Fixed frequency Variable duty cycle with 555 -
This circuit is based on a very old application
note from exar, in this the frequency is fixed by IC1
and IC2 -P1 controls the duty cycle. you need to
compute the R and C values to get what you need, IC -
Even a Toy can be made with 555 -
Electronic Fifteen LED Roulette
You have to study the circuit and do something
more innovative perhaps, just copying is ok for
learning but it will get you nowhere, so learn and
then innovate, the eagle circuit is given below so
you can learn by editing it, also design a PCB with
it, and you can even make a PCB at home to learn, but
it is always good to get PCBs done by a PCB vendor,
but you should understand his problems, then you will
design well, so make a few PCBs.
Here is my main 555 circuits stub at my Schematics
555-Circuits - delabs Schematics
In this 555 circuit the first 555 is the
fixed-frequency oscillator that sends a train of
trigger pulses to the 2nd 555. The 2nd 555 can
stretch the on duration pulse by the width you set
using the preset. This can be used as a Manual motor
speed or heating control.
See Larger Circuit.
555 Variable Duty Cycle
Oscillator. Edit the Eagle file
The Duty cycle is the "On" Time in
terms of the Total Cycle or Time Period (1/f). The
limit of 50% can be bypassed by adding a Diode across
Rb a duty-cycle of less than 50% can be done. A
asymmetrical square wave is possible.
NE555 Applications and Specifications
This also helps in creating PWM circuits and
Pulse shaping circuits. Power Supplies and Closed
Loop Systems can be made by using the powerful drive
of 555 to On/Off Relays at 20 Seconds Cycle Time for
large heater/fan/compressor control. It can also be
used to drive opto-couplers which in turn operate
Thyristors and Mosfets for DC Motor Control and
Process Control with cycle times of 1 Second or less.
In real-time controls, a cycle-time of 1 mS or less
it can be used to make Switching Power Supplies and
mini motor control too.
Image above from the Original
Signetics 555 Datasheet