Electronic – Anyway to avoid the startup delay on square wave generator (e.g. 555 astable) circuit

Your schematic shows no decoupling capacitor across the supply. High-current spikes, like the 555 generates on transitions, require a supply that is low-impedance at high-frequencies. The inductance and resistance of the cable connecting to your supply is too high for it to supply those high-frequency currents.

Try adding a capacitor of a hundred or so microfarads across the supply, as near to the 555 power pins as you can get.

Your scope shot is shows noise with a period of 3.5 * 200 nS, or about 1.4 MHz. This is a higher frequency than any noise generated by the 555 circuit. It could be residual switching noise from your power supply, but is most likely sloppy scope probing. If you are using the alligator-clip-on-a-wire ground lead on the scope probe, you have made a little loop antenna which will show you any magnetic field in the vicinity.

To see how the 555 operation is affecting the power you need to be triggering synchronous to it. Connect one channel to the LED and trigger off of it, then look at the supply with the other channel.

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}