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Wednesday, October 6, 2010

Static 0 to 9 display

The circuit shown here is of a simple 0 to 9 display that can be employed in a lot of applications. The circuit is based on asynchronous decade counter 7490(IC2), a 7 segment display (D1), and a seven segment decoder/driver IC 7446 (IC1).
The seven segment display consists of 7 LEDs labelled ‘a’ through ‘g’. By forward biasing different LEDs, we can display the digits 0 through 9. Seven segment displays are of two types, common cathode and common anode. In common anode type anodes of all the seven LEDs are tied together, while in common cathode type all cathodes are tied together. The seven segment display used here is a common anode type .Resistor R1 to R7 are current limiting resistors. IC 7446 is a decoder/driver IC used to drive the seven segment display.
Working of this circuit is very simple. For every clock pulse the BCD output of the IC2 (7490) will advance by one bit. The IC1 (7446) will decode this BCD output to corresponding the seven segment form and will drive the display to indicate the corresponding digit.

4W Fluorescent lamp driver.

This is a simple 4 W fluorescent lamp driver circuit that can be operated from a 12 V supply.The first part of the circuit includes a NE555 timer IC wired as an astable multivibrator.The output pulses from the IC are amplified by the transistor Q1.The transformer steps up the collector voltage to around 1KVto drive the fluorescent lamp. Before using the circuit, set the R2 at full resistance and switch on the supply, now adjust R2 so that the collector current is 300mA (use a multimeter) and this is the optimum setting for the lamp.Operating the lamp in this setting will give a better life.

  • Assemble the circuit on a general purpose PCB.
  • The IC 1 must be mounted on a holder.
  • Use heat sink for transistor Q1.
  • Use a 3 V primary , 230 V secondary, 5W transformer for T1.
  • Power the circuit from a 12V battery or 12V DC power supply.
  • The L1 can be a 6 inch, 4W fluorescent lamp.

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13V 5A adjustable regulator using LM338

Here is the circuit diagram of a 30V/3A adjustable regulator using the LM723 IC from the National Semiconductors. LM723 is an integrated series regulator whose output voltage can be adjusted between 2V and 37V. The IC by itself can deliver an output current of 150mA and the maximum input voltage to the IC is 40V.
Here 3A output current is attained by adding a pass transistor (Q1) to the ICs output. The pass transistor used here is a Darlington transistor MJ3001. The internal reference voltage of the IC is 7.15V and it is available at pin6. POT R1 can be used to adjust the output voltage.
  • Assemble the circuit on a good quality PCB.
  • T1 can be a 230V primary, 25V secondary, 5A step down transformer.
  • Q1 must be fitted on a proper heat sink.
  • Output voltage can be adjusted by using the POT R1.

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13V 5A adjustable regulator using LM338

This 13V/5A power supply is based on the famous LM338 IC from the ST Microelectronics. The IC has time dependent current limiting, thermal regulation and is available in 3 lead transistor package. The IC can easily supply well over 5A at an output voltage range between 1.2V and 30V.
In this circuit the output voltage is determined by the two resistors R1 and R2.The output voltage can be varied by adjusting the R2.Diodes D2 and D3 are protection diodes. Capacitors C1 and C5 are filter capacitors while C2 and C3 are decoupling capacitors.

Voltage Controlled Oscillator

In most cases, the frequency of an oscillator is determined by the time constant RC. However, in cases or applications such as FM, tone generators, and frequency-shift keying (FSK), the frequency is to be controlled by means of an input voltage, called the control voltage. This can be achieved in a voltage-controlled oscillator (VCO). A VCO is a circuit that provides an oscillating output signal (typically of square-wave or triangular waveform) whose frequency can be adjusted over a range by a dc voltage. An example of a VCO is the 566 IC unit, that provides simultaneously the square-wave and triangular-wave outputs as a function of input voltage. The frequency of oscillation is set by an external resistor R1 and a capacitor C1 and the voltage Vc applied to the control terminals. Figure shows that the 566 IC unit contains current sources to charge and discharge an external capacitor Cv at a rate set by an external resistor R1 and the modulating dc input voltage. A Schmitt trigger circuit is employed to switch the current sources between charging and discharging the capacitor, and the triangular voltage produced across the capacitor and square-wave from the Schmitt trigger are provided as outputs through buffer amplifiers. Both the output waveforms are buffered so that the output impedance of each is 50 f2. The typical magnitude of the triangular wave and the square wave are 2.4 and

The frequency of the output waveforms is approximated by
fout = 2(V+ - Vc)/R1C1V+

Figure shows the pin connection of the 566 unit. The VCO can be programmed over a 10-to-l frequency range by proper selection of an external resistor and capacitor, and then modulated over a 10-to-l frequency range by a control voltage, VThe voltage controlled oscillators (VCOs) are commonly used in converting low-frequency signals such as EEG (electro-encephalograms) or ECG (electro-cardiograms) into an audio­frequency (AF range).

