GATING THE OSCILLATOR

The 555 oscillator can be turned on and off via a control line. This is called "Gating the Oscillator" or "Controlling the Oscillator."
When designing this type of circuit, two things need to be considered:
1. The oscillator to be switched off with the output HIGH
2. The oscillator to be switched off with the output LOW
The next consideration is:
3. The oscillator (block) to be switched off
4. The oscillator (block) to remain in circuit.
There is an enormous difference between these designs. The main difference is the current consumption of the load, but the actual consumption of the chip can also be important.
Take for example, these two circuits:

In the first circuit, the key is in the output and the chip draws current all the time. If the project is battery operated, it will need an on-off switch. The second circuit uses the key as the switch and the circuit will not need a switch. The difference is only 10mA but if the first circuit is left on, the battery will

The next circuit shows one way to turn off a 555 after a period of activation:

The only problem with this circuit is the gradual lowering in volume as the electrolytic discharges. The 1,000u to 4700u determines the length of time the circuit is activated AFTER the Bell-Push is pressed. The circuit drops to zero current (the only current is the leakage of the 1,000u electrolytic).

In the following circuit the first 555 gates the second 555.

The second 555 is not turned off. The circuit inside both 555's are always drawing current.
It is not practical to "turn off" a 555 as shown in the next diagram:

The output of a 555 is 1.7v less than rail voltage. This means the second 555 is receiving 10.3v from the output line of the first 555 if the rail voltage is 12v. The maximum output of the second 555 will be 10.3 - 1.7 = 8.6v This may be too low for many output devices and the result may be disappointing.
The Schmitt Trigger can be gated too.
The first point to note is the hex Schmitt trigger IC contains 6 identical gates and the chip is normally permanently connected to the supply rail. If any of the unused inputs are tied HIGH, the particular gate draws very little current (less than 1uA), making the total for the chip about 6uA.
There are two ways to GATE a Schmitt Trigger and prevent it from oscillating.
The diagrams below show a Schmitt Trigger being gated so that the output is:

1. LOW,
2. HIGH.

Mouseover the animations below and see how the GATING LINE inhibits the oscillator:

Mouse-over to INHIBIT the Left Oscillator

For the left circuit, if the gating diode is taken HIGH, the capacitor charges quickly. This inhibits the operation of the oscillator and the output goes LOW. If a load is connected to the output of this gate, it will not be driven and the gate will consume the least current.
For the right circuit, when the gating diode is taken HIGH it does not have any effect on the operation of the circuit and the oscillator continues to operate.

Mouse-over to INHIBIT the Right Oscillator

For the left circuit, if the gating diode is taken LOW, it does not have any effect on the operation of the circuit and the oscillator continues to operate.
For the right circuit, if the gating diode is taken LOW, the capacitor is discharged and the oscillator is INHIBITED. The output goes HIGH and the load will be driven. The circuit will draw maximum current.

If NO LOAD is connected to the output, an inhibited gate will draw more current than when it is oscillating. Both arrangements will draw a similar current when inhibited. The current taken will be about 1uA for the gate plus the current through R.