555 timer with a 50% duty cycle

Question

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.

Answer 1

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.

Answer 2

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.)

Answer 3

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 R_A should be 0 Ω. However, the 555 will not cycle with 0 Ω for R_A; you must have some resistance.

I got my 555 cycling about 4 seconds on and 4 seconds off using 10 kΩ for R_A, 330 kΩ for R_B, 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 R_A and the 500 Ω resistor is R_B. 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 R_A that is insignificant to R_B but is large enough to let the 555 oscillate.