Hi all

I am working on a basic project, just for personal use. I am attempting to build a pulsating/glowing Kryptonite Crystal, it will be approx 6-7" tall and 3-4" wide. There is a purpose for the Crystal which is another matter so I will not go into it here.

I am hoping to build a circuit that I can sit in the bottom of the crystal and have 2 Green hyper bright 5mm LED's that pulsate alternately which will hopefully be bright enough to be seen by the audience from the stage/performing area. Ideally, I would like it so that neither LED actually goes off completely and as one is fading out the other is fading in so there is always some light been generated.

I have a circuit that I will attach and I would just like to know if what I have will work or if I have made any error's. I'm only an amateur when it comes to electronics but I'm trying at least LOL.

The circuit uses a 555 timer and I have also included a potentiometer so the speed of the pulsating can be adjusted if required. The Schematic shows a 9v battery in use but ideally, I would like to use a smaller battery that is easy to find, maybe a 12v N-type, like in some car remote key fobs but I don't know if it would be sufficient to power the circuit or what the battery life would be, I've used a 7555 timer as these can apparently work at a slightly higher voltage to a standard 555.

The crystal will be made from a silicone mould using clear Epoxy with a touch of green coloring.

Anyway, have a look and let me know what you think.

 

Just noticed my self that I have not included a power switch, I'll amend that

 

Personally, I'd go with an Arduino driving a couple of FETs to PWM the LEDs so you can easily fiddle with the lighting patterns to get the most pleasing results. That would allow you to dynamically change the lighting as needed.

 

I think that circuit will fade the LEDs up and down alternately.

 

I think it will just flash them alternately
Well, maybe not. I just noticed it has the cap on the bases. It will be better to use PWM to limit dissipation of the transistors. Build it and see if it works.

 

I think it will just flash them alternately

No. The LEDs are driven by the voltage ramp on the capacitor.

 

Personally, I'd go with an Arduino driving a couple of FETs to PWM the LEDs so you can easily fiddle with the lighting patterns to get the most pleasing results. That would allow you to dynamically change the lighting as needed.

I had thought of this, however, I am trying to keep the board to an absolute minimum in relation to size. Also, to be fair, once the fade rate is set it is very rarely going to need to be changed, it's more like a "Set and Forget" thing.

I think that circuit will fade the LEDs up and down alternately.

That is the effect I am after, I was just really wanting confirmation that the circuit WILL indeed work and that I was not missing anything stupid before designing & printing the board.

 

You just beat me to it. I edited my previous post. (A couple of times to fix typos that I usually do when using the iPad)
Derek, build it on a proto board or breadboard first to get it working, then do a board.

 

If your circuit behaves erratically, try adding a ceramic capacitor (.01 uf of higher) between pins 1 and 8 as recommended by the manufacturers.

 

Well I built the above on breadboard and it works just how I want it when powered at 5v.

However, when I power it from a PP3 9v it does not work very well at all, take a look at the videos below.

5 volt

9 volt (ignore the audio, it's my work colleagues who were unaware that I was recording)

What do I need to do to make the 9v one operate like the 5v?


Cheers

 

Try placing a resistor across the LED, or add additional LED's in series to get it to go dark in the down stroke.

 

Sensacell beat me to it I would put normal silicon diodes in series with each Led rather than another led. and not sure about placing a resistor across the led.

 

I would re-configure the 555, use pin 7 as the discharge for the capacitor rather than pin 6 which is threshold. Your trouble seems to be your timing capacitor has a residual charge sufficient to bias on the transistors at 9V supply.
I suppose as it is, put a low voltage (3.3V or so) in series with each transistors base.
Always decouple pin 5 with a 10nf to 100nf capacitor.
From experience, I prefer to take the output from pin 3, keeping the timing part separate.
pin 2..trigger, pin 6 threshold, pin 7 discharge. There are a thousand ways of using this little i.c but take a look at astable circuits using it and most follow the above convention.
https://en.wikipedia.org/wiki/555_timer_IC
http://www.circuitbasics.com/555-timer-basics-astable-mode/

 

LTspice says the additional diodes in series with the LEDs, as suggested above, will do the job, but give a minimum brightness of zero. Here's an alternative if you want some control over the minimum brightness :-

R3 provides the control. Increasing it increases the minimum , but also introduces some asymmetry between the brightnesses of the two LEDs .

 

From experience, I prefer to take the output from pin 3, keeping the timing part separate.

TS wants the LEDs to fade in and out. That is why the '555 is configured the way it is and why the drive to the transistors comes from the timing capacitor.

The problem of the LEDs no going completely out when running from 9V is because the voltage across the timing capacitor goes from 1/3 VCC to 2/3 VCC and so the transistors are still conducting at both ends of that range so a circuit change like that in post #14 should fix the problem.

 

TS wants the LEDs to fade in and out. That is why the '555 is configured the way it is and why the drive to the transistors comes from the timing capacitor.

The problem of the LEDs no going completely out when running from 9V is because the voltage across the timing capacitor goes from 1/3 VCC to 2/3 VCC and so the transistors are still conducting at both ends of that range so a circuit change like that in post #14 should fix the problem.

Ah yes, I admit i missed the point about the fading, I saw he was taking the drive from threshold and that it why it was holding the transistor on.

 

Just thought I would update you all as I went down a different route.

I built the circuit on breadboard and for no other reason, other than it's all I had lying around, I used Bi-Colour LED's and everything appeared to work as I wanted it to. I ordered some White SuperBright LED's and changed the Bi-colours for the white ones and the LED that it triggered by the 3906, did not illuminate but if I swapped out the white LED for the Bi-Colour it was working fine. After much hair pulling, I discovered that the 3906 was not dropping to Zero volts, according to the spec's it was only getting to .7v. I decided to use a diode at the base to pull it down to Zero.

My next issue was when I tried the circuit with a fresh battery as the one I was using was down to about 7-7.5v, the LED's would not flash and were simply illuminated with the fresh Batt so I stuck a 100 Ohm resistor in there and all is good with a fresh batt and is still good if I put the partially drained batt back on.

I have also added some additional LED's, all white.

I've added a schematic if anyone is interested.

thanks for your help and advise.