Electronic – How to enable/disable a running 555 astable oscillator with a control signal

The first scheme is the astable configuration as descibed by the datasheet: it's a monostable multivibrator which triggers itself.

• When the output is low, C is charged through R1 and R2 until it reach the threshold voltage (2/3 Vcc);
• then OUT is turned high, DIS is turned on and the capacitor starts discharging through R2 to DIS, until it drops below the triggering voltage (1/3 Vcc); then OUT goes low and it starts again.

Note that the duty cycle is limited to 1/2.

The other two schemes have the advantage that the duty cycle can go from almost 0 to almost 100%, making them more suitable for PWM purposes.

The second schematic is very ugly, but basically you change the duty cycle varying the ratio of the resistances that charge and discharge the capacitor C1, turning the potentiometer R1.

For some reason, the charge and discharge of C1 is driven by the output Q, while the real output is through the DIS pin. The bad thing about this is that the output through the DIS pin is open collector, so it will be strongly driven low by the Dis pin but pulled up only by the 10k resistor.

Note that if you turn it fully to one of the rails, you are basically charging/discharging the capacitor through a diode and the output stage of the 555. This is not exactly a good idea.

The third one is based on the same concept, but a resistor is put to the OUTput to avoid the aforementioned problem, and the Vout pin is used properly.

If you need a PWM control, I would suggest looking at the datasheet, where you can find a good PWM modulator:

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}