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Function Generator Circuit

The ICL8038 is a function generator chip, able of generating triangular, square , sine, pulse and sawtooth waveforms . From these sine, square & triangular wave forms can be made simultaneously.There is the option to control the parameters like frequency ,duty cycle and distortion of these functions.This is the best function generator circuit for a beginner to start with and is of course a must on the work bench of an electronics hobbyist.The circuit here is designed to produce waveforms from 20Hz to 2o kHz.The ICL 8038 has to be operated from a dual power supply.

Notes .
  • The circuit needs a dual power supply . A +15 -15 power supply as shown in the circuit is enough for the purpose.
  • The frequency of the output wave form can be adjusted using R7.It must be a 100K Log POT.
  • The duty cycle can be adjusted using R3 , a 1K POT.
  • The distortion of the wave form can be adjusted using R5 , a 100K POT.
  • Square,triangle & sine waveforms can be obtained simultaneously at pins 9,3,2 respectively.
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IC 723 Voltage Regulators

Figure shows the basic circuit of an IC 723 voltage regulator. This IC has a voltage reference source, an error amplifier, a series pass transistor, and a current limiting tran­sistor all contained in one small package. The device can be connected to operate as a positive or negative voltage regulator with an output voltage ranging from 2 V to 37 V, and output current levels upto 150 m A. The maximum supply voltage is 40 V, and the line and load regulations are each specified as 0.01%.

Figure shows an IC 723 connected to operate as a positive voltage regulator. The output voltage can be set to any value between approximately 7 V (reference voltage) and 37 V by appropriate selection of resistors R1 and R2. A potentiometer may be included between R1 and R2, of course, to make the voltage adjustable. An external transistor may be Darlington connected to Q1 (as shown in earlier post) to handle large load current. The broken lines in the figure shows connections for simple (non-foldback) current limiting. (Foldback current limiting can also be used with IC 723). A regulator output voltage less than the 7 V reference level can be obtained by using a voltage divider across the reference source [terminals 6 and 7 in earlier figure]. The potentially divided reference voltage is then connected to terminal 5.
It is important to note that the supply voltage, at the lowest point on the ripple waveform, should be at least 3 V greater than the output of the regulator and greater than VREF; otherwise a high-amplitude output ripple may occur.

555 Timer as Monostable Multivibrator

A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network,connected externally to the 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable (unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capaci­tor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accom­plished by external triggering. The schematic of a 555 timer in monostable mode of operation is shown in figure.
Pin 1 is grounded; pins 4 and 8 are shorted and then tied to supply +Vcc, output (VOUT is taken form pin 3; pin 2 and 6 are shorted and the connected to ground through capacitor C, pin 7 is connected to supply + VCC through a resistor RA; and between pin 6 and 7 a resistor RB is connected. At pin 5 either a bypass capacitor of 0.01  F is connected or modulation input is applied.

Monostable Multivibrator Circuit details
Pin 1 is grounded. Trigger input is applied to pin 2. In quiescent condition of output this input is kept at + VCC. To obtain transition of output from stable state to quasi-stable state, a negative-going pulse of narrow width (a width smaller than expected pulse width of output waveform)  and  amplitude of greater than + 2/3 VCC is applied to pin 2. Output is taken from pin 3. Pin 4 is usually connected to + VCC to avoid accidental reset. Pin 5 is grounded through a 0.01 u F capacitor to avoid noise problem. Pin 6 (threshold) is shorted to pin 7. A resistor RA is connected between pins 6 and 8. At pins 7 a discharge capacitor is connected while pin 8 is connected to supply VCC.

555 IC Monostable Multivibrator Operation.

For explain­ing the operation of timer 555 as a monostable multivibrator, necessary in­ternal circuitry with external connections are shown in figure.

The operation of the circuit is ex­plained below:

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls below +1/3 VCC, the output of comparator goes high which resets the flip-flop and consequently the transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable to quasi-stable state, as shown in figure. As the discharge transistor is cut­off, the capacitor C begins charging toward +VCC through resistance RA with a time constant equal to RAC. When the increasing capacitor voltage becomes slightly greater than +2/3 VCC, the output of comparator 1 goes high, which sets the flip-flop. The transistor goes to saturation, thereby discharging the capacitor C and the output of the timer goes low, as illustrated in figure.
Thus the output returns back to stable state from quasi-stable state.
The output of the Monostable Multivibrator remains low until a trigger pulse is again applied. Then the cycle repeats. Trigger input, output voltage and capacitor voltage waveforms are shown in figure.

Monostable Multivibrator Design Using 555 timer IC

The capacitor C has to charge through resistance RA. The larger the time constant RAC, the longer it takes for the capacitor voltage to reach +2/3VCC.
In other words, the RC time constant controls the width of the output pulse. The time during which the timer output remains high is given as
tp = 1.0986 RAC
where RA is in ohms and C is in farads. The above relation is derived as below. Voltage across the capacitor at any instant during charging period is given as
vc = VCC (1- e-t/RAC)
Substituting vc = 2/3 VCC in above equation we get the time taken by the capacitor to charge from 0 to +2/3VCC.
So +2/3VCC. = VCC. (1 – e-t/RAC)   or   t – RAC loge 3 = 1.0986 RAC
So pulse width, tP = 1.0986 RAC s 1.1 RAC
The pulse width of the circuit may range from micro-seconds to many seconds. This circuit is widely used in industry for many different timing applications.

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