Silicon Junction

I was looking for an adjustable oscillator that I could use as a frequency source for my Tesla coil. I already knew that the coil resonates at approximately 2 MHz, since I had previously built a Slayer-exciter circuit and measured its operating frequency.

A common first choice for a simple oscillator is the 555 timer. However, when I built a 555-based oscillator, I found that 2 MHz was a little too fast for the standard device. While faster CMOS 555 variants are available, I didn’t have any on hand in the lab.

As an alternative, I considered using an astable multivibrator:

The multivibrator would certainly be fast enough. Unfortunately, adjusting its frequency requires changing the values of R1 and R2 simultaneously. This is somewhat inconvenient, as I don’t have any double-ganged potentiometers on hand. In addition, the circuit shown above does not produce particularly sharp edges without adding several extra diodes. Altogether, this feels like a lot of extra complexity just to generate an adjustable square wave.

A Schmitt Inverter To The Rescue

A commonly used circuit in digital applications is this simple oscillator:

This looks great because it can produce square-waves with only 3 components, plus you can adjust the frequency by varying a single resistor. You can find specific details of how this oscillator works here:
http://electronics-course.com/schmitt-trigger-oscillator

There’s an informative video about these oscillators here:

I built the circuit using a Texas Instruments SN74AC14N, which I happened to have left over from a previous project. The SN74AC14N is particularly well suited to this application because of its very short switching time—on the order of 1.5 to 11 nanoseconds. As a result, it can produce a square wave with very sharp edges, even at my target frequency of 2 MHz.

The video above demonstrates tuning the oscillator over a fairly wide frequency range by adjusting R1, which I’m quite happy with. The circuit produces clean square waves across a very broad range of frequencies. I was able to get it running up to around 4–5 MHz, as shown, and down to about 0.3 Hz—slow enough to blink an LED.

Another advantage of this approach is that the SN74AC14N contains six inverters in a single package, allowing you to build up to six oscillators if desired, or to repurpose the remaining gates for other functions. The SN74AC14N is also an inexpensive part, costing around 60 cents from RS Components.

Issues

I encountered a problem where the oscillator would not start reliably on power-up. One commonly suggested fix is to add a relatively large resistor (around 50 kΩ) in series with the timing capacitor. This helps pull the inverter input toward ground at startup, forcing the output high and kick-starting the oscillation. However, in my case, this approach did not resolve the issue.

The solution that completely eliminated the problem was to run the oscillator from a 3.3 V supply instead of 5 V. This worked well for my application and resulted in reliable startup behavior.

Have fun!