D.I.Y Clock Oscillator.

This page contains diagrams for building a variable frequency/duty clock oscillator of your own.

Variable duty oscillator using an LM555. C2 and R4 are just for simulation and should be omitted. C1, R1 and R2 determine timing.

Introduction

I've recently found I need a bunch of equipment that I previously did not have. So I'm posting schematics here. If anybody wants any premade versions of these, or if you find these diagrams useful, just drop me a line (address on front page) and let me know.

Variable Duty Clock Oscillator

This is a modification to the standard 555 timing, overcoming virtually all the shortcomings of the 555 for use as a digital clock generator. This circuit can have virtually any duty cycle from 0% to 100% by varying R1 and R2. I personally built one for use as a clock generator, for my digital circuit designs. The timing of this circuit is determined by:

Ton=R1*C1

Toff=R2*C1

F=1/(Ton+Toff)

These equations seem to hold in reality. Good.

It seems that there is some confusion about R1 and R2. Yes, R1 and R2 are meant to be different pots (not center-tapped) in my case, since I wanted a variable frequency. You could replace them with a single center-tapped pot, but then your clock would be fixed frequency with variable duty cycle. I wanted both, therefore I needed two knobs. If you don't have non-center-tapped pots, you can just use an end and center tap of a center-tapped one (or connect one end to the center and use that with the other end).

If you should have requirements for fixing the minimum or maximum duty cycles, fixed resistors in series with R1 or R2 will do the trick. Just use the formulas.

Oh, feel free to use any general-purpose transistor you want, I'm probably going to use a BC547 and a BC557 myself (because they're really cheap). Oh, another thing I forgot to mention, you can change the VCC voltage to virtually anything you want, as long as it won't pop your 555 (up to about 12V or so, I forget exactly). I'm pretty sure that the EBJ of the transistors will never be pushed below it's limit (about 6V actually) in this way.

Now for gratuitous pictures of the one I built. I wish I could put up a scope picture of this, but so far I have yet to figure out how to take pictures in my workshop.

Front panel. I actually built mine as a desktop instrument. You can see the bank of capacitors hanging off the switches. Left to right is power switch, period adust knobs, range switches, and output posts.

Back panel. Not much more than connections for VCC and Ground.

Circuit board. It's a little scrap I had lying around, hence the rough edges. This board is just standard "prototype PCB", which is wired like breadboard. Two header connectors on top and bottom allow for interconnect to be added later (just in case I need to change the design).

Innards. This is a shot of the internals after assembly. This is actually post test. The gray wire seen in the foreground is actually the camera wire.

Outside shot. Isn't the modem-style case pretty? I got these from Radio Shack a few years back, and I've been wanting something to do with them ever since.

Revamped outside, since I just got a cute little Dymo labelmaker.

All material on these pages is Copyright (c) Jennifer E. Elaan.