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### Topic: Circuit Design Help (Read 2262 times)previous topic - next topic

#### Wawa

#30
##### Nov 27, 2016, 03:50 am
I would not rely on 100pf being 100pf on a breadboard.
And there could be some unwanted coupling between IC pins.
A significant higher capacitance, e.g. 1nF will drown out the 10pF? breadboard/wire capacitance.
Leo..

#### Shandy

#31
##### Nov 27, 2016, 03:21 pm
It does seem a very temperamental frequency to generate for some weird reason i have never had these problems with any other frequency i have tried different online calculators and brought more accurate components (with expense of long delivery times) i think however i may have found a circuit which seems to fit the criteria perfectly its a modified 555 circuit for 50% duty cycle.

i tested with the transistor to make sure it worked then removed as i will replace that part of the circuit with a constant current driver, i modified the values using the formula given on the website...
"Theoretical frequency can be calculated by:

f=1/(1.4 RC)

where f is frequency in kilohertz, R is resistance in kilohms, and C is capacitance in microfarads.

38 kilohertz = 1/(1.4 * 18.7969 kilohms * 0.001 microfarads)"

so 56.24 kilohertz = 1/(1.4 * 12.7 kilohms * 0.001microfarads) i used a 8.2 kilohm resistor with a 5k pot and once again i could tune it to 55.56 and it then jumps to 57.14 completly skipping 56 i tried adding a 1k pot in and using that to further tune it and it still functioned the same, i added a 500pf capacitor nope still missing then tried dozens of combinations, am i not using this formula correctly?

I do realise the bandwidth on a infrared receiver is 1khz so it can be off by that amount however 55.56 could drift down below the 1khz threshold through thermal drift and other factors so i would like to get it atleast 55.90 or 56.50 that way i dont have to worry about this.

the website with this information is: Infrared555

#### tinman13kup

#32
##### Nov 27, 2016, 04:08 pm
I wouldn't use that schematic. I think your chip is upside down in relation to the circuit
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

#### tinman13kup

#33
##### Nov 27, 2016, 04:10 pm
Never mind. It was me. I built your previous circuit to test values and my circuit is just laid out the opposite direction

MORE COFFEE!!!
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

#### Shandy

#34
##### Nov 27, 2016, 04:23 pm
Nice to hear that someone is giving it a go besides me will be intereasting to hear your results and how you did it differently love the more coffee comment.

#### Shandy

#35
##### Nov 27, 2016, 04:41 pm
may i ask why you wouldn't recommend that schematic?

#### tinman13kup

#36
##### Nov 27, 2016, 06:06 pm
may i ask why you wouldn't recommend that schematic?
I had an earlier post where I DID build the circuit as was attached with your calculations. I didn't have the exact values you figured, so I had to put resisters/capacitors in series to get what your calculations indicated. I have 10 capacitors in series to get the 100pF value. My breadboard is pretty busy looking getting that particular circuit to the values you posted.

When I initially looked at your latest circuit, it was laid out the opposite direction, and a glance at my spaghetti bowl breadboard looked like your IC was backwards in comparison, when it is essentially a different circuit.

I achieved the 56Khz (56. without a pot on R2. My indicated (by markings) and actual (by dvm on breadboard) are posted if you missed them
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

#### Shandy

#37
##### Nov 27, 2016, 06:16 pm
tinman13kup thank you when you say my IC is backwards the top cut out is at the top im not entirely sure what you mean could you post a diagram you used or further elaborate?

#### tinman13kup

#38
##### Nov 27, 2016, 06:31 pm
tinman13kup thank you when you say my IC is backwards the top cut out is at the top im not entirely sure what you mean could you post a diagram you used or further elaborate?
You are not following what I'm saying. Your IC is fine. The IC circuit that was in the attachment of the figures is fine. When I wired up my breadboard to achieve the values your figures indicated, I had to use multiple resistors and capacitors. That meant jumping to different parts of the breadboard to have enough room. For C1, that capacitor bank took up 1/4 of the breadboard to get the 100pF value. (Mine actually came out to 98pF). By the time it was done, my breadboard looks pretty busy, but more to the point, it is just laid out the opposite physical direction than either of the circuits you posted, so a quick glance between your circuit and what's sitting in front of me looked opposite, or backwards.

It was just an illusion. I needed more coffee.

The fact is I achieved the 56KHz with your figures. Perhaps your 100 pF cap is more like 180pF.
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

#### Shandy

#39
##### Nov 27, 2016, 07:53 pm
Oh i understand do you think then my problem could be im trying to compact everything a little to much and maybe i should not put capacitors directly next to the pins of the 555? is there some techniques i should use when laying out my circuit.

I also have become quite happy with the circuit in post #32 it gives me a perfect 50% duty cycle and is very tidy once again though even with the pots the frequency just jumps away from the 56khz frequency do you think this is also caused by my compact layout? or do you think im using the formula wrong?

#### tinman13kup

#40
##### Nov 27, 2016, 09:11 pm
Oh i understand do you think then my problem could be im trying to compact everything a little to much and maybe i should not put capacitors directly next to the pins of the 555? is there some techniques i should use when laying out my circuit.

I also have become quite happy with the circuit in post #32 it gives me a perfect 50% duty cycle and is very tidy once again though even with the pots the frequency just jumps away from the 56khz frequency do you think this is also caused by my compact layout? or do you think im using the formula wrong?
I don't think it's due to a compact layout. Maybe a cheap/old/too big pot causing headaches. If you are still hooked up like the original circuit, try putting another small value capacitor in series with C1 and see where it ends up. By small value, I'm talking 10nF. Putting caps in series will lower the capacitance and increase the frequency.  Perhaps pull the pot for now. I had 14.7K using a 10K+4.7K 1/4W resistors.
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

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