I have seen different site using 556 set up as one timer in astable controlling a second timer in monostable mode. Other sites show PWM by using only one 555 timer set up in astable mode. I would really rather use only one 555 timer, but what is the upside to using a setup with a
556 timer?
========================================================== Maybe a wider PWM range or more stable frequency can be obtained with the separate trigger timer? What range of duty cycle do you need? 99%? or 10% to 90%?
Without seeing the circuit in question, my guess would be to have a timed set of pulses then a rest and another set of pulses. Maybe to fast charge a battery so there is a cool down period ? Tell the link the the circuit in question.
The one with the dual timer gives you "pulse frequency modulation" or PFM, while the single timer gives you "pulse-width modulation", or PWM. One is a stream of equal pulses at a varying frequency, and the other is a stream of variable-width pulses at a constant frequency.
Both ways have advantages and disadvantages - it depends on the application.
Good Luck! Rich
-- Using a single 555 in astable mode will only allow duty cycles of >50%, at any frequency, while using an astable to determine rep rate
With a diode between pins 7 and 6 you can obtain duty cycles < 50% as it only uses one resistor to charge instead of two.
Mark
-- Damn! I forgot about the "magic diode"! Good catch. But, it goes in parallel with R2 and with the anode on pin 7, no?
That is correct. And for people like me who always have to google to remember which one is the anode i'll draw a little zoomed in picture
IC1 - 555
| 7 6 2 | ----------------------- | | | \\/\\-+-/\\/\\/-+---+-||-+ R1 | R2 | C1| +-->|---+ 0v D1 etc.
------8
nix is easy linear technology give away ltspice for windows.
*nix is harder berkely spice is free by works from a command-line... theoretically it can be coupled with oregano (an X-based gui for drawing circuits) but last time I tried I couldn't figure it all out.Apparently ltspice works well under wine. I haven't tried installing wine yet.
Bye. Jasen
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