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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 μF capacitor, a 150 Ω resistor and a 1 kΩ 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

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    \$\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, 2013 at 5:14
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    \$\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, 2013 at 8:36
  • \$\begingroup\$ possible duplicate of variable 555 circuit with equal LOW&HIGH frequency \$\endgroup\$
    – Dave Tweed
    Nov 30, 2013 at 11:44
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    \$\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, 2013 at 21:11

3 Answers 3

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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:

schematic

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.

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    \$\begingroup\$ D-flipflops are more common. Connect Q* to D. \$\endgroup\$
    – radagast
    Nov 30, 2013 at 9:04
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    \$\begingroup\$ @radagast Yeah but JK are more like magic. I prefer magic ;o) \$\endgroup\$
    – jippie
    Nov 30, 2013 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, 2013 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, 2013 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\$
    – Olshansky
    Nov 30, 2013 at 14:42
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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.)

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I assume you are trying to replicate Figure 14 in the TI LM555 specification that you linked. It is a sample circuit that allows "Astable Operation" or in other words, it self oscillates.

According to the TI formulas for Figure 14 (shown below Figure 15) it says that to have a 50% duty cycle RA should be 0 Ω. However, the 555 will not cycle with 0 Ω for RA; you must have some resistance.

I got my 555 cycling about 4 seconds on and 4 seconds off using 10 kΩ for RA, 330 kΩ for RB, and 22 μF for the capacitor.

Note: The TI formulas say my times should be closer to 5 seconds, but because of tolerances the resistors and capacitors values are not exactly what the manufactures say they are.

It doesn't look like you replicated Figure 14 exactly and that is probably why your circuit is not oscillating. I see that the 555 pins 2 and 6 go to both resistors as well as the capacitor. Pins 2 and 6 should go to the capacitor and ONE resistor. You show pin 7 going to one resistor; it should go to both resistors.

Since your 1 kΩ resistor goes to + I assume it will be RA and the 500 Ω resistor is RB. Your capacitor is 470 μf so the TI formulas would give:

t1 = 0.693·(1000+500)·0.00047 = 0.49s
t2 = 0.693·(500)·0.00047 = 0.16s

Obviously that is nowhere near a 50% duty cycle. To get close to a 50% duty cycle you are going to have to pick an RA that is insignificant to RB but is large enough to let the 555 oscillate.

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