# 555 frequency issues

Discussion in 'General Electronics Chat' started by nzemke, May 12, 2012.

1. ### nzemke Thread Starter New Member

May 12, 2012
9
0
I have been looking around everywhere trying to figure out what I did wrong with my calculations and I'm hoping that you can help me.

I'm trying to create a square wave oscillator at a frequency around 31,000 hz. I put together a little excel spread sheet to do the calculations for me. I took the calculations from the http://www.kpsec.freeuk.com/555timer.htm website. The calculations are as follows:
F = (1.4/((R1 + (2 * R2)) * C1))
R1 = 10000
R2 = 17700
C1 = 0.001 uf (102 cap)
I have this setup on a breadboard and it works but it gives me a frequency of 11.8Khz.
One question I have is if the C1 has to be an Electrolytic or if you can use a ceramic?
Really confused and hoping that someone can point out my error.

Mar 24, 2008
21,838
3,047
3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
I don't know what your problem is, but it is probably either a measurement error or a component value error.
I don't think you will find a 1nF electrolytic capacitor. 100nF is the lowest value i have seen.
What kind of cap are you using, i.e., ceramic, mica, polyester, etc.?

4. ### tracecom AAC Fanatic!

Apr 16, 2010
3,940
1,428
The circuit below works for me.

Change C1, R1, and R2 to the values shown in the following table.

File size:
16.8 KB
Views:
96
• ###### 31 KHz Astable.png
File size:
2 KB
Views:
101
nzemke likes this.
5. ### MrChips Moderator

Oct 2, 2009
19,114
6,145
The 555 formulas are guidelines only. I would not be too concerned if you do not get exact results. Remember, your capacitance can be off by as much as 20%.

You should not use electrolytics for the timing capacitor, C1, if timing is critical.

Try to reduce R2 to around 10K.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
11.8kHz vs 31kHz is not just a component tolerance problem.

7. ### MrChips Moderator

Oct 2, 2009
19,114
6,145
Geez Ron, somehow I suspected you will notice this.

8. ### nzemke Thread Starter New Member

May 12, 2012
9
0
Below is the circuit that I'm using.

9. ### nzemke Thread Starter New Member

May 12, 2012
9
0
Tracecom, I'll put together that circuit this afternoon and see if I'm getting a better result.
Thanks everyone for the assistance!

10. ### MrChips Moderator

Oct 2, 2009
19,114
6,145
What are the colors on those resistors? They don't seem right.

11. ### nzemke Thread Starter New Member

May 12, 2012
9
0
Red-Purple-Red
brown-green-orange
brown-black-orange

12. ### nzemke Thread Starter New Member

May 12, 2012
9
0
In case anyone was interested I just attached the excel spreadsheet I was using to calculate everything.

File size:
22.6 KB
Views:
19
13. ### tracecom AAC Fanatic!

Apr 16, 2010
3,940
1,428
Here's a photo of my layout.

• ###### NE555N Astable Photo (lo res).jpg
File size:
97.4 KB
Views:
36
nzemke likes this.
14. ### Wendy Moderator

Mar 24, 2008
21,838
3,047

R1 is a 10KΩ, R2 is a 2.7KΩ (red violet red). Is it in parallel with the 15KΩ? I see no 17.7KΩ resistor. C2 is for noise control, you can remove it in a small circuit with no effect what so ever.

15. ### nzemke Thread Starter New Member

May 12, 2012
9
0
I have them running in series.

16. ### nzemke Thread Starter New Member

May 12, 2012
9
0
tracecom, I noticed that you are using the ne555n chip and I'm using the ne555p, will this cause issues?

17. ### tracecom AAC Fanatic!

Apr 16, 2010
3,940
1,428
No, the two are functionally identical.

BTW, if you don't need a reset function, you can eliminate SW1 and R3 in my schematic and tie pin 4 directly to Vcc.

18. ### Wendy Moderator

Mar 24, 2008
21,838
3,047
I find it interesting the OP is getting 11.8Khz, and should be getting 5.04Khz. It is a X2 difference. I am really suspicious of instrumentation error, the frequency counter. Have you looked at the waveforms with an oscope? I'm thinking it is that or a hard to spot wiring error. 555's are extremely predictable.

19. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
Bill, I think you have combined two posts. In post #1, he says he is expecting around 31kHz, and is getting 11.8kHz. There is nothing wrong with his equation, and if you plug in the component values, you get 30.8kHz. A sim gave me 31.5kHz.

20. ### takao21203 AAC Fanatic!

Apr 28, 2012
3,682
491

I changed components:

2 diodes from a television PCB, don't know 1n4148 or whatever
5K adjustable resistor not 2x 220 Ohms

Is there any reason why you need to calculate frequency?

Just any kind of capacitor will do.

File size:
4.6 KB
Views:
32
File size:
12.9 KB
Views:
21