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.
There is little to be said about this circuit. All the work is done by the regulator. The 78S09 can deliver up to 2 amps continuous output whilst maintaining a low noise and very well regulated supply.
The circuit will work without the extra components, but for reverse polarity protection a 1N5400 diode is provided at the input, extra smoothing being provided by C1. The output stage includes C2 for extra filtering, if powering a logic circuit than a 100nF capacitor is also desirable to remove any high frequency switching noise.
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.
This sensitive vibration sensor is exclusively made for shops to protect against burglary. It will detect any mechanical or acoustic vibration in its vicinity when somebody tries to break the shutter and immediately switch on a lamp and sound a warning alarm. A 15-minute time delay after switch-on allows sufficient time for the shop owner to close the shutter.
This circuit conditions different signals of frequency below 1 kHz and displays their waveforms on the PC’s screen. The hardware is used to condition the input waveform and convert it to the digital format for interfacing to the PC. The software for acquiring the data into the PC and displaying the same on its screen is written in Turbo C.
The input waveform (limited to 5V peak-to-peak) is first applied to a full-wave rectifier comprising op-amps A1 and A2 of quad op-amp LM324 (IC4) and a zero crossing detector built around LM3914 dot/ bar display driver (IC8) simultaneously.
The full-wave rectifier rectifies the input signal such that the negative half cycle of the input signal is available in the positive side itself, so both the half cycles are read as positive when it is given as input to the ADC. During positive half cycle, diode D3 is on and diode D4 is off, and op-amps A1 and A2 act as inverters. Thus the output is a replica of the input. During the negative half cycle, diode D3 is off and diode D4 is on. With R2=R3=R4=R5=R6=R=330 ohms, the voltage (V) at inverting pin 2 of op-amp A1 is related to the input voltage (Vi) as follows:
Vi/R +V/(2R)+V/R=0
V= - (2/3)Vi
The final output voltage (Vo) at pin 7 of op-amp A2 is given by the following relationship:
Vo=(1+R/2R)(-2Vi/3)= -Vi
As Vi is negative, the output voltage is positive.
The zero-crossing detector detects whether the cycle is positive or negative. It is the most critical part of the circuit and if it operates improperly, the symmetry of the analogue signal displayed in the PC monitor gets affected. At the zero-crossing instant when the input signal transits to negative side, the zero-crossing detector informs the PC by taking pin 15 of 25- pin ‘D’ connector of the parallel port high.
The input at pin 15 of ‘D’ connector goes low when the input signal transits to positive side. The zero-crossing detector communicates with the PC through bit D3 of the status port 379Hex.
The zero-crossing detector has been realised using LM3914 IC. You may adjust VR1 such that the last LED (LED10) goes off when the input signal transits negative side of the input waveform. The LM3914 itself rectifies the input signal and allows only positive half of the cycle.
The output from the full-wave rectifier is applied to the input of a sample-and-hold circuit comprising op-amps A3 and A4 of the LM324 (IC5), capacitor C3, transistor T1 (SL100), and analogue switch IC6 (CD4016). This circuit samples the input signal, i.e. it divides the waveform into a number of voltages or points and inputs each voltage level (with a delay) to the ADC for conversion into the digital format. Op-amps A3 and A4, along with a switch from IC CD4016 and a 1500pF capacitor with sampling time of 20 μs, are used as voltage followers/buffers.
The inbuilt clock generator circuit produces a frequency of about 640 kHz with R1=10 kilo-ohms and C4=150 pF, which are the externally connected timing components. The conversion time obtained is approximately 100 μs. The functions of other pins are given below:
Pin 1 (CS): This is activelow chip-select pin.
Pin 2 (RD): This active-low pin enables the digital output buffers. When high, the 8-bit bus will be in Hi-Z state.
Pin 3 (WR): This active-low pin is used to start the conversion.
Pin 9 (Vref/2): This is optional input pin. It is used only when the input signal range is small. When pin 9 is at 2V, the range is 0-4V, i.e. twice the voltage at pin 9.
Pin 6 (V+), Pin 7(V-): The actual input is the difference in voltages applied to these pins. The analogue input can range from 0 to 5V.
