Ok.... I have been reading a lot of sites and how-to to do this.

I'm wanting to make several green led's throb from very dim to full bright. I don't want them to go completely out. I want to use a TLC555 or if i have to 556CN. the green leds are 3.5v @20mA. I would also like to have a switch or button implemented to activate it and turn it off.
I'm thinking of setting the 555 up with a 50% duty cycle and using a trimpot to set the desired rate for the throbbing. im just unsure of the correct uF caps and NpN or PnP to use. I have a breadboard for set up and testing. i would like to either use a 6v drycell for power or impliment a wall wart that is 5vdc @ 2000mA output.

 

By a switch to activate it, do you mean leave the LED blinking, or turn it all the way on or off?

The subject has come up before, in this thread...

http://forum.allaboutcircuits.com/showthread.php?t=2389

among others.

My contribution, which I have never built, was this.

Aim for a 50% duty cycle. Get the frequencies on each oscillator close to each other, the difference in frequencies will be the fade in/fade out duration. Basically a form of hetrodyning, but it meets your requirements exactly, full on, full off. Pin 4 of the 555 is a reset pin, which could also be called enable. If one oscillator is turned off the LED is on ½ brightness. If both are turned off using pin 4 then the LED is dark.

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

Bill's Index

You may have noticed I tend to be one of the 555 guru's around here. Here's a little sheet that may help in the future.

 

Bill,

I think that you could replace the full-wave bridge with a CMOS Exclusive "OR" gate such as the MC14070 and get the same result.

I too have never built this particular circuit but it has a sound operating principle behind it.

hgmjr

 

My thought with the diode bridge is you get to keep the 555's excellent drive characteristics, this could handle 4 or more 555s, each with it's own diode bridge and LED, all criss crossing each other and having it's own flash pattern. Something similar to my From 4, 20 article.

The logic works out identical as with an exclusive OR gate, oddly enough, and diodes are cheap. You can even get the bridge as a single component, as opposed to a chip. The downside is you loose 1.4V through diode drop, combined with the 1.4V drop from the 555 and a 3 Vf drop from the green LED you'll need at least 6.5 volts on the power supply, even though a conventional 555 minimum power supply spec is 4.5VDC.

The 6V power supply might work, but it will be tight, and depend on the LED somewhat. If this is unacceptable then the OP would have to use a MC14070 and add a transistor driver.

 

My thought with the diode bridge is you get to keep the 555's excellent drive characteristics, this could handle 4 or more 555s, each with it's own diode bridge and LED.

The logic works out identical as with an exclusive OR gate, oddly enough, and diodes are cheap. You can even get the bridge as a single component, as opposed to a chip. The downside is you loose 1.4V through diode drop, but even at 4.5VDC for a conventional 555 (minimum power supply spec) it would work.

I agree with your comments. In particular, taking advantage of the drive levels from the 555 does keep the design simple and easy to troubleshoot if it happens to misbehave.

hgmjr

 

I was editing... It happens. Got to thinking about where the voltages were going. There are good and easy ways to still use my schematic and a 5V power supply (original OP specs), such as using a type of diode called Schottky's, which drop a lot less voltage.

Things that use voltages...

555 are 1.4VDC less than power supply when on....1.4V
Diode Bridge...................................................1.4V
Schottky Diode Bride.........................................0.3V
Modern Green LED.,...........................................3.0Vf
Modern Red LED................................................2.5Vf
Old Green LED..................................................2.0Vf

By picking and choosing your components it can go pretty low on the power supply.

 

For years I have used a linear circuit to slowly fade and brighten LEDs.
It is similar to Bill Bowden's Fading Red Eyes circuit where two opamps form a triangle-wave generator. I used three of the generators driving a red, a green and a blue LED at slightly different frequencies. The colours add and make slowly changing millions of colours on a white ceiling.

The power supply voltage and the 33k and 47k voltage divider determine the amount of dimming and brightening of each LED which are different for the different colours.

