other sites

Well 3V isn't much more than a LED takes but the circuit to the right in my 2nd attachment can probably manage with it, the constant current config isn't much nessecary anyway without the caps being directly on the LEDs. This link should be of help in sorting out the AM and the second stages can be fiddled with by using a dpst switch to simulate the rest of the circuit, given a value for the collector resistors in the AM.

BTW in this case the discharge diodes definitely should be Schottky barrier.

##### Share on other sites

hmm, im unsure where to start with the calcs because I dont really understand the aim of the circuit.. I get that its supposed to change the current running through the diodes - increasing and the shut off, but Im not sure how it is supposed to do that..

Can you give me an order of magnitude estimate for the ratings of the componants? milli/micro farad, kilo ohms?

##### Share on other sites

• 3 weeks later...

I think a microcontroller with several resistors and switches will do this job perfectly. You can set the 3 seconds delay precisely by some simple programming.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.