See Page 49 for the first section of this discussion. Each inverter can be used as a separate "block" or "building block" or "circuit" in a project. You can use 1, 2 or all 6 "gates." We can call the inverter a "gate." This is a very loose term as a "gate" normally has more than one input and the second input is used to activate or de-activate the gate. The action of a Schmitt Trigger is shown in the following animation. The action shows the output changing rapidly as the input rises and falls. The output always changes from one state to the other VERY QUICKLY. The input can rise and fall slowly - the output is always a fast action. This is because the chip does not know if the voltage is a HIGH or a LOW. Here are some basic building blocks: In the diagram above, the input goes HIGH and remains HIGH. It can be detecting a piece of equipment being turned on, for example.
If a diode is added across the input resistor, the capacitor "C" will be discharged when the input goes low, so the "Delay Time" will be instantly available when the input goes HIGH: The following circuit produces a PULSE when the input goes HIGH: To invert the output, add an inverter:
To produce a pulse after a delay, the following circuit is required: The following circuit produces a tone during the HIGH period. When the output of the second inverter is HIGH, it places a high on the input of the third inverter, via the diode. The oscillator above can be set to produce a 100Hz tone and this can activate a 2kHz oscillator to produce a 2-tone output. A "jamming diode" is needed between the third and fourth gates to allow the high-frequency oscillator to operate when the output of the low-frequency oscillator is HIGH. The output can be buffered with a transistor: Extending the action of a push button The action of a push button can be extended by adding the following circuit:
To produce a pulse of constant length, (no matter how long the button is pressed), the following circuit is needed: GATING Gating is the action of preventing or allowing a signal to pass though a circuit. In the following circuit, buttons "A" and "B" are gated to allow the oscillator to produce an output. The first two inverters form an "OR-gate." When the output of the gate is HIGH it allows the oscillator to operate.
The second diode is called the gating diode. When the output of the second inverter is LOW, the capacitor is prevented from charging as the diode will not allow it to charge higher than 0.7v, and thus the oscillator does not operate.
2 MINUTE TIMER Some of the features we have discussed have been incorporated into the following circuit. The relay is energized for a short time, 2 minutes after the push-button is pressed. The push-button produces a brief LOW on pin 1, no matter how long it is pushed and this produces a pulse of constant length via the three components between pin 2 and 3. This pulse is long enough to fully discharge the 100u timing electrolytic on pin 5. The 100k and electrolytic between pins 6 and 9 are designed to produce a brief pulse to energize the relay.
The 74c14 can be used for lots of different circuits. In the following design, the output produces 3mS pulses every second. The circuit is adjustable to a wide range of requirements. TRIGGER TIMER The next design interfaces a "Normally Open" and "Normally Closed" switch to a delay circuit. The feedback diode from the output prevents the inputs re-triggering the timer (during the delay period) the so that a device such as a motor, globe or voice chip can be activated for a set period of time.
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