SCR Based SSS Solar Charge Control

A force-commutated SCR makes a novel Solid State Switch in a solar charge regulator control. Prior art includes relay and transistor switches, but an SCR switch in this type of DC application may be new to the world (normally SCRs are applied in line-commutated AC applications). Advantages include robustness and the requirement of only one conductive device that performs (3) functions: switch, latch and reverse polarity diode (reverse blocking thyristor). The main SCR conducts the charging current from the solar panel to the battery while a 2nd commutation SCR performs the function of commutating (turning off) the main SCR current at the end of the conduction period. All circuitry consists of readily available discrete components. Note that this is old technology and may not be the best or cheapest solar charge control solution, but it makes a great, mind-expanding tech school lab experiment.

This circuit is applied in fashion similar to

Secure Power Switch Circuit

Most of the diagnostic, treatment, and life support devices used in healthcare require regular power supply in order to function properly. Accidentally turning off a medical device can have serious consequences. How can you prevent an accidental or unexpected equipment shutdown?

Here is a little circuit to prevent such costly disasters. It is nothing but a digital toggle switch with an additional switching protection logic. Here two momentary push button switches (S1 &S2) are used to control the switching of the output load, ie one switch is for normal power swtching & other switch is for enabling the

Low Power Ozone Generator

Source:-http://www.electroschematics.com/10859/low-intensity-ozone-generator/

Generating ozone and negative ions in the air can be done by a corona discharge. For this we can use a sharp tip of negative polarity, acting against a flat metal part of positive or neutral potential. The device generates negative air ions and ozone. The ozone acts against rot and mould, and can refresh the unhealthy air of humid cellars or whatsoever air charged with bacteria, fungus or bad smell.

A cascade, generating a high DC voltage from grid voltage, is simple, silent and has a far higher efficiency than the typical flyback converters. Here is what we call in Germany a Greinacher- rectifier. In the English-speaking world the schematic is rather called a Cockroft-Walton multiplier, probably independently invented.

Schematic of the ozone generator circuit

The capacitors were put on the upper side and the diodes on the underside of perforated boards without copper islands, just the pure plastic board. In the negative branch are 15 Capacitors 68 nF 630V (0.068uF), in

3V to 24V Variable Power Supply

This 3V to 24 volt variable-regulated power supply can be adjusted from 3 to 25 volts and is current limited to 2 amps as shown, but may be increased to 3 amps or more by selecting a smaller current sense resistor (0.3 ohm). The 2N3055 and 2N3053 transistors should be mounted on suitable heat sinks and the current sense resistor should be rated at 3 watts or more.

Voltage regulation is controlled by 1/2 of a 1558 or 1458 op-amp. The 1458 may be substituted in the circuit below, but it is recommended the supply voltage to pin 8 be limited to 30 VDC, which can be accomplished by adding a 6.2 volt zener or 5.1 K resistor in series with pin 8. The maximum DC supply voltage for the 1458 and 1558 is 36 and 44 respectively. The power transformer should be capable of the desired current while maintaining an input voltage at least 4 volts higher than the desired output, but not exceeding the maximum supply voltage of the op-amp under minimal load conditions.
The power transformer shown is a center tapped 25.2 volt AC / 2 amp unit that will provide regulated outputs of 24 volts at 0.7 amps, 15 volts at 2 amps, or 6 volts at 3 amps. The 3 amp output is obtained using the center tap of the transformer with the switch in the 18 volt position. All components should be available at Radio Shack with the exception of the 1558 op-amp.

Variable Power Supply Circuit Diