oscillator using a 7414 inverter...

[C Bradley]

I came across a schematic for a clock pulse generator (oscillator) in a textbook of mine. It's a 7414 inverter with a cap and a resistor to set the frequency. Frequency = (0.8/RC)

I figured that since an inverter is a transistor wired common emitter that I could substitute a 2n2222 or similar for the inverter. A 100k and a 1uF cap would get down to 8Hz, and adding a pot in series with the 100k would get even lower.

The circuit is wired like this. The inverter has a resistor from the output to the input. The capacitor is from the input to ground. The normal 5V supply is used to power the 7414. I'm trying to save board space or I'd just go buy a 7414. Can this be done with a 2n2222 or some other single transistor?

[Peter Snowberg]

The 7414 is an inverter with hysteresis called a "Schmidt trigger".

In amplifiers you hear a lot about negative feedback. Normal inverters switch states at a specific voltage. The Schmidt trigger uses positive feedback to reduce noise by making the signal swing higher than mid-level to produce a low output or lower than mid-level to produce a high output. The voltage range between these higher and lower than mid-level thresholds is called the "dead zone" which is not to be confused with the parking lot vendor area at a Grateful Dead show.

If you want an oscillator with just a simple transistor, Google for "phase shift oscillator" which is what Fender uses in their tremolo oscillators.

There are many DIY transistor examples, but I don't have any links for you. I hope somebody else will chime in.

You should be able to find more here with the search function.

[Paul Perry (Frostwave)]

For something to oscillate, you need to have a feedback loop, and the gain around the feedback loop has to be in phase and greater than 1, for some frequency.
Since a single transistor amp stage (the common emitter configuration that is normally used) inverts the signal, if you are only usign one transistor then you need to make a phase shift of 180 deg from resistors & caps in the feedback.
Or you can use a two stage transistor amp (you got positive feedback now).
But (depending what you are doing & what waveform you want) for small space think 7555 cmos timer. Or if you are comitted to transistors maybe a flip flop.