In this circuit, pins 1 and 2 are always made low, so the IC and the buses are always enabled. Pin 9 is made open, as we use analogue input with 0-5V range.
Pin 7 is grounded.
Pin 5 (INTR): This active-low pin indicates the end of conversion. It is connected to pin 17 (bit D3 of I/O port 37A) of ‘D’ connector. (Note that this bit is inverted.)
The start-of-conversion command via pin 16 of ‘D’ connector is applied to pin 3 of the ADC0804. Since we cannot read 8- bit digital data output from ADC through the 4-bit status port at a time, we divide it in two 4-bit parts and read. Hence the ADC data output is multiplexed through two 4-bit sections of octal buffers of IC1 (74244) with the help of output-enable signals
The software, written in C programming language, is user-friendly and easyto-understand. It gets data from the developed hardware circuit and displays it in the graphical screen with some changes.
The C program includes two user-defined functions with the main function: graphics( ) and settings( ). The settings( ) function is used to adjust the voltage and time scale. The graphics( ) function is used to display the waveform on the screen. The sample control signal is used to close the switch in the sample-and-hold circuit, so the capacitor charges towards the analogue input voltage. After the sampling is over, the switch is opened using the same signal. Then the start-of-conversion control signal is given to start the conversion. The sampling time is approximately 20 μs and the conversion time is approximately 100 μs.
A simple way to see if a fuse has blown without removing it from its holder.Its not often you can design a circuit using just two components, but with just one resistor and an LED this circuit provides visual indication of when a fuse has blown. LED 1 is normally not lit, being "short - circuited" by the fuse, F1. Note that the LED will only light under fault conditions, i.e. a short circuit or shunt on the load. In this case the supply current is reduced to a safe level by R1.
Manual stabilisers are still popular because of their simple construction, low cost, and high reliability due to the absence of any relays while covering a wide range of mains AC voltages compared to that handled by automatic voltage stabilisers. These are used mostly in homes and in business centres for loads such as lighting, TV, and fridge, voltage fluctuates between very low (during peak hours) and abnormally high (during non-peak hours). Some manual stabilisers available in the market incorporate the high-voltage auto-cut-off facility to turn off the load when the output voltage of manual stabiliser exceeds a certain preset high voltage limit. The output voltage may become high due to the rise in AC mains voltage or due to improper selection by the rotary switch on manual stabiliser.
This ring generator will ring a telephone once every 10 seconds. The interval between rings can be lengthened or shortened by varying the value of the 1 Meg resistor. The 70 volt/30 Hz ring voltage is produced from the 120 volt side of a small 12.6 V AC power transformer. Both capacitors connected across the transformer windings are non-polarized 100 volts. Circuit draws about 300mA from the 12 volt DC power supply during the ringing interval.
Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment. A non-polarized capacitor is placed in series with the transformer line connection to prevent DC current from flowing in the transformer winding which may prevent the line from returning to the on-hook state. The capacitor should have a voltage rating above the peak ring voltage of 90 volts plus the on-hook voltage of 48 volts, or 138 volts total. This was measured locally and may vary with location, a 400 volt or more rating is recommended. Audio level from the transformer is about 100 millivolts which can be connected to a high impedance amplifier or tape recorder input. The 3 transistor amplifier shown above can also be used. For over voltage protection, two diodes are connected across the transformer secondary to limit the audio signal to 700 millivolts peak during the ringing signal. The diodes can be most any silicon type (1N400X / 1N4148 / 1N914 or other). The 620 ohm resistor serves to reduce loading of the line if the output is connected to a very low impedance.
The circuit for a powerful AM transmitter using ceramic resonator/filter of 3.587 MHz is presented here. Resonators/filters of other frequencies such as 5.5 MHz, 7 MHz and 10.7 MHz may also be used. Use of different frequency filters/resonators will involve corresponding variation in the value of inductor used in the tank circuit of oscillator connected at the collector of transistor T1. The AF input for modulation is inserted in series with emitter of transistor T1 (and resistor R4) using a transistor radio type audio driver transformer as shown in the circuit.
