Very new guy needs help designing an LED circuit.

[CS1726]

Hello everyone,
I am EXTREMELY new to electronics. I am trying to figure out how to put together an LED circuit that will be a part of a larger (non-electrical) hobby project I am building. The design is to have a total of 12 LEDs, 1 illuminated on/off push button. When the pwr is turned on, 4 of the LEDs and the illuminated button will light up and stay on. The other 8 LEDs will fade on and then slowly "pulse" or "breath". After MUCH frustration and countless YouTube videos, I have been able to breadboard a circuit to accomplish the pulsing effect on 1 LED. It's now time to figure out how to make everything else work correctly too. This is why I have respectfully come to your online community for help. If someone could figure this out for me, it would save me a ton of time and you would have my humble gratitude. I will attach a photo of what I have come up with so far. The area circled in pencil is how I got 1 Led to "pulse" on and off at the rate I wanted. Green indicates the LEDs I need to pulse and yellow shows the illuminated button and other 4 LEDs that I need to simply light up and stay on. On top of each LED I have place a small "1" or "2". This is because I will be using two types of blue LEDs. [Type 1: 5mm Blue, Vf=3.2, If=30mA]. [Type 2: 3mm Blue, Vf=3.5, If=30mA]. Power supply will be a standard 9V "square" battery. (But that can be changed if need be. ) THANK YOU so much for you time and any help/advice you can offer. Remember: this photo is just me putting something down. Feel free to do whatever you need to in order to make it simple and functional. -Cheers!

[pebe]

Hi CS,
Generally, the basis of your circuit is OK, buy these are the points that should be noted.

The switch is wired in the wrong place. As it stands, it will put a short circuit on the battery as soon as it is switched on. It needs to be moved so that it is in series with the +ve lead out of the 9V battery. Its LED should be wired between –ve battery terminal and the 9V supply ‘rail’ so it lights up as soon as the unit is switched on.

When the LEDs are pulsing, pins 2&6 of the 555 vary between 1/3 and 2/3 battery voltage, so allowing for the battery to fall to 8V and allowing for the 0.6V BE voltage drop of transistor, the available voltage for the LEDs is 2.06V to 4.7V. That is not enough to maintain the LED on because it needs 3.5V to stay lit. If you use 2 x 9V batteries in series, then you have a voltage swing between 4.7V and 10V and the LEDs will stay lit when dimmed.

For the ‘2’ LEDs, it is not recommended to wire several directly in parallel as you have done, because there will be small differences in Vf for each LED and the ones with the lowest Vf will take most of the available current. If all LEDs come from a single batch they may have well matched Vf, in which case you may get away with it. If not, you will need a separate resistor for each LED.

If you use 2 x 9V batteries and each of the ‘1’ LEDs has a Vf of only 3.2V, then you could rearrange the 4 LEDs in series plus a single resistor in a single string. That would save about 90mA of battery current.

Will you let me know if that all makes sense and if you can sort out the values of the resistors?

[CS1726]

Hello Pepe,
Thank you so much for offering the advice. I have tried to rework things a bit to better match up to your suggestions. I see how the duel 9V batteries would work better. However I would like to keep the size down. So if it is possible, I would like to try to stay with a single 9V supply.

The pulsing that I have figured out so far has the LED fading in from 0 to being very bright. Then it fades out again back to 0. This is the effect that I wanted.

I have redrawn the circuit and tried to figure out the resistors. However I still feel like I'm fumbling my way through the dark.

Check out round 2 and let me know what you think...

[CS1726]

"Pebe" -not- "Pepe". I got your name wrong in my response above. ....Sorry about that.

[pebe]

Hi CS,
No problem with the name - " 'er indoors" calls me all sorts of names

The switch is now in the right place so your circuit is now workable. But I would still make the point I made before, ie. it is not advisable to run LEDs in parallel through a common resistor.

Also, the NPN transistor supplies a triangular waveform at its emitter that goes down to 2.06V which, according to the Vf information you supplied, would leave the LEDs switched off for a period in the waveform.

But if you have tried it out and are happy with the results then go ahead.

[CS1726]

Pebe,
Understood. I'll change it to add a resistor to each each LED that is in parallel. Once I have done so, I am going to see if I can get it set up on a breadboard and finally test it out today or tomorrow. I will certainly let you know how it turns out ASAP. You have been a HUGE help. I really appreciate the time you have taken to look at this and offer your assistance. Thank you!

[pebe]

I am pleased to have been of assistance.

[CS1726]

Well, things have been really busy lately and I have not had time to actually set everything up on the breadboard yet. Here is where I'm at so far. I have changed the design so that every LEDA has its own resistor. I then drew a breadboard layout of the circuit. This way, when I DO find time, I won't have to think about how to build it. If it works, then I will move on to figuring out a printed circuit board (pcb). I'm still open to any comments or suggestions that anyone may have.

[CS1726]

Here is the breadboard layout I mentioned...

[pebe]

Hi CS,
Yes, that looks about right. Two points to consider though.

You may need a limiting resistor in series with the LED in the switch, if one is not already fitted.

You may have to reduce the value of the eight resistors in series with the fading lamps, because there will be a maximum of only about 1.8V across each resistor. That would give you a max of 8mA through each LED. Changing to a 100ohms will give you about 18mA.