Problem with 555 Monostable Circuit

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Rob 20 years ago

I am trying to create a long high pulse (around 10 minutes), then falling edge. Then have the process start over.

I did not find any astabel circuits that could produce a very long pulse without additional ICs. I did find some long pulse monostable circuits, but am having trouble getting the timing to be consistent.

In my sample circuit I have decreased the RC values just for testing. Sometimes the pulses are consistent, other times they appear to be at fairly random intervals.

For testing I am using 22K for the resistor, and 33uf for the cap My actual values will be 1.6M and 33uf

My pinouts look like this:

Pins 1,2,5 are grounded Pins 1,2,5 also go to the - on my cap Pins 4,8 +5V Pins 6,7 go through my resistor to +5 Pins 6,7 also go to the + on the cap Pin 3 is my output

(sorry I tried to draw a schematic, but it did not come out too well)

Why is the timing in my circuit not stable? Is there a better design I can use?

Thanks

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Joe McElvenney 20 years ago

Hi,

With the long time constants needed capacitor leakage can become a problem. You might try using use a CD4060 (a quite common IC or its later equivalents). This is a 14-stage binary counter with an internal oscillator so that you may make use of a much higher oscillator frequency (shorter time constants) and the easier adjustment so afforded.

Cheers - Joe

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John Fields 20 years ago

How long after the falling edge do you want it to start over?

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Rob 20 years ago

but

fairly

John,

Thanks for your help. The circuit is running better now. How would I get the circuit to trigger a reset and start over again, after the output goes low (re trigger itself)?

BTW: after the falling edge (which will trigger something else) the output can go high almost instantly.

Thanks

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John Fields 20 years ago

There are two ways. The first is: +V +--------|------+ | |8 | [22k] +---+---+ | Rt| 2|_ Vcc |3 | +---O|T OUT|--+----OUT> | 6| _|4 +----|TH R|----+V | 7|_ 7555|5 | O|D C| +| | GND | [33µF] +---+---+ Ct| |1 +--------+ | GND and the second: +V | +------------+ | |8 [Ra] +---+---+ | 7|_ Vcc |3 +-------O|D OUT|------OUT> | 6| _|4 [Rb] +---|TH R|----+V | | 2|__ 7555|5 +----+--O|TR C| +| | GND | [33µF] +---+---+ Ct| |1 +------------+ | GND The first way will get you a 50% duty cycle square wave out if you use a CMOS 555 (7555) and the second way will allow you to adjust the duty cycle of the output by adjusting Ra and Rb. Rather than going through it all here, there are a lot of excellent references on the web. Go to: http://www.uoguelph.ca/~antoon/gadgets/555/555.html and if that doesn\'t work for you, Google "555 timer" for something that surely will. One caveat with using 555\'s for long timing intervals is that the leakage current of the capacitor can become a problem if the timing resistance can\'t supply enough current to charge the cap. For best results you should use a low leakage capacitor like: http://www.mouser.com/catalog/specsheets/llrl.pdf

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Harold Ryan 20 years ago

Hi Rob:

The discharge of the RC is most likely constant. The problem is that the voltage sits too long just above the trigger threshold of the 555. Any disturbance like RF or a non perfect power supply or even a temperature change will make the circuit trigger randomly. My guess is that you are experiencing about a 5% jitter. The best solution is to minimize the time the voltage resides at the trigger level and shield everything. In other words speed up the process and divide the output frequency.

Harold

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John Fields 20 years ago

None of the above is relevant, it\'s just wired wrong. Also, learn to bottom post.

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Rob 20 years ago

the

low

output

Thanks. I am going to play for a few days and post back...

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JeffM 20 years ago

I'm assuming that you *were* using a monospaced font (Courier).

Easier than trying to do it in Outbreak Express

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or in a text editor with a monospaced font is this tool:

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(Word-for-word language translation is always entertaining.)

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Problem with 555 Monostable Circuit

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