I'm trying to build a 555 timer with a 50% duty cycle.

I followed the instructions given here (pg 12).

I used a 470 microfarad capacitor, 150 ohm resistor and a kohm resistor. The output voltage is always low.

I've attached an image of my current circuit and would appreciate it if someone could pinpoint what I'm doing wrong.

enter image description here

  • 3
    \$\begingroup\$ It might be a good idea to crop the image just to the region of interest. It looks to me like the cap on pin 5 would be shorted out by that arrangement or is the black wire soldered to it? \$\endgroup\$
    – PeterJ
    Nov 30 '13 at 5:14
  • 1
    \$\begingroup\$ Indeed looks like pin 5 is connected to ground, if that is the case, thresholds for the RC oscillator are entirely wrong and it won't work. The thresholds normally are 1/3 and 2/3 times Vcc, in this case 0 and 1/2 times Vcc. You can never cross the 0V threshold. \$\endgroup\$
    – jippie
    Nov 30 '13 at 8:36
  • \$\begingroup\$ possible duplicate of variable 555 circuit with equal LOW&HIGH frequency \$\endgroup\$
    – Dave Tweed
    Nov 30 '13 at 11:44
  • \$\begingroup\$ A picture of a wired-up breadboard is not nearly as useful as a schematic, unless trying to help you determine if you've wired the breadboard incorrectly from the schematic. \$\endgroup\$
    – JYelton
    Nov 30 '13 at 21:11

If you need an accurate 50% duty cycle, a simple 555 circuit is not the way to go. A better method is to have an oscillator run at double frequency, where duty cycle is unimportant. Then feed that signal in a divide-by-two circuit and you'll have a perfect 50% duty cycle.

A simple divider can be made using a JK-flip flop where the J and K are high, like in this circuit:


simulate this circuit – Schematic created using CircuitLab

There are countless ways to create a 2-divider, all with their own advantages and disadvantages. A simple Google image search will give you pages full of them.

  • \$\begingroup\$ D-flipflops are more common. Connect Q* to D. \$\endgroup\$
    – radagast
    Nov 30 '13 at 9:04
  • 1
    \$\begingroup\$ @radagast Yeah but JK are more like magic. I prefer magic ;o) \$\endgroup\$
    – jippie
    Nov 30 '13 at 9:07
  • \$\begingroup\$ @radagast Actually I first wanted to draw a circuit with a D-FF, but CircuitLab lacks the \$\overline{Q}\$ output on the D-FF symbol. \$\endgroup\$
    – jippie
    Nov 30 '13 at 9:09
  • \$\begingroup\$ I won't deny that JKs are more universal (magical), but I guess their more complex internals would make them more expensive as well. (In any case, I +1'd for the best solution to get 50.00% duty cycle) \$\endgroup\$
    – radagast
    Nov 30 '13 at 9:12
  • \$\begingroup\$ Thanks for the detailed solution to get an exact 50% duty cycle! Otherwise, my issue was indeed shorting pin 5 which numerous people mentioned. \$\endgroup\$
    – Olshansk
    Nov 30 '13 at 14:42

It's hard to see exactly, but it looks like that black wire connects gnd to pin 5. That'll kill any chance of the 555 doing something interesting. Yank the black wire and put the cap from pin 5 to the gnd bus. (Or, since it's not critical for ordinary experimental purposes, omit the cap and leave pin 5 unconnected.)


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.