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Fixed frequency Variable duty cycle with 555 - del00012


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 - LM555.

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 blog - 555-Circuits - delabs Schematics

Fixed frequency
                          Variable duty cycle with 555




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 del00012.zip.

Duty Cycle

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.

555 Duty Cycle

Image above from the Original Signetics 555 Datasheet