Electronic – Using 555 to generate square signal with fixed duty cycle / variable frequency

555capacitorduty cyclefrequencytimer

I am attempting to design a circuit whose output is a square wave with the following properties:

Fixed duty cycle, as high as possible (let's say 99%)
Variable frequency, between 0.2 – 20 Hz (I'm somewhat flexible about this range)

I imagined (naively?) that such a thing would be possible using a 555 timer, wired up in astable configuration. I began exploring the problem space using an online tool that calculates duty cycle / frequency for given values of R1, R2, and C (see diagram).

Astable 555

It would appear that, at least in theory, I could achieve the desired effect with the following setup:

Some fixed, reasonably high ratio of values for R1:R2 (e.g., R1 = 1 megohm / R2 = 1 kilohm); this sets the duty cycle high
Varying the capacitor between 0.1 and 10 microfarads; this varies the frequency within my desired range without affecting duty cycle

The big problem would be in realizing this circuit with actual components. At least to my knowledge, there is no such thing as a variable capacitor with that sort of range.

I am left wondering if there is some other way to configure a 555 (besides the typical layout given above), which would provide a more realistic way to tweak frequency while holding duty cycle constant.

Alternatively, I'd be open to any suggestions on how to achieve variable capacitance from 0.1 to 10 microfarads.

Finally, I'm open to any non-555 implementations that achieve my desired result. The simpler, the better.

Best Answer

If the duty cycle itself does not matter and the requirement is for pulses of, say, 0.5ms, then you could send the output of your circuit to a monostable 555 circuit. Dual-555 chips are available: the 556.

Related Solutions

Electronic – how to create a square wave with large duration and high duty cycle

I am not going to answer your question, instead I'll answer a question that you didn't ask :)

You could implement a timer-based solution. However, it is actually going to be cheaper and more effective to use a microcontroller for your problem. The advantages of a microcontroller solution are:

Cheaper to build (I know it sounds crazy but it is true)
More flexible in the logic: you can change the logic with a few mouse clicks
More supportable: for many people it is easier to see the application code than understand why the RC time constant is not behaving as it should.

Some solutions to think about in the microcontroller world:

Arduino: starts at about $11 for Arduino Nano on ebay/DealExtreme, programmable over the USB port, more input and outputs than you'll know what to do with. Programmed in C/C++
Pure AVR: about $2 for the minimum chip, but requires ~$20 investment for an ISP programmer. Programmed in C/C++ or Assembly.
PICAXE. Do they make them anymore? Programmed in BASIC or Flowchart. About $3-$5 to get started. Programmed with nothing more than a serial port.
PIC Microprocessors. Similar to AVR, but it's kind of a Mac vs. PC thing from a few years ago. I am an AVR person, but there is a number of PIC people on this board, so if you go this route there is plenty of help too.

Basically, Arduino is an easier-to-use AVR\$^*\$, and PICAXE is an easier-to-use PIC. Between Arduino<->PIXAXE, Arduiono wins in popularity hands down: Arduino is extremely popular and well-supported. Between the underlying platforms (PIC/AVR), it isn't clear: both are popular. There are also other microcontroller options(Cortex M, Propeller, and many others) but the options mentioned in the bullets above are the easiest to get started with, IMHO.

* This is only 99.9% true because a few Arduino's use non-AVR Atmel chips.

By the way, I am not suggesting the you ditch your existing circuits and use a microcontroller for everything (whether that's a good idea or not is not relevant to your question). You can use a microcontroller for the purpose that you don't flood your house or overwater your plans as stated in your design goals. Here's what your solution would look like in microcontroller pseudocode:

loop:
   have 12 hours passed?
      yes: turn on output for 3 minutes
      no: don't do anything
   go back to loop:

There are all sorts of sophistications you might add, such as sleep, implementing the wake 12 timer hit as an interrupt, etc.

Electrical – How to balance between frequency and duty cycle in this circuit

How can I achieve the required range of frequency and duty cycle?

Don't make life hard, use an LTC6992: -

It's operating frequency can be varied between 3.81 hertz and 1 MHz. Duty cycle can be controlled (with a voltage or a potential divider) from zero to 100% with options to limit at 5% and 95%).

It works from supplies of 2.25V to 5.5V.

Related Topic