I’m new to electrical engineering and this is my first time working with an oscillator. I'm trying to setup a 555 timer ic as an oscillator with a low time of 1s and a high time of 0.5s. I've been using the following wire diagram, with the exception I am currently not using D2, though the results are the same with it as without it.

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I'm using the following: C1 = 10uf R1 = 150kohm R2 = 75kohm A TLC555CP IC.

Every thing works except the LOW and HIGH times are the reverse of what I had expected from all the reading I have done. I got a result of 1s on and 0.5s off. If I reversed R1 and R2 then I got what I wanted 1s off and 0.5s on.

This seems counter to everything I've read that said R2 had to be less than R1 to get a less than 50% duty cycle. If anyone can tell me why this is I would be most appreciated.

I’ve also noticed that when power is first applied to the ic I get an instant ON that is longer than desired. All successive ON cycles are okay. Any suggestions for dealing with this are much appreciated.


1 Answer 1


When the output is high the capacitor charges through R1 (and D1) so the high time is proportional to R1.

When the output is low the discharge pin is also low, so the capacitor discharges through R2 (and D2, if present).

If you want high_time/low_time = 0.5/1 then R1/R2 = 0.5, so R2 should be double R1.

During astable operation the capacitor charges and discharges between Vcc/3 and 2Vcc/3. When first powered, the capacitor can be discharged to 0V or something near to that so the initial cycle tends to be considerably longer.

  • \$\begingroup\$ Thanks I guess I was misinterpreting what charging through R1 meant, I was thinking it meant the output would be low then. Thanks for clearing that up. \$\endgroup\$
    – CyF
    Jan 29, 2019 at 0:28

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