 

i was talking switch to activate it and then pressing the switch again would deactivate it.
i have seen some set ups using the 555 and a NpN to help reveres or dissipate the current for the Led making it dim. I idea for the project is to make it look like the thing is glowing or breathing inside. So the Diode bridge will help with the variable voltage to make the Leds full on and fade to almost off and then back on again as the Cap in the 555 charge and discharge. what is a good size cap for the 555 I have seen use of 10uF

 

If you are refering to my design (you need to be specific) the diode bridge is used to multiplex two high frequency signals, the difference between them is the beat signal. Look up hetrodyning for more info, we're rejecting the sum, but using the difference in frequencies.

Advantages include small parts, including capacitors and being digital, fair efficiency. Disadvantages include freq stability is a must, if it drifts the beat frequency will also drift.

AudioGuru's method also works, but is a more analog approach. It is also simple like the 555 method, but does require larger capactors. There will be more heat generated as a result of being analog, but not too much.

If you refer to the original post I linked to early on you'll see some other methods.

The switch method you refer to can be mechanical or electronic. I have seen push on/push off switches. You could also use a simple flip flop to control the 555.

If you were going with my design you can use 0.1µF caps, with Ra being 1KΩ and Rb being 10KΩ (referencing to the datasheet I posted). One of the oscillators is going to have a variable resistor to adjust it's frequency. Something like Rb using a 9.1KΩ in series with a 1KΩ 10 turn pot to adjust the flash rate.

I will be formalizing my schematic for my 555 projects when I get caught up, but this could take a while.

The 555 Projects

 

I understand the diode bridge. What diodes do you recommend for the bridge. I picked up some 1N914 switching diodes from radio shack. and a 1K ohm 15turn trim pot.

 

The 1N914 diodes will work but remember that they will introduce a combined voltage drop of approximately 1.4V off of the voltage you are using to power you 555 timer. You will need to take this into account when selecting the LED current limiting resistor.

hgmjr

 

ok another question would adding a larger say 10uF cap in the circuit help for the power out put loss and adding several leds on the out put side?

 

I would suggest you start by getting the circuit to work as originally designed. Once you have it operating then you can explore what design changes you can get away with.

hgmjr

 

You mentioned your power supplies. I suspect 5V isn't enough, and 6V is going to be close. How much voltage does your LED drop when you give it 20ma? You mentioned green, so I am assuming 3.0Vf (Vf is the forward dropping voltage of the LED), but we need to pin this number down.

I'll redraw the schematic when we have all the numbers, such as the LED and power supply, to real world values.

 

i set my 556 up tonight on my board like the schematic on the right. I added a 1kΩ 15turn trim pot at Ra1 and a 8.2kΩ at Rb1. I set the other side up per below With 1kΩ @Ra2 and 10KΩ @Rb2. each has a .1uF cap.
Ill post some pics of the set up on my bread board later if anything needs clarification.
The Led comes on but their is no variation or dimming no matter which way I adjust the Trip pot.

Aim for a 50% duty cycle. Get the frequencies on each oscillator close to each other, the difference in frequencies will be the fade in/fade out duration. Basically a form of hetrodyning, but it meets your requirements exactly, full on, full off. Pin 4 of the 555 is a reset pin, which could also be called enable. If one oscillator is turned off the LED is on ½ brightness. If both are turned off using pin 4 then the LED is dark.

 

Is the 1K resistor the one you are using in series with the LED? If so then you need to drop that to 0.6V/0.01mA = 56 Ohms.

hgmjr

 

no the 1kΩ is Ra on the top part of the 556

 

What resistor value are you using in series with your LED?

hgmjr

 

I have a 51Ω in series at the Led. Are there any good component and schematic drawing software I could download? So I can draw what I have set up. If not I'll have to do it on paper and then scan it.

 

Linear Technology's LTSpice is available for free on their website:
http://www.linear.com