are there any problems with having lots of resistors in series

Thread Starter

lotusmoon

Joined Jun 14, 2013
227
I am using a 555 timer circuit similar to this one but using it to drive an LED flasher - http://forum.allaboutcircuits.com/showthread.php?t=84399
I will have 12 separate frequencies available on it all run to a twelve way switch.
I wish to get the frequencies very accurate so for each frequency i may have up to three fixed resistors - {1-10r) (10-100r)(100-1k)(1k-10k)ect
this means that for the last frequency in the line I could have up to 36 resistors in series. Are there any problems with this?
 
Last edited:

ScottWang

Joined Aug 23, 2012
7,397
You said that you want to drive an LED flasher, but the LED flasher is for our eyes to look, do you really need the accurate frequencies?
How accurate you want?
What's the frequencies?
I wish to get the frequencies very accurate so for each frequency i may have up to three fixed resistors - {1-10r) (10-100r)(100-1k)(1k-10k)ect
this means that for the last frequency in the line I could have up to 36 resistors in series. Are there any problems with this?
it's ok, or you want to spend more money to buy small pcb type Potentiometer, there are many kinds of types, you can choose the type you like from internet or local stores.

 

shortbus

Joined Sep 30, 2009
10,045
<snip>I wish to get the frequencies very accurate <snip>

If your desire is to have "very accurate frequencies", your wasting your time with the 555. They shift frequency with variance in heat and other outside influences. Your better off to start with a crystal oscillator and electronically divide it to your desired frequencies. http://en.wikipedia.org/wiki/Crystal_oscillator
 

wayneh

Joined Sep 9, 2010
17,496
+1

You'd also be wasting time if you were not using high precision resistors, but those would be a waste since the 555 itself will drift as noted.
 

Thread Starter

lotusmoon

Joined Jun 14, 2013
227
I was going to use a frequency counter to set the frequencies what kind of % drift is there in a 555 circuit.?

thats very interesting about the crystal oscillator, do you know where could I find a schematic and information how to do this?
 

Thread Starter

lotusmoon

Joined Jun 14, 2013
227
Thank you I guess the only way to know is for me to build one and test it to see the % variance.
 
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wayneh

Joined Sep 9, 2010
17,496
No, these things are known and predictable. Resistors have temperature coefficients and specified precision ranges. The 555 will have a specification. OK, it may do better than the spec, but that's not something you want to rely on.

There's a reason that guitar tuners, for instance, use quartz crystal oscillators. A 555 would never allow adequate accuracy.
 

THE_RB

Joined Feb 11, 2008
5,438
A couple of percent variance is typical with a lot of caps in 555 circuits.

Just the temperature rise in the cap in the time from first being switched on to being running for a few minutes can give you 2% freq variation. Room temp changes or temp changes inside your enclosure can make that worse still.

Do you know the actual frequencies you need?
 

WBahn

Joined Mar 31, 2012
29,979
You've got a fundamental problem in that let's say that I want to create a resistance of 2487Ω using a 2kΩ, a 400Ω, an 80Ω, and a 7Ω resistor. Let's say that I want it to be accurate to ±1Ω. So my last resistor can be a 10% resistor. Doesn't sound too bad. But the 80Ω has to be a 1%, the 400Ω has to be a 0.1%, and the 2kΩ has to be a 0.01%. And even that won't quite do it as it would give me an uncertainty of roughly ±2Ω.

If the 555 has short-term variability of a percent or two, then you are gaining nothing by using multiple resistors instead of a single 1% resistor per frequency (or perhaps 0.5% since you would still have twelve in series for the slowest frequency).

I think I am picturing the circuit you have in mind for your switching. Are you putting a resistor (a series of three, in your case) between each pair of adjacent contacts and the using the wiper as one end of your adjustable resistor and one of the end contacts as the other? I think you can do much better with a slight modification so that each frequency selection uses a single, independent resistor. Just put each frequency's resistor on a separate pole of the switch and tie the other end of all resistors together. Now you can change one frequency without affecting all of the frequencies lower than it.
 

Thread Starter

lotusmoon

Joined Jun 14, 2013
227
Hi I have been trying to make a new thread for a few days now but it shows up server error each time. is there something i can do to achieve this?
 

Thread Starter

lotusmoon

Joined Jun 14, 2013
227
thank you for all of these new posts lost of great information.
The error I am getting when trying to start a new thread is --
Server error
The website encountered an error while retrieving http://forum.allaboutcircuits.com/newthread.php?do=postthread&f=6. It may be down for maintenance or configured incorrectly.
Here are some suggestions:
Reload this web page later.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfil the request.
 

Thread Starter

lotusmoon

Joined Jun 14, 2013
227
I am just experimenting at this stage. I will build this circuit and give it time to warm up and then fix and test the frequencies when it is warm. The accuracy is not a problem for me but I would like to be able to give a % variance to the people I am giving these to.
 
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Metalmann

Joined Dec 8, 2012
703
If your desire is to have "very accurate frequencies", your wasting your time with the 555. They shift frequency with variance in heat and other outside influences. Your better off to start with a crystal oscillator and electronically divide it to your desired frequencies. http://en.wikipedia.org/wiki/Crystal_oscillator


Thank you, I just thought those 555s were more accurate than that.

Always helps to read from the Pros!:cool:
 

WBahn

Joined Mar 31, 2012
29,979
The first thing you need to do is determine how accurate you need your frequencies. Don't be qualitative about it. It does cut it to say, "I need it to be very accurate," or "I need it to be as accurate as possible." You need to say something like, "I need it to be accurate to within 0.01%, " or "I need it to be within 1Hz from 100Hz to 10kHz."

And it is important that you determine how accurate you NEED it to be, not how accurate you would LIKE it to be.
 
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