need some help with throbbing leds

SgtWookie

Joined Jul 17, 2007
22,230
no i only have this one. i have 9v at pin 3 0v at pins 2&6, 6v at pin 5.
OK, so pins 2 and 6 are connected together?
And R1 is connected between pin 3, and the junction of pins 2 and 6?
And C1 is connected between the junction of pins 2 and 6, and the battery negative terminal?
 

Thread Starter

McGuffin

Joined Sep 19, 2009
52
yes everything is connected like that. I just tried reading voltage at Q2 emitter going to the leds and with the meter connected there and ground the led comes on and blinks but it is very very dim all I can see is the green center of the led light up. Im thinking now that the resistance of 220ohm at the led might be too much
 

Wendy

Joined Mar 24, 2008
23,415
Wookie is on track on how to trouble shoot it, there isn't too much I can add. If you look at the 555 Hysteretic Oscillator article, it goes into theory of operation for this kind of oscillator. Pay special attention to this illustration...



Pins 2 and 6 are effectively one pin in this design, and are very high impedance (don't interact with the RC much). This circuit is slow enough that you can see the voltage changes with the DVM.

The LED drops 3 volts when on, the transistors 1.2. With a 9V battery it is 9V-3V-1.2V=4.8V. To figure current 4.8V/220Ω=21.8ma. 220Ω is actually too small, not too large, but it works.

You either have a transistor wired wrong, or the LED backwards, for it not to be lit. If you put the meter across the LED what voltage do you get? Should be 3V. If you put the meter across the resistor you should get 4.8V, and the transistor should be 1.2V across the collector/emitter.

Wait a minute, you said it blinks? How fast? It should blink (this is the whole point), around one cycle per 3 seconds. If it is blinking then try adjusting R2 down. It is possible it is averaging the pulse too much, as you adjust R2 pot down it will start to get brighter and dimmer in a pulsating fashion.

Seems like things are changing on the readings too much (as in the voltages won't stay put, and now it is blinking), how did you wire this circuit? On a protoboard or soldering it up?

Like I said, this is a slow oscillator (again, the whole point). You should be able to see the output (pin 3) alternate between 7.8V and ground (0V) every second or so on the DVM.
 
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SgtWookie

Joined Jul 17, 2007
22,230
OK, see the attached; it's a simulation of the circuit.

You'll see the waveforms of various points, and you can see how they change over time.

What's the Vf @ current rating for your LED again? I don't recall seeing it.

[eta]
OK, 3.5v@20mA - was in your first post. :rolleyes:

That's the problem. If it were a 1.7v@20mA or 2.1v@20mA, it would be much brighter. However, with it being 3.5v (maybe even higher) it won't be very bright with the circuit the way it is. Changing R3 to a lower value won't help much.

The 555 timer is spending too much time high, and not enough time low. This is due to the drop across the Darlington voltage follower in the 555 IC itself.
 

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Wendy

Joined Mar 24, 2008
23,415
Don't forget, that is maximum settings, you can adjust the intensity of the LED brightness even wider by adjusting R2 down.

I liked your idea about making R1,R2 100KΩ and C1,C2 22µF. You could then translate the 555 to the CMOS version, and really drop the current requirements. It would make the 9V battery last several days or more, as well as squaring up that wave form more.

BTW, thanks for the assist Wookie!
 
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SgtWookie

Joined Jul 17, 2007
22,230
Don't forget, that is maximum settings, you can adjust the intensity of the LED brightness even wider by adjusting R2 down.
Well, sort of. The problem here is that the 555 timer was spending too much time high, and not enough time low; the average voltage on C2 was staying too high to turn on Q1/Q2 and enable current flow through the LED.

It's that "Darlington follower gotcha" again. Might've been fixed by simply pulling pin 5 down a bit by using an 18k to 22k resistor between it and ground...

I liked your idea about making R1,R2 100KΩ and C1,C2 22µF. You could then translate the 555 to the CMOS version, and really drop the current requirements. It would make the 9V battery last several days or more, as well as squaring up that wave form more.
Well, R1 and C1 could definitely be changed, but when I posted that initially, I was looking at the dual 555 circuit, not this later one. Whoops!

BTW, thanks for the assist Wookie!
You're welcome ;)

Here's an idea - ever think of using a voltage follower from C1?
You already had a rough triangle wave; no need to make things more complicated.
 

Thread Starter

McGuffin

Joined Sep 19, 2009
52
Ok here is the video of what it does when Ii use the meter @ Q2 emitter to read voltage. you can see the led around the 40sec mark.
 

SgtWookie

Joined Jul 17, 2007
22,230
OK, so at the 40 second mark, the LED starts flashing on and off; not fading in and out.
That tells me that C2 is not connected, or R2 is set far to low.
 

SgtWookie

Joined Jul 17, 2007
22,230
I've looked at the video three times now.
It's so dark, that I really can't see much except for the fact it's a Radio Shack breadboard, then you pan away for 10 seconds or so, then there's a probe stuck on something and no way to tell what you're measuring, or if you might be shorting something out with one of the probes.

Something that you should know about that particular breadboard, is that the power busses (the two running horizontal at the top and bottom) are not connected in the middle. I leave permanent jumpers in mine to ensure that they are connected.

You may have some intermittent or bad spots in your breadboard. Try moving the components to a different location.
 

Thread Starter

McGuffin

Joined Sep 19, 2009
52
reworked it to Wookies design and it still does not work unless I check voltage at Q2 emitter to ground. like it does in the video it blinks not fade.
 

Wendy

Joined Mar 24, 2008
23,415
Well, sort of. The problem here is that the 555 timer was spending too much time high, and not enough time low; the average voltage on C2 was staying too high to turn on Q1/Q2 and enable current flow through the LED.

It's that "Darlington follower gotcha" again. Might've been fixed by simply pulling pin 5 down a bit by using an 18k to 22k resistor between it and ground...


Well, R1 and C1 could definitely be changed, but when I posted that initially, I was looking at the dual 555 circuit, not this later one. Whoops!


You're welcome ;)

Here's an idea - ever think of using a voltage follower from C1?
You already had a rough triangle wave; no need to make things more complicated.
As to the latter idea, I thought of it, but you couldn't get the adjustment out of it you need. With R2 it should be able to go completely dark before lighting again.

The voltage divider off the 555 output does have some merit. I was (and still am) trying to keep it simple, which is why I left the secondary resistors off of both R1 and R2.
 

SgtWookie

Joined Jul 17, 2007
22,230
OK, looks like you're trying to use the upper busses, but they are not connected from end to end like I mentioned a couple of posts before.

Add jumpers, shown in pink:
 

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McGuffin

Joined Sep 19, 2009
52
Ok Wookie mentioned something that Made me do a big :Facepalm: The runners are the separated ones. So Bill you original design works 4.0 now. Wookie your design make the led Flash. Im going to get some Cmos 555's and work it again with the low power consumption for the final product. Now to add 2 more leds in this design wouldIi just need to remove R3 since the other two will be droping voltage. Should they be series or parallel?
I also need some different 10k pots. the two i got like to pop out of the breadboard when you want to ajust it or read it out with the meter. these two will be fine in the final soldered board.

I should have remebered to K.i.s.s It
 

SgtWookie

Joined Jul 17, 2007
22,230
Don't beat yourself up about those power busses. I had to find it out the hard way myself. Those particular Radio Shack boards are the only breadboards I've seen that were like that. RS hasn't stocked that particular breadboard in years. Trust me, just keep jumpers where I have illustrated above, and you'll be OK.

If my design makes your LED flash, then you have connected something on the wrong side of R2.

Be careful with the CMOS 555 timers; they have far less current sourcing/sinking ability than the bjt 555 timers, and they are very static-sensitive. You will have to increase the size of the resistors, and decrease the cap sizes.

If you want to have more LEDs, you will need to add them with their own resistors in parallel to the 1st LED and resistor.
 

Thread Starter

McGuffin

Joined Sep 19, 2009
52
I forgot about the two types. I used the other style in labs. I had to have Rs call to different stores to find these two. Yeah im going to make a wrist strap or try and find one at rs before I start with the ESD stuff. reducing the caps and getting different pot wont be a problem. Also I mentioned earlier post about adding a push button to turn it on and off. Would I add it inline to the Vcc side or would it work better on the ground side? Also would the extra leds need the same resistor? Im trying to remember my kindergarten lessons on paralell Leds and resistance.
 

SgtWookie

Joined Jul 17, 2007
22,230
I forgot about the two types. I used the other style in labs.
When I started in electronics, the 555 timer hadn't been introduced yet. ;)

I had to have Rs call to different stores to find these two.
RS used to be a hobbyist experimenter's dream back in the 70's. You could go into a Radio Shack, and they would have aisles filled with all kinds of IC's, tools, etc - you could find most things you needed, and the people in the store actually knew their stuff about electronics.

Nowadays, they know all about cell phone plans (and are more than happy to sell you on one), practically nothing of electronics, and have a very limited selection of components.

For small purchases, Digikey is hobbyist-friendly. They will actually ship a SMALL order via USPS 1st Class, which will save you money, and they don't have a minimum purchase requirement.

Yeah im going to make a wrist strap or try and find one at rs before I start with the ESD stuff.
ESD is a big problem in arid climates. Here in FL, it's so humid whether you're inside or out that ESD is of little concern. Wrist straps are a good idea, but unless you have a way to test them, you don't know if they are effective or not. Just having dry skin will make them ineffective.

reducing the caps and getting different pot wont be a problem.
R1/C1 can be very easily exchanged by shifting decimal points. R2/C2 is a different matter. You might consider going to MOSFETs instead of a Darlington configuration.

Also I mentioned earlier post about adding a push button to turn it on and off. Would I add it inline to the Vcc side or would it work better on the ground side?
You would need to find a push-on, push-off switch somewhere. Radio Shack hasn't carried that kind of thing in many years. You'd be better off with a toggle or slide switch; one side on, one side off.

With power switches, it's customary to interrupt the "hot" side. With battery powered equipment, it's necessary to declare one side "ground", or the 0v reference - even though it may not be a true "ground". We have declared the negative terminal of the battery as "ground", so in this case it would be more proper to switch the connection to the positive terminal. In reality, it doesn't matter - but it WILL matter to the person trying to figure out the schematic later on.

Also would the extra leds need the same resistor? Im trying to remember my kindergarten lessons on parallel LEDs and resistance.
Yep, they'll each need the same value resistor. Don't forget that since they are in parallel, the current draw will go up.
 
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