Question about MSI PM8M3-V Motherboard
I have a device which is controlled by a Fairchild KA555 timer. At present it operates for 5 minutes at each hour, so I am presuming it's set up in astable configuration.
I need to replace the timing resisters so that I can vary both the On time and the Hold time. It's been 20 years since my electronics training, and I simply can't remember the formulas.
The Capacitor on the board which is currently hanging off of Pin #6 is a 1000 microfarad electrolytic.
I would like to achieve an "On" time of 1 to 10 minutes.
I would like to extend the "Off" time variably from 1 hour up to 23 hours if possible, with one potentiometer.
If it is not possible to extend the "Off" time to that extent using only one potentiometer (due to the range of adjustment that would be required) then I need several values for RB that would give ranges from 1-4 hours, 4-8 hours, 8-16 hours, and 16-24 hours, so I can modify 4 different devices and proceed with some evaluative testing.
Yes, it is in an astable configuration.
You can find the 555 information here http://www.national.com/ds/LM/LM555.pdf
Look on page 8 for the astable information
in a nut shell, the charge time is determined by the addition of both resistors (Ra+Rb) while the discharge time by the one between the capacitor and the discharge pin (pin7), Rb.
Such long durations might be difficult to achieve due to capacitor leakage.
It would be better to add a counter at the output of the 555.
Posted on Oct 11, 2007
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