# Completely Dumb Question

Discussion in 'General Electronics Discussion' started by Nathon, Feb 1, 2012.

1. ### Nathon

9
0
Feb 1, 2012
I am completely green when it comes to building circuits... as can be gathered from this question. I am an experienced software developer, systems administrator, system builder, etc. but that does me absolutely no good here.

I am building my first circuit which in the development world would be called my "Hello World" application. As is suiting, it's a 555 timer (don't laugh please). That went in just fine and is working no problem. Now I'm trying to add an LED to it to indicate power and to blink on the output. However, as has ALWAYS happened to me since the beginning of time, I cannot seem to figure out the right resistor to use and I just keep flowing LEDs.

I've calculated that I need about a 800Ω resistor (work shown below), but I measured the voltage and it's barely changed at all. I have a 20KΩ Trimmer that I used to test (no LED) and even with it all the way up it gives me less than 1/2v drop. I only have the resistor on the positive side. Am I doing something wrong (I guess the answer to that is obvious)?

For this I have 12V supply and am assuming my resistors are 1.7v at 15mA since they're not marked and that's the safest assumption I can find based on the various resistors I see online.

Formula: V=I*R -or- 12=.015*R
Rearrange for R: R=V/I -or- R=12/.015

Thank you in advance for not laughing me out of the forum! Have a great evening!

2. ### Raven Luni

798
8
Oct 15, 2011
Greetings from another software developer. I hope your sense of humour is as good as mine

Anyway - your calculation seems fine (you should also subtract the voltage drop from the LED before dividing but its always better to go with a higher resistor value to be safe).

All I can think of in the way of an aswer is another couple of daft questions: do you have the right information on the LEDs you are using and are you reading your resistor values correctly?

3. ### Nathon

9
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Feb 1, 2012
Raven,

Thank you so much for replying! I appreciate your time very much. I don't know the exact value of my LED since they were purchased in bulk quite some time ago. I found a site that lists the various voltage and amperage values for LEDs though and chose the smallest voltage and amperate values available. I figured the worst that would happen is it would barely illuminate and I could go from there, rather than what always happens which is I immediately blow the LED in one brilliant millisecond long display. I swear I'm an LED graveyard.

Anyway, I think the whole problem is stemming from the fact that even though I put a large value resistor or trimmer on the positive lead to the LED, the voltage isn't dropping. I've put a picture of my circuit online so you can see it and tell me what I might be doing wrong. I could also put it together in Spice and post the file if you'd rather have that. Here is the link to the image of the circuit.

This doesn't show the resistor & LED that I am trying to add, but I'm coming off of switch 1 of the DIP switch. This is positive 12v and it continues to run across the whole top row. I just continue this to the next section to the right. This goes into the center pole of a 20kΩ trimmer. I'm trying to determine what I should set this to, but even at 20KΩ, I still only get a voltage drop of less than 1/2 volt from that 12v. I'm measuring this between the 2nd switch in the DIP switch, which is the negative side of my power supply. I have no ground, but not sure what I would connect a ground to in a 12v DC system.

I hope I'm not sounding too dumb with all this. lol

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
The LED should have an (about) 1k resistor in series and be connected between pin 3 of the 555 and either V+ or Gnd.

When fiddling I always use 1k resistors because in the worst case it gives you 1mA per volt which, for a power supply less than 20V is never going to give you more than 20mA. You can always calculate a "better" value later if you wish.

V+ is the +ve side of your power supply, gnd is the negative side. It gets more complex if you have a double-ended power supply, but let's cross that bridge when we come to it.

edit: Gee you're neat! It must take you forever to wire things like that up, but it does make it easy to follow.

edit2: My, what a large breadboard you have...

edit3: Nice test equipment too. (are you sure you're a beginner)

Last edited: Feb 1, 2012
5. ### Nathon

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Feb 1, 2012
Thanks Steve!

Steve,

Thanks. Yea, I'm definately a beginner with circuitry and electronic components. I've picked up these things over the years so the cost has been spread out. I was an employee at an electronic component warehouse so I got the breadboard for cheap. The power supply (20A 1-15v +12v steady) I think I picked up cheap from some garage sale. The voltmeter/oscilloscope was a pretty penny though. I was in the market for another meter and decided to just buck up and spend the big bucks on it. However, I figured it was a lot cheaper than trying to get my hands on a nice oscilloscope and I got a nice meter along with it.

The first time I tried setting up the LED, I did it with what the pack said was a 1K resistor and it blew instantly. However, I do remember now that I tested those with my meter and they were way off, so I looked up the color code and they mis-packaged them. I had set it up straight off the 3pin and the other side of the LED to ground/-. Is there a maximum frequency LEDs can handle? I have my 555 running at about 105Hz right now. Is that a problem for LEDs? If so, what is the maximum frequency you can operate them at?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
No, frequency isn't a problem.

Check that the resistors are not 1 ohm.

Might be time to start learning the resistor colour code

Can your meter measure resistance? If not, pick up a cheap meter that can.

7. ### Nathon

9
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Feb 1, 2012
Thanks

Yea, it can measure resistance, but unfortunately I trusted the package. It turns out they were like 300 and whatever ohm (or something close, don't remember right now). I found that out after measuring them with my meter and then verifying with the color codes. I'll take another stab at it with a verified 1kΩ resistor.

Thanks for all your help everyone!

8. ### Raven Luni

798
8
Oct 15, 2011
Heres a good page on colour codes. Reading them becomes second nature before long

http://www.reuk.co.uk/Resistor-Colour-Codes.htm

P.S. When it comes to blowing LEDs few things are more impressive than putting one across a fully charged 4700uF capacitor. I accidentally did this with a 3mm and it was literally blown to pieces

Last edited: Feb 1, 2012
9. ### Nathon

9
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Feb 1, 2012
LOL!

That MUST have been exciting! I watched a fun YouTube video just a little while ago where they were messing with a cap in that range. Welding stuff, vaporizing circuit traces, etc. It was pretty fun. My dad works in a mill and he told me a story once about when he and some buddies were messing with some HUGE capacitors. They charged them up, dropped them in 55 gallon drums (so they were shorted) and said they made a heck of an boom. Not sure if it's typical father-son stories or not, but was fun to think about!

10. ### Raven Luni

798
8
Oct 15, 2011
Look for 'photonic induction' on youtube - that guy likes to blow stuff up with extreme high voltages and currents