Experiment in PWM using 555 Timers

[Ed/KB1HYS]

I had a couple of the ubiquitous 555 timer chips around so I dug out the bread board and tried to build a PWM circuit. It takes two of them to be able to produce a pulse width modulator circuit.  The first is set up in an astable circuit (oscillator) producing pulses around 100 khz.  The second is setup in a monostable circuit with the oscillator output used to trigger the monostable chip.  The input wave form (audio sine wave) is supplied to pin 5 of the monostable chip.

Well, I built it, and it works, sort of.  Aliasing is pretty bad (no output filtering in the breadboard).  Also, the pulses always maintain a 50% duty cycle, and just the frequency changes.  Found out I had totally the wrong values for R & C in the PWM chip.  Still kinda fun.   Maybe I'll clip lead together a PWM QRP rig for when the AMQRP net opens up again...

[Radiohead]

There some nice PWM schematics with 555 timers on this page, and complete PWM Class E FET/Tube tx's
http://www.lu8jb.com.ar/Amplitud%20Modulada.htm

[Ed/KB1HYS]

That fellow does nice work, creative too, using the wire spools as forms for coils/transformers like that.

I also liked the link (dead unfortunately) for 'Loco por El AM' or "Crazy for AM" nice...   

[KB3DKS]

That is a Phase Linear 700 chassis and original power supply he is using here including one of the meters!
http://www.lu8jb.com.ar/PWM_CX8%20JD.htm.
Wild!!
Bill

[Ed/KB1HYS]

He's definitely a "parts is parts" kind of guy.

[WA1GFZ]

The PWM steve uses will work much better over a wider duty cycle.
I use the UC1846 current mode with current transformer feedback.
555 will work but will not give you a full range of duty cycle.

[Ed/KB1HYS]

Yes, they're not a real high performance ciruit. I did sweep it with an audio sine wave.  Very interesting results, but not real usefull.  Besides there are single package chips that provide the pwm output with out all the added complexity.  Still fun to experiment with some of those  lil' black bugs every once in a while.