Electronic – Astable 555 timing circuit (0.5 Hz and 50% duty cycle)

Without knowing what your power supply configuration is, I can offer a few suggestions with reasonable caveats.

• If you need to ensure that there is no overlap between your current two transistors and the third, then your circuit will be more complicated than a few more 555's. You're then looking for something called a "dead time generator" and you probably would need to define your minimum dead time requirements.

• If you are driving MOSFETs directly, and VDD for the 555 is the same as the load you are switching, you can use a PFET instead of an NFET, and that will work reasonably well. This will not work if your 555 is powered separately from your load. This is the simplest option and requires no additional parts. See schematic below:

^{simulate this circuit – Schematic created using CircuitLab}

• If you need to use all the same type of transistor (all NFETs or all PFETs) then the next best thing would probably be to use an inverter (or other integrated logic gate) that comes in a small package. This makes for a small circuit and easy to troubleshoot. If you're set on through-hole or breadboard-able parts, then there are a lot of chips with 6 inverters on them that will work great. Tons of options exist if you can live with some flavor of surface mount (you can have as few as 1 inverters in a package). This also has the advantage of working if your load is switched from a different VDD than your 555.

^{simulate this circuit}

• You could also use 555's to synthesize any logic function, such as an inverter.

^{simulate this circuit}

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.