Modulated RF output is developed across the tank circuit which can be tuned to resonance frequency of the filter/resonator with the help of gang condenser C7. The next two stages formed using low-noise RF transistors BF495 are, in fact, connected in parallel for amplification of modulated signal coupled from collector of transistor T1 to bases of transistors T2 and T3. The combined output from collectors of T2 and T3 is fed
to antenna via 100pF capacitor C4.
The circuit can be easily assembled on a general-purpose PCB. The range of the
In the circuit below, a quad voltage comparator (LM339) is used as a simple bar graph meter to indicate the charge condition of a 12 volt, lead acid battery. A 5 volt reference voltage is connected to each of the (+) inputs of the four comparators and the (-) inputs are connected to successive points along a voltage divider. The LEDs will illuminate when the voltage at the negative (-) input exceeds the reference voltage. Calibration can be done by adjusting the 2K potentiometer so that all four LEDs illuminate when the battery voltage is 12.7 volts, indicating full charge with no load on the battery. At 11.7 volts, the LEDs should be off indicating a dead battery. Each LED represents an approximate 25% change in charge condition or 300 millivolts, so that 3 LEDs indicate 75%, 2 LEDs indicate 50%, etc. The actual voltages will depend on temperature conditions and battery type, wet cell, gel cell etc.
This circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about 30 mA from a 6-9 volt battery and can be received to about 100 yards. A 555 timer is used to produce the tone (about 600 Hz) which frequency modulates a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna so that the antenna position and length has less effect on the frequency. Fine frequency adjustment can be made by adjusting the 200 ohm resistor in series with the battery.
Oscillator frequency is set by a 5 turn tapped inductor and 13 pF capacitor. The inductor was wound around a #8 X 32 bolt (about 3/16 diameter) and then removed by unscrewing the bolt. The inductor was then stretched to about a 3/8 inch length and tapped near the center. The oscillator frequency should come out somewhere near the center of the band (98 MHz) and can be shifted higher or lower by slightly expanding or compressing the inductor. A small signal diode (1N914 or 1N4148) is used as a varactor diode so that the total capacity in parallel with the inductor varies slightly at the audio rate thus causing the oscillator frequency to change at the audio rate (600 Hz). The ramping waveform at pins 2 and 6 of the timer is applied to the reversed biased diode through a large (1 Meg) resistor so that the capacitance of the diode changes as the ramping voltage changes thus altering the frequency of the tank circuit. Alternately, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator but it may require an additional pull up resistor to reverse bias the diode. The N channel JFET transistors used should be high frequency VHF or UHF types or similar.
Here is a low-cost teleconferencing system that lets you talk to two persons at a time in any part of the world over two telephone lines. The circuit makes use of a coupling transformer and some passive components.
The circuit is connected between the two telephone lines. It works like this: When ‘X’ calls ‘A’ on the first telephone line, ‘A’ puts this call on hold, dials ‘Y’ on the other telephone line (which is free) and keeps this call too on hold, and slides switches S1 and S2 to ‘on’ position. Now ‘X,’ ‘A’ and ‘Y’ can talk to one another simultaneously over the two telephone lines.
Nothing critical here. To get a bit of tuning out of the coil you could put a 4-40pF trimmer capacitor (optional) parallel over the 1 μH coil, L1. C1/C4 and C5/C6 are ceramic capacitors, preferably NPO (low noise) types. C2/C3 are electrolytic or can be tantalum types. The antenna is nothing more than a piece of 12" wire or a piece of piano wire from 6" to 12".
To find the signal on your receiver, make sure there is a signal coming into the microphone, otherwise the circuit won't work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. I'm sure you get the idea... Or you can just lightly tap the microphone while searching for the location of the signal on your receiver.
