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I've built a low duty cycle 555 timer (astable) on a breadboard and on Multisim (here).

enter image description here

When the switch is closed, the LED turns on for about 1 sec (1.4 secs in Multisim) then turns off for 50 secs (not sure how long in Multisim since it's so slow), then continues that cycle until the switch is opened.

What I'd like to know is:

  1. Why does the LED turn on immediately after the switch is closed? Isn't it supposed to be powered by the discharging of the large capacitor?
  2. How does the circuit need to be changed so the LED doesn't turn on until after the 50 secs?
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  • \$\begingroup\$ For Q1, the charge in the timing cap in a typical 555 circuit is not used to power anything. It's just discharged through the 555 to ground, not to the output. \$\endgroup\$
    – brhans
    Commented Jul 5, 2023 at 10:48

1 Answer 1

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  1. When threshold and trigger are low (as when power is first applied, with C1 discharged) the output will be high, so the LED will be illuminated.

  2. Flip D1, add 10kΩ in series, and connect the LED + 680Ω resistor between Vcc and output (with the correct polarity so that when the output is high the LED is off). The 555 sinks current better than it sources current anyhow.

n.b.

  • The initial off time (assuming C1 has fully discharged) will be considerably longer than the subsequent off times because of the way the 555 works.

  • Also, at 4V, you're below the minimum supply voltage for the bipolar version of the 555 so it may not work well, or at all.

  • Leakage on a 100uF electrolytic capacitor may be high enough to significantly affect the timing or even prevent it from operating with 1MΩ resistance.

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    \$\begingroup\$ Thank you for all that, Spehro! Is this what you mean?: multisim.com/content/EfuP5eP9bAEvcCSdPeNfsn/… \$\endgroup\$
    – Terry
    Commented Jul 5, 2023 at 10:41
  • \$\begingroup\$ Yes, that's it. \$\endgroup\$ Commented Jul 5, 2023 at 11:05
  • \$\begingroup\$ OK, thanks Spehro. On the breadboard that outputs about 90 secs low (previously 50 secs), then 1.5 secs high (previously 1 sec), then repeats. (The first cycle timings seem to be the same as subsequent cycles, which is good). Any suggestions how to make it about 60 secs low, then 0.5 secs high, then repeat? I assume I need to change resistor values, but so far I'm not getting anywhere (I tried decreasing R1). Also, it's going high momentarily (maybe 0.05 secs) initially. Not a big problem for me, but why does it happen and is it easy to avoid? Thanks again. \$\endgroup\$
    – Terry
    Commented Jul 6, 2023 at 6:09
  • \$\begingroup\$ 2nd schematic. R2 controls off time, R4 on time. Reduce the resistance to reduce the time, so for 60 seconds you'd want about 680kΩ. Similarly for 'on' time. Removing C2 and adding a bypass capacitor across the power may help with the glitch. That will slightly change the timing. \$\endgroup\$ Commented Jul 6, 2023 at 6:20
  • \$\begingroup\$ OK, I'll experiment with R2 & R4, thanks Spehro. Re my statement "The first cycle timings seem to be the same as subsequent cycles", I found I was mistaken after I drained the 100uF cap (by shorting its legs) before my test, so you were right. The 10nF cap at C2 is intended to be a decoupling cap, which is the same as a bypass cap, right? C2 is currently a KEMET Multilayer Ceramic Capacitor MLCC 100V dc±10%, C315C103K1R5TA. Should I change that to something else, if so what type & value? Or do you mean I should just move this cap to being "across the power", if so between what & what, exactly? \$\endgroup\$
    – Terry
    Commented Jul 6, 2023 at 22:45

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