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Digital Electronics Message Board
programmable duty for 555 timer
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programmable duty for 555 timer |
For Mr. Surtell or anyone who can help. I recently came across a schematic showing a 555 timer in astable mode with a programmable duty cycle. There was a calculator accompanying the website and I found by eperimenting with values that the duty cycle could be anything between 1 and ~100% with this duty being directly related to the ratio between R1 and R2. The circuit layout is the same as yours for astable operation with the only addition being a diode paralleled across R2. I can't seem to find any information on this scheme of astable operation. Is this design authentic? Reliable? Where would I find the math for such a circuit? Thanks! |
Posted by petoskey (garfield@dundee.net) on 15/07/2003, at 21:24 GMT |
555 astable circuit with diode |
The design is authentic! The diode is connected between pins 7 and 6 of the 555 in parallel with R2, with the diode anode at pin 7. This bypasses R2 during the charging period so the mark (on) time is determined only by R1 (rather than R1+R2 as in the normal circuit without the diode). The diode has no effect on the space (off) time which is determined by R2 as normal. With this arrangement the time periods are:
Mark (on) time Tm = 0.7 x R1 x C Space (off) time Ts = 0.7 x R2 x C
So the Mark to Space ratio = Tm/Ts = R1/R2
These equations assume a 'perfect' diode, real silicon diodes have 0.7V across them when conducting which will slightly increase Tm but the difference can be ignored in most cases. If great precision is required you should use preset (variable) resistors.
The diode should be a signal diode such as the 1N4148.
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Posted by John Hewes on 16/07/2003, at 14:24 GMT |
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