I'm wanting to make a tone generator with a 555 timer that is a frequency humans can't hear. I'm going to follow this schematic. My R1 (potentiometer) will be 10k. R2 will be 33k. C1 will be .001mF. What should capacitor 2 be? And would the resistors i mentioned work for making a tone within the 17-20 Khz range? Also, how and where would I attach the power source? I'm thinking about using a 9 volt battery. Or would something else work better? I want it to be able to run for several hours straight. Thanks! http://www.reconnsworld.com/audio_tonegenerate.html?

Thanks you!

LargeMouthBass5 years ago
I'm sorry about my earlier post, I totally glossed over the link to the schematic.

The frequency will be F = 1.44/[(R1+2R2)*C1]

I assume by 0.001mF you mean 0.001 microfarad? It is more common to use uF, where lower case u is used in place of the greek letter mu. Using mF would mean "millifarad".

That equation for frequency is from the datasheet I reffered to.
The values you list will produce a frequency of about 19 kHz if the pot is at 10k. As the pot is turned and the frequency decreases, the frequency will increase.

One potential problem with this circuit is that you don't want R1 to be 0 ohms (or a very low resistance), as pin 7 of the 555 is switched to ground to discharge C1 through R2. If the resistance is too low, the 555 can be damaged. There shoud be a fixed resistor in series with R1 so that there is no possibility of damage.

The capacitor C2 is for decouping of the VCC supply to the 555 timer IC. It is typically a small ceramic capacitor with a value of 0.1uF or something near that. It is not part of the timing function, but instead it stabilizes the VCC voltage when the part draws short spikes of current during switching events.
LargeMouthBass5 years ago
It is very straightforward to generate square waves in the frequency range you list using a 555 circuit. Its hard to make an assesment of your circuit values when you don't provide a schematic showing how each part is connected.

I recommend you get a copy of the 555 timer datasheet. It contains many sample circuits and the basic formulas relating the output frequency and pulse width to the component values. One easy way to get the datasheet is by going to www.digikey.com and searching on the part. The detail page for most parts contains a hyperlink to a pdf of the manufacturers data sheet.

Also, there are tons of 555 articles on this site and elsewhere on the web.