Using the versatile L200 voltage regulator, this power supply has independent voltage and current limits. The mains transformer has a 12volt,2 amp rated secondary, the primary winding should equal the electricity supply. The 10k control is adjusts voltage output from about 3 to 15 volts, and the 47 ohm control is the current limit. This is 10mA minimum and 2 amp maximum. Reaching the current limit will reduce the output voltage to zero.
A basic full wave rectified power supply is shown below. The transformer is chosen according to the desired load. For example, if the load requires 12V at 1amp current, then a 12V, 1 amp rated transformer would do. However, when designing power supplies or most electronic circuits, you should always plan for a worst case scenario. With this in mind, for a load current of 1 amp a wise choice would be a transformer with a secondary current rating of 1.5 amp or even 2 amps. Allowing for a load of 50% higher than the needed value is a good rule of thumb. The primary winding is always matched to the value of the local electricity supply.
Having multiple extension telephones at home is very convenient. You can make or receive phone calls practically anywhere in the house. This circuit disables other telephones connected to the phone line whenever a telephone (either the master or any extension phone) is in use. The circuit is inexpensive and is guaranteed to keep the phone conversation private. The circuit does not need an external power supply. It gets its power from the telephone line. The no-load voltage at the telephone line, when the telephone handset is ‘on-hook,’ is around 48 volts. However, when the handset is off hook, terminal voltage drops to between 5 volts and 15 volts. This is due to the impedance of telephone line and the telephone set. The voltage of the telephone line is the key factor that controls the operation of this circuit. diodes D1, D2, D3 and D4 are connected as bridge rectifier to make the circuit non-polarised. Lifting the handset causes the terminal voltage to drop from 48V to about 10V. The drop in voltage does not, however, occur rapidly. therefore while the terminal voltage is still high (above the threshold voltage level), both zener diodes D5 and D6 are turned on. Current flows through resistor R3, triggering SCR1 and providing a link to the telephone set connected to lines L1(a) and L2(a). When the terminal voltage drops below the threshold voltage of the zener diode, diode D5 reverts to its nonconducting state, cutting off the gate drive to SCR1. However, once the SCR is on, it will remain in that state as long as the current flowing through it does not fall to near zero level. Thus the link continues. Zener D6 maintains the voltage across resistor R2 and LED1/LED2 indicates as to which telephone is in use. The low off-hook voltage of the line will disable the other extension phones. The line voltage will not turn on zener diodes D11 and D12, even if the handsets of the other extension phones are lifted. Use the following procedure to check up the system after wiring:
1. Lift the handset of each telephone to see whether the corresponding LED lights up. Return the handset back in its cradle; the LED should turn off. Use the same procedure to check the other phones.
2. Lift the handset of phone ‘A;’ its corresponding LED should light up. The other phones should be cut-off (no dial tone).
3. Lift the handset of phone ‘B’, then return the handset of phone ‘A’ to its cradle. Now ‘B’ telephone’s LED should light up and the dial tone should be heard through the ear-piece.
Here is a simple circuit which can be used for decoration purposes or as an indicator. Flashing or dancing speed of LEDs can be adjusted and various dancing patterns of lights can be formed.
The circuit consists of two astable multivibrators. One multivibrator is formed by transistors T1 and T2 while the other astable multivibrator is formed by T3 and T4. Duty cycle of each multivibrator can be varied by changing RC time constant. This can be done through potentiometers VR1 and VR2 to produce different dancing pattern of LEDs. Total cost of this circuit is of the order of Rs 30 only. Potentiometers can be replaced by light dependent resistors so that dancing of LEDs will depend upon the surrounding light intensity. The colour LEDs may be arranged as shown in the Figure.
In an audio amplifier the quality of sound depends upon a number of factors, e.g. quality of active and passive components, circuit configuration, and layout. To an extent, the selection of components depends on the constructor’s budget. The discrete active components like transistors have been increasingly replaced by linear ICs, making the task of designer easier. With the passage of time, the general-purpose op-amps like LM741, which were being used in audio/hi-fi circuits, have become The preamplifier circuit presented here is based on a dual precision op-amp for the construction of a low distortion, high quality audio preamplifier.
A dual op-amp OPA2604 from Burr-Brown is used for all the stages. The FET input stage op-amp was chosen in this context it is worth wile to mention another popular bi-polar architecture op-amp, the NE5534A. It has, no doubt, an exceptionally low noise figure of 4nV/ÖHz but rest of the specifications compared to OPA2604 are virtually absent in this IC. Also This IC is also capable of operating at higher voltage rails of ± 24V (max.). Also its input bias current (100 pA) is many orders lower than its bipolar counterpart’s. This ensures a multifold reduction in noise.
The simplest lamp dimmer circuit consists of a rheostat, in series with the lamp, which one may adjust to obtain the required brightness. Such linear regulators are quite inefficient since a lot of power is wasted in them. Moreover, in the rheostat the moving contacts are likely to get damaged in the long run, as its value is frequently adjusted by moving the slider. Such linear control circuits provide an overall efficiency of no more than 50 per cent. This wastage of power can be avoided if one uses pulse width modulation (PWM) which can be made to control an electronic rheostat. The circuit shown here is based on PWM principle. Gate N1 and its associated components constitute an oscillator producing oscillations of approximately 200 Hz with a pulse width of 0.1 ms. This output is fed to transistor T1 for level shifting. At the output of this transistor is a potentiometer VR2, using which a DC component can be added to the pulses emerging from transistor T1. By adjusting this potentiometer/trimmer, one can have a good linear control of the lamp brightness from completely off state to 100 per cent on state. The signal is inverted by gate N2 and fed to MOSFET 12N10. IC CD40106 provides six inverting buffers with Schmitt trigger action. The buffers are capable of transforming slowly changing input signals into sharply defined jitter-free output signals. They are usually used as wave and pulse shapers. IC CD40106 possesses high immunity and low power consumption of standard CMOS ICs along with the ability to drive 10 LS-TTL loads. In this circuit loads up to 24W can be connected between MOSFET drain and 12V supply without using a heatsink. The loads can even be DC motors, miniature heating elements, etc. If one uses a low RDS (on) MOSFET, a higher efficiency can be achieved. By using the components as shown in the circuit, an efficiency of approximately 95 per cent can be achieved. The flexibility of the design makes it possible to change the MOSFET with a similar one, in case of non-availability of 12N10. The circuit by itself does not draw much current when the load is disconnected. Ensure proper ESD protection while handling the MOSFET to prevent damage. Lab note: The circuit was tested using MOSFET IRF640 with RDS (on)=0.18 ohm.
