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Subject: Re: 555 & 7555 Timers

Date: 02/07/00 at 6:31 AM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: 555 & 7555 Timers posted by Jeff on 02/04/00 at 7:31 PM:

: Hello,
: I have built a small 5W xtmr usinf 3 - 3v cells. When I switch the power on I would like the signal to run 440 ms and then hold for 50 sec. This sequence should continuously repeat itself until the power is turned off. Will a 7555 timer circuit run in a loop or just one time? If so where can I find info on this? If not what should I use?

: Thanks In advance

Hi, Jeff.

What you propose is to make a free running astable circuit using a 555 or 7555 at a duty cycle of less than one percent at a cycle time of nearly one minute. There are two difficulties invovled.

1) A duty cycle that low is, I feel, difficult to achieve with a 555 based design.
2) Also, a cycle time that long using a 555 would require an inordinately large timing capacitor value which leads to excessive bulk and, with some electrolytics, leakage problems.

If a 555 type design is absolutely necessary, consider using a CMOS version rather than a bipolar version. I forget the nomenclature of the CMOS devices, but their higher impedance characterisitics would help mitigate the leakage and capacitance difficulties.

However, if you are not committed to a 555 design, you could run a pair of digital counters to develop your timing. Then, achieving your intended duty cycle would be no problem at all and the accuracy of your 440 mSec and 50 Sec intervals would only be a function of the frequency stability of your clock.

Good luck.

John Dunn - President
ambertec@ieee.org



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