Parts List:
R1,R3 = 100K
R2 = 10K
R4 = 470 ohm
C1,C4 = 470pF
C2,C3 = 4.7μF, 16V, electrolytic
C5,C6 = 4.7pF
C7 = 4-40pF trimmer cap (optional, see text)
L1 = 1μH
Q1,Q2 = 2N2222, NPN transistor
Mic = Electret Microphone
B1 = 9 Volt, Alkaline battery
Most of the telephone security devices available in market are simple but quite expensive. These devices provide blinking or beeping type line-tap/misuse indications. Quite often they do not offer guaranteed protection against unauthorized operation. A very simple and unique circuit of a telephone watch-dog to safeguard subscriber telephone lines against any fraud is described here. This little circuit keeps continuous watch over the telephone lines and sounds an alarm in case of any misuse. In addition it transmits a loud tone through the telephone lines to prevent further misuse. When switch S1 is turned on, the normal (on-hook) telephone line voltage at the output of bridge-rectifier diodes D1 to D4 is approximately 48 volts, which being well above the break-down voltage of zener diode D5, the diode conducts. As a result transistor T2 gets forward biased. This effectively grounds the base of transistor T1 which is thus cut off and the remaining circuit does not get any power supply. In this state, only a small (negligible) current is taken by the circuit, which will not affect the telephone line condition. However, when handset of any telephone connected to the telephone lines is lifted (off-hook), line voltage suddenly drops to about 10 volts. As a result, transistor T2 is switched off and transistor T1 gets forward biased via resistor R1. Now, the astablemultivibrator built around timer IC1 starts oscillating and the speaker starts sounding. Output of the astablemultivibrator is also connected to the base of transistor T1 through capacitor C5. As a result, only a loud (and irritating) tone is heard in the ear-piece of the unauthorized telephone instrument. This circuit can be constructed on a veroboard using easily available low cost components and it can be connected to any telephone line without the fear of malfunctioning. No extra power supply is required as it draws power from the telephone line for operation. Note: Please disconnect the gadget when you are yourself using the telephone as it cannot distinguish between authorized and unauthorized operation.
The circuit shown here for clap operated switch is inexpensive and easy to assemble by hobbyists of any level. Clap sound signals picked up by condenser microphone are first amplified by transistor T1, which is a simple common-emitter amplifier. Amplified signals are rectified by diode D1. Positive half cycle of clap signal is applied to a 3-stage DC amplifier formed by transistors T2, T3 and T4. Output from transistor T4 is used as a clock for flip-flop IC1 (7472). For each clap, the output of IC 7472 toggles alternately to on/off state. Transistor T5 functions as relay driver since output level of IC1 is insufficient to drive a relay directly. The entire circuit, except the relay driver, operates at 5 volts regulated.
This is basically a Schmitt Trigger circuit which receives input from a photo cell and controls a relay that can be used to switch off and on a street lamp at dawn and dusk. I have built the circuit with a 120 ohm/12volt relay and monitored performance using a lamp dimmer, but did not connect the relay to an outside light
The photocell should be mounted above the light on top of a reflector and pointed upward at the sky so the lamp light does not strike the photo cell and switch off the lamp
The switching points are about 8 volts and 4 volts using the resistor values shown but could be brought closer together by using a lower value for the 7.5K resistor. 3.3K would move the levels to about 3.5 and 5.5 for a range of 2 volts instead of 4 so the relay turns on and off closer to the same ambient light level. The potentiometer would need to be readjusted so that the voltage is around 4.5 at the desired ambient condition.
Circuit description:
This circuit is a simple IR detector for testing IR remote controllers. The circuit is based on one phototransistor which receives the IR beam. The NPN transistor works as an amplifier which feeds current to the led. When this circuit detects IR or light, the LED is on. So you need to shield the phototransistor from ambient light if you don't want to do your tests in the dark. The best way is to fit the phototransistor in a small black tube.
Component list:
Q1 BP109 (or similar phototransitor)
Q2 BC238C or BC547
D1 RED LED
R1 390 ohms 0.25W
The circuit below responds to sound pressure levels from about 60 to 70 dB. The sound is picked up by an 8 ohm speaker, amplified by a transistor stage and one LM324 op-amp section. You can also use a dynamic microphone but I found the speaker was more sensitive. The remaining 3 sections of the LM324 quad op-amp are used as voltage comparators and drive 3 indicator LEDs or incandescents which are spaced about 3dB apart. An additional transistor is needed for incandescent lights as shown with the lower lamp. I used 12 volt, 50mA lamps. Each light represents about a 3dB change in sound level so that when all 3 lights are on, the sound level is about 4 times greater than the level needed to light one lamp. The sensitivity can be adjusted with the 500K pot so that one lamp comes on with a reference sound level. The other two lamps will then indicate about a 2X and 4X increase in volume.
