I’m trying to figure out how to build a simple circut that will allow me to ocsillate the brightness up and down on an LED. I’m trying to simulate a rotating beacon for my next AC project.
I’d like to have control (either real-time once it’s built or at least in choosing the resistor properties while building it etc.) over two seperate aspects.
The cycle time (rate between peak flashes)
The ramp up/ramp down (the time it takes to fade from off to full brightness and back down again.)
If any of you have watched an airport beacon or a lighthouse before, you’ll be familliar with the effect I’m trying to achieve. Rather than a sharp ON/OFF like you get with a standard blinking LED, I’m trying to recreate the soft glow of light you see before the main beacon sweeps across the field of view. It slowly increases until the main beam is visible, then fades back again slowly.
I’m assuming I’ll need some form of timing circut (like a 555) and perhaps a capacitor to achieve this. Any help offered would be MOST appreciated! [bow]
WOW. OK, now to decipher it all! [:O] I gave up years ago with trying to have the local Radio Shack wiz kids explain anything. I wouldn’t even trust them with explaining a watch battery!
I may be able to hack this one out though I think I will keep looking to see if I can find any simpler means of doing the same thing. Thanks though CK!
That second one had me running for cover! What a frightening circut! LOL I think the first link I got was a little closer to the type of effect and control I am looking for - but I still have to track down the IC chip to test it. I’m most greatful for your work in looking these up for me though - THANKS!
According to the article I should be able to get them to fade in and out at about a half second rate - but nothing. Any guesses as to what I may have done wrong? (I understand it may be difficult to diagnose without seeing the actual board)
I’ve used all the exact parts stated in the article. The only things I can think of might be the use of a polarized capacitor? I was unable to find a non-polarized cap at that size, but I wired it so the current flow was consistant with the rest of the circut. Or perhaps I got the transistor in backwards ie: the emmitter where the collector should be?
Would either of these things cause the LEDs to trun on but not to cycle do you think??
ANY help most graciously appreciated! [banghead][banghead][banghead]
If the transistor was wired backwards, there would be no current flow.
To make a 22 microfarad non-polarized, use two 47 microfarad in series wired back to back. ( The second circuit shown with the two 22’s would equal 11 microfarads. )
Does it mean something bad if the circut runs both with and without the IC chip? It does the same thing - just stays on without cycling. So if I’m getting any light at all, can I assume that the transistor isn’t the problem here and it must be something else? Would a burnt out capacitor keep things from cycling?
Are you sure the base of the transistor is connected to Pin 1 and not pin 3? For the LED to stay on, the base must have a bias voltage on it.
Do you have a digicam to take a pic of your circuit?
I’m not sure (I’m not home at the moment, but will be soon) if I got the pins switched…but I’ll double check it. I do have a cam at home and I’ll take a few shots and see if it helps to show what I have very likely done wrong! LOL
As you are looking at the flat side of the transistor, it goes emitter, base, collector (left to right) for the transistor. As you are looking at the circuit diagram, the base is connected to the +9V. The emitter is the lead connected to the 100 ohm resistor and the LEDs. The collector comes from the op-amp/capacitor.
However, I would suggest looking at the specs of your op-amps. Have you connected them correctly, and do they have power and a ground?
Ah now I’m even more confused than before! I don’t know WHICH way the CBE or EBC goes on theis stupid thing! LOL I switched it around the other way and got nothing at all - not even the steady on glow.
Have I burned it out when I soldered it maybe? I seem to recall that certain components are extremely sensitive to heat (like IC chips).
When looking at the transistor with the flat side facing you, it goes EBC.
If it is wired backward ( CBE ), there won’t be any current flow and the led won’t light. If the led is on continuously, then it is likely that the base always has a
bias voltage on it. If you have a multimeter, measure the voltage between ground and the base ( middle lead ) of the 2n3904. And yes, they are sensitive to excessive heat. At least the 2n3904 is cheap and easily found. BTW , I built the circuit myself on a breadboard, and it does work, however you might want to play with the value of the capacitor between pins 1 & 2 to get the desired delay rate.[^]
yah sorry, Copierguy is right, i was wrong about the circuit diagram. thats what is what i get for posting at 3:30 in the mourning. I would still make sure you have the op-amps hooked up correctly though. That would be my best guess…
WOW, didn’t think of that. It worked great. Variable seed baby!! LOL!! Honestly, I have not designed many circuits; just put them together for lab and I have done a lot of “what the H#LL is wrong with this thing!!”
So, let me see… The greater the capacitance, the more the capacitor has to charge, making it slower, but the light should be brighter, right? I am using about 20uF and it is pretty dim, but I also had to use 55k resistors for the bridged input, and I’m not sure how this affected the current. Hmmm, I have gotten too used to Pspice… Now i sound like a real geek!
Brian, from the sound of it, you will be doing a lot of lighting for your models. It might be worth investing in a proto, or bread board for testing the design.