Gate Oscillator

CD 4011 contains four 2-input NAND gates in a single package. All the gates are independent in operation except for the common power supply and ground. CD 4011 comes in the 14 pin DIP (Dual In Line Package). Unlike other digital integrated circuits, all the unused inputs of the IC should be either connected to the positive rail or ground. No inputs should be left floating as this may cause erratic operation of the IC.

The circuit shown above is a simple oscillator using CD 4011 and only few passive components. This forms a simple RC oscillator. Resistor R2 (18K) turns gate 1(N1) into a linear amplifier with positive feed back. The circuit can give 2.2 kHz pulses from pin 4.


The formula to determine frequency is

Frequency of Oscillation f = 1 / 2.5 x R2 x C1

R2 = 18 K ( 18x 10 3 ) C1 = 0.01 ( 0.01x 10 -6 )

f = 1 / 2.5 x 18 x 10 3 x 0.01 x 10 – 6

1 / 2.5 x 0.18 x 10 -3

= 2.2 kHz

Knight Rider

In the Knight Rider circuit, the 555 is wired as an oscillator. It can be adjusted to give the desired speed for the display. The output of the 555 is directly connected to the input of a Johnson Counter (CD 4017). The input of the counter is called the CLOCK line. 
The 10 outputs Q0 to Q9 become active, one at a time, on the rising edge of the waveform from the 555. Each output can deliver about 20mA but a LED should not be connected to the output without a current-limiting resistor (330R in the circuit above). 
The first 6 outputs of the chip are connected directly to the 6 LEDs and these "move" across the display. The next 4 outputs move the effect in the opposite direction and the cycle repeats. The animation above shows how the effect appears on the display. 
Using six 3mm LEDs, the display can be placed in the front of a model car to give a very realistic effect. The same outputs can be taken to driver transistors to produce a larger version of the display.

555 Amplifier

The 555 can be used as an amplifier. It operates very similar to pulse-width modulation. The component values cause the 555 to oscillate at approx 66kHz and the speaker does not respond to this high frequency.  Instead it responds to the average CD value of the modulated output and demonstrates the concept of pulse-width modulation. The chip gets very hot and is only for brief demonstrations.

THE SIMPLEST 555 OSCILLATOR

The simplest 555 oscillator takes output pin 3 to capacitor C1 via  resistor R1.
When the circuit is turned on, C1 is uncharged and output pin 3 is HIGH. C1 charges via R1 and when Pin 6 detects 2/3 rail voltage, output pin 3 goes LOW. R1 now discharges capacitor C1 and when pin 2 detects 1/3 rail voltage, output pin 3 goes HIGH to repeat the cycle.
The amount of time when the output is HIGH is called the MARK and the time when the output is LOW is called the SPACE.
In the diagram, the mark is the same length as the space and this is called 1:1 or 50%:50%.
If a resistor and capacitor (or electrolytic) is placed on the output, the result is very similar to a sinewave.