UNIT-IV Application of Special I.C’s. The 555 Timer, 555 as Monostable, Astable Multivibrator and Applications, Phase Locked Loops, Operating Principles.

Inside the 555 timer there are Over 20 transistors, 15 resistors, 2 diodes, depending of the manufacturer. Supply voltage between 4.5 and 18 volt, supply current 3 to 6 mA, Sinking or sourcing 200 mA of load current. Rise/Fall time of 100 nSec. The Threshold current determine the maximum value of Ra + Rb. For 15 volt operation the maximum total resistance for R, i.e. (Ra +Rb) is 20 Mega-ohm. The temperature variation is only 50ppm/°C (0.005%/°C).

Inside the 555 S Q R Note the voltage divider inside the 555 made up of 3 equal 5k resistors

Salient Functional Features of 555. Trigger (Pin2) <1/3 Vcc, sets Vo (Pin3) to “1”. Once triggered even pin2 is made 1 or 0 there is no change in the output. Threshold (Pin6) > 2/3 Vcc sets Vo (Pin3) to “0” Reset “0” (Pin4) sets Vo to “0” any time. Usually it is at “1” Initially (Pin7) is set to ground as the power supply is on.

Monostable Operation or Timer. R-S Flip Flop Initial Condition: 1. Threshold Comparator. Ref: Inv(-) at 2/3 Vcc Trigger Comparator. Ref: non Inv(+) at 1/3 Vcc . Q2 is a PNP transistor used to reset and make a short the NPN transistor when Pin.4 (Base) is grounded. Usually Pin.4 is connected to + of power S Q R Q S R OUTPUT Q REMARK No Change 1 FF Resets FF Sets Not allowed Supply so that transistor is open. Pin.5 is at 2/3 Vcc. The comparator output is –ve, Q is 1 making short of pin.7 to ground.

Note that the trigger pulse must actually be of shorter duration than the time interval determined by the external R and C. When pin 2 is held low longer than that, the resultant output will remain high until the trigger input is driven high once again. In the mean while even pin2 is made 1 or 0 there is no change in the output.

Monostable Operation or Timer.

DERIVATION The time t1 taken by the circuit to charge from 0 to (2/3) Vcc is t1 = 1.098RC The time t2 taken by the circuit to charge from 0 to (1/3) Vcc is t2 = 0.405 RC The time to charge from (1/3) Vcc to (2/3) Vcc is tHigh = t2-t1 = 1.098RC - 0.405 RC = 0.693 RC

Connecting a Load. Current Sinking and Sourcing.

Charging a Capacitor Capacitor C1 is charged up by current flowing through R1 As the capacitor charges up, its voltage increases and the current charging it decreases, resulting in the charging rate shown

Charging a Capacitor Capacitor Current Capacitor Voltage Where the time constant

Charging a Capacitor

Charging a Capacitor The time that it takes for the capacitor to charge to 63.7% of the applied voltage is known as the time constant (t). It takes approximately 5 complete time constants for the capacitor to charge to almost the applied voltage.

Astable Multivibrator

Initially when the output is high capacitor C starts charging towards  Vcc through RA and RB. However as soon as the voltage across the capacitor equals 2/3 Vcc , comparator1 triggers the flip-flop and the output switches to low state.    Now capacitor C discharges through RB and the transistor Q1. When voltage across C equals 1/3 Vcc, comparator 2’s output triggers the flip- flop and the output goes high. Then the cycle repeats.

Working Principle Threshold(6) and trigger comparator(2) inputs are joined together. A capacitor is connected to ground from 2&6. If the power is on, Pin2 will be < 1/3 Vcc. Pin3 is “1”. Discharging transistor is off and the capacitor starts charging. When it is > 2/3 Vcc, the FF is set, Pin3 is “0” Now the transistor is short and capacitor starts discharging through RB. When it is < 1/3Vcc. Output is again “1” and cycle repeats. Hence the charging and discharging is between 2/3Vcc and 1/3Vcc .

Astable Multivibrator

Tc = 0.693(RA+RB) Td + 0.693RBC T + tc + td = 0.693(RA + 2RB)C   The output frequency, f is independent of the supply voltage Vcc.   Therefore the frequency of oscillation Thus the total time period of the output waveform is

Minimum component Astable (50% Duty Cucle) This is a cheap and cheerful astable using just one resistor and one capacitor as the timing components: Minimum. However, if you build this circuit, it is probable that the HIGH time will be longer than the LOW time. (This happens because the maximum voltage reached by the output pulses is less than the power supply voltage.) Things will get worse if the output current increases. The charging and discharging path is through Pin-3 of 555.

BISTABLE CIRCUIT.

At the beginning of the cycle, C1 is charged through resistors R1 and R2. The charging time constant is The voltage reaches (2/3)Vcc in a time  = (R1+R2)C1  = 0.69(R1+R2)C1

The capacitor voltage cycles back and forth between (2/3)Vcc and (1/3)Vcc at times, and 1 = 0.69(R1+R2)C1 2 = 0.69(R2)C1

The frequency is then given by

APPLICATIONS OF 555

1. Extended duty cycle astable Duty Cycle = ton/(t1+t2).

2. Pulse width Modulation. One shot triggered by trigger pulses at Pin-2. Modulating Sine wave is given at Pin-5. PWM output is at Pin-3. Used for DC motor speed control.

PWM Waveforms

Optical Transmitter Circuit Astable is used to produce carrier pulses at a frequency we cannot hear (well above 20kHz)

3. Pulse Position Modulation (PPM) +ve Pulses Suppressed Differentiator In pulse position modulation, the amplitude and width of the pulses are kept constant, while the position of each pulse with reference to the position of a reference pulse, is changed according to the instantaneous sampled value of the modulating signal.

Pulse position modulated waveform

A 555 IC timer can be used to build a Pulse position modulator A 555 IC timer can be used to build a Pulse position modulator. This pulse position modulator (PPM) is different from pulse width modulation (PWM) which keep constant frequency.The PPM does not keep constant frequency.

4. FSK Generator

Digital Modulation Only.

5. Linear Ramp Generator When a capacitor is charged with a constant current source then linear ramp is obtained. This concept is used in linear ramp generator.

The circuit is used to obtain constant current Ic is a current mirror circuit, using transistor Q and diode D. The current Ic, charges capacitor C at a constant rate towards + Vco But when voltage at pin 6 i.e. capacitor voltage Vc becomes (2/3Vcc), the comparator makes internal transistor Qi ON within no time. But while discharging when Vc becomes (1/3 Vcc)/ the second comparator makes Qi OFF and C starts its charging again. As discharging time of capacitor C is very small, the time period of ramp is assumed practically same as that of charging time of capacitor.

Fan/Motor Speed Control By adding a comparator to the ramp generator we can create a very nice variable duty-cycle pulse generator, much like we did in the previous section. We will use this for a speed controller for our little DC brushless fan.

VARIABLE DUTY CYCLE

555 Timer as a Schmitt Trigger When a Sine wave is applied Tripping points are 1/3Vcc &2/3Vcc. R1=R2

The upper comparator will trip at 2/3 Vcc while lower comparator at 1/3 Vcc.

The frequency of square wave remains same as that of input The frequency of square wave remains same as that of input. The Schmitt trigger can operate with the input frequencies up to 50 kHz.

555 Timer Applications 555 timer is used to produce an oscillating signal whose voltage output is increased by the transformer to a dangerous level, producing sparks. DO NOT DO THIS WITHOUT SUPERVISION