555 Timer ( Ast able)

Can the 555 timer(Astable) be operate below 50% duty cyle

Posted by greentea on 19/10/2004, at 10:48 GMT

555 timer duty cycles less than 50%

You can achieve 555 astable duty cycles of less than 50% by adding a signal diode (eg 1N4148) in parallel with R2. The diode's anode should be connected to pin 7 so that it conducts from pin 7 to pins 6 and 2. This bypasses R2 during the Mark time (Tm) so the timing equations are:

Mark time: Tm = 0.7 x R1 x C
Space time: Ts = 0.7 x R2 x C
Total time period: T = Tm + Ts = 0.7 x (R1+R2) x C
Duty cycle: Tm/Ts = R1/R2 (so almost 0% to almost 100% is possible).

The above equations assume the diode is 'perfect'. Real silicon diodes have about 0.7V across them when conducting, but this will have negligible effect on the values in most cases.

Note that R1 and R2 should be in the range 1K to 1M ohm.

I hope that helps.

Posted by John Hewes on 19/10/2004, at 14:09 GMT