The 2N3392 transistor is a low-noise type in a TO-92 housing and can be replaced by a NTE199 or ECG199. Potentiometer R5 of 100K is a linear type with an on/off switch attached. The value of C1 may need to be between 0.05μF and 0.1μF (47nF/100nF). Experiment with the value for best performance.
Here is a really simple game show timer designed with the beginner in
mind! The power source is an ordinary 12 volt lantern battery or battery
pack made up of C or D cells. The lamps may be ordinary flashlight bulbs;
the prototype uses 750 mA Krypton types (KPR112) with wires soldered
directly onto the bulbs. The control devices may be just about any nonsensitive
gate SCRs or triacs with a current rating of a few amps. SCRs are
shown and triacs may be substituted by connecting MT1 to the negative
battery terminal and MT2 to the lamps. In fact, the two types of devices
could probably be mixed if that is what the junk box has to offer! The
zener is an ordinary 9 volt, 1 watt type like the 1N4739. The 100 ohm resistor
may be any type with a rating of 1/8 watt or more. The reset/power
button is an ordinary toggle switch and the pushbuttons are any
convenient normally-open type.
The entire circuit may be built into one box with two wires running out to
each contestant's pushbutton switch or the SCR, lamp, and switch may be
in a remote box for each contestant with three wires running back to the
base unit. Many more than three contestants may be added by repeating
the pattern.
The brake lights of the automobile trigger this circuit on and off. This save the annoyance of the alarm when it is not needed.
Mobile phone chargers available in the market are quite expensive.The circuit presented here comes
as a low-cost alternative to charge mobile telephones/battery packs with a rating of 7.2 volts, such as Nokia 6110/6150.
The 220-240V AC mains supply is downconverted to 9V AC by transformer X1. The transformer output is rectified by diodes D1 through D4 wired
in bridge configuration and the positive DC supply is directly connected to the charger’s output contact, while the negative terminal is connected through current limiting
resistor, R2.
LED2 works as a power indicator with resistor R1 serving as the current limiter and LED3 indicates the charging status.During the charging period, about 3 volts
drop occurs across resistor R2, which turns on LED3 through resistor R3.
An external DC supply source (for instance, from a vehicle battery) can also be used to energise the charger, where resistor R4, after polarity protection diode D5, limits the input current to a safe value.The 3-terminal positive voltage regulator LM7806 (IC1) provides a constant voltage output of 7.8V DC since LED1 connected between the common terminal (pin 2) and ground rail of IC1 raises the output voltage to 7.8V DC. LED1 also serves as a power indicator for the external DC supply.
After constructing the circuit on a veroboard, enclose it in a suitable cabinet.A small heat sink is recommended for IC1.
Here’s a circuit to display the song number in an audio system for quick reference to songs. It also serves the purpose of an extra visual indicator in modern audio systems
When the power is switched on, the power-on-reset circuit comprising 3.3k resistor R20 and 1μF, 25V capacitor C6 resets the counters, showing ‘00’ in the display. One can also reset the display to zero at any time by pressing reset switch, S1.
A touch switch is a switch that is turned on and off by touching a wire contact, instead of flicking a lever like a regular switch. Touch switches have no mechanical parts to wear out, so they last a lot longer than regular switches. Touch switches can be used in places where regular switches would not last, such as wet or very dusty areas.
Parts List:
C1 - 10uF 16V Electrolytic Capacitor
R1, R2 2 - 100K 1/4 Watt Resistor
R3 - 10 Meg 1/4 Watt Resistor
U1 - 4011 CMOS NAND Gate IC
MISC - Board, Wire, Socket For U1
Notes:
1. The contacts can be made with just two loops of wire close together, or two squares etched close together on a PC board.
2. When activated, the output of the circuit goes high for about one second. This pulse can be used to drive a relay, transistor, other logic, etc.
3. You can vary the length of the output pulse by using a smaller or larger capacito for C1.
