555 frequency issues

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: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.

We need a schematic for reference. I tend to be one of the people who really know the 555 around here, and have made several references.

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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.?

The circuit below works for me.




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

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.

MrChips said:

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.

Click to expand...

11.8kHz vs 31kHz is not just a component tolerance problem.

Ron H said:

11.8kHz vs 31kHz is not just a component tolerance problem.

Click to expand...

Geez Ron, somehow I suspected you will notice this.

Below is the circuit that I'm using.

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

What are the colors on those resistors? They don't seem right.

Red-Purple-Red
brown-green-orange
brown-black-orange

In case anyone was interested I just attached the excel spreadsheet I was using to calculate everything.

nzemke said:

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

Click to expand...

Here's a photo of my layout.

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.

I have them running in series.

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

nzemke said:

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

Click to expand...

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.

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.

Bill_Marsden said:

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.

Click to expand...

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.

w/o transistor this circuit will readily blink.

I changed components:

2 diodes from a television PCB, don't know 1n4148 or whatever
68K/1uF -> fast blinking, 10Hz
5K adjustable resistor not 2x 220 Ohms

Is there any reason why you need to calculate frequency?

Just any kind of capacitor will do.