# Variable Frequency Op-Amp Astable

I am trying to design a variable frequency Op-amp astable which works ranging from 0 to 2kHz. I found an useful lead which I attached below.

According to the reference, the frequency can be calculated using the following formula:

Where C=C1 and R=VR1

My question is: How can I adjust that circuit to make it work from 0 to 2kHz? Also, I am planning to use a TL072 instead of LM324.

• the reference you mention is pretty good. it explains the entire operation of the astable oscillator. if you use a TL072, are you planning on using that in a bipolar power circuit? you know with $\pm$15 volts or something? the R1 R2 voltage divider can be replaced with a single 50K resistor to ground, in that case. i dunno how well the TL072 will work in a unipolar power context, but i am sure someone has done it. – robert bristow-johnson Feb 12 '17 at 4:53
• zero Hz is an unreasonable lower limit of frequency range. With R1 at its maximum resistance, frequency only goes down to 72 Hz. – glen_geek Feb 12 '17 at 5:06
• So, do you suggest not to modify that circuit to get a reasonable range? – Blue_Electronx Feb 12 '17 at 5:11

That's a very useful circuit for an astable oscillator. However, for a useful wide range oscillator, I would recommend a couple of alterations.

The circuit gets its range from adjustment of VR1. If it's adjusted right down to zero, the frequency and adjustment become silly. Add a small resistor in series with it to set a minimum value.

If you use a linear potentiometer, the adjustment sensitivity becomes very twitchy when you are at the 10% end of the travel. I would recommend using either a logarithmic potentiometer, or another pot in series of perhaps 10% of the value, to give you coarse and fine controls.

To extend the adjustment over more than 10:1 on the pot alone is impractical. I would recommend using a rotary switch to select capacitors for C1 in a 1:10 ratio, so 100nF, 1uF, 10uF etc. This type of oscillator keeps a DC bias across the capacitor, so it's OK to use electrolytics for the larger values. Warning, electrolytics often have poor tolerance, so your frequency calculations may be wrong, and poor leakage, so a really large capacitor may not work at all. You can expect RC combinations down to one second to work, to 1000s won't work, somewhere in the middle, try it.

There is a fundamental problem with 0Hz, in that it would take an infinite capacitor to generate, and you an infinite time to see it. Something more practical like 1Hz, or 0.1Hz might be a better lower limit.

While LM324 is specifically designed for low voltage, TL072 is not. However, it is specified down to a 10v supply, so it may still work OK at 9v, you'll have to try it.

• Okay, I get it. So I think if I just try to assemble the exact circuit without modifying it I would get a reasonable range. The lower limit for that circuit is about 72 Hz but I do not know how much will be the higher limit. – Blue_Electronx Feb 12 '17 at 12:58
• If you adjust VR1 down to zero, the niave theory says the frequency goes to infinity. That's where the adjustment sensitivity will be poor, and where it's recommended to put a resistor in series with VR1 to limit its minimum setting, a 10k in series would give you >10:1 range. – Neil_UK Feb 12 '17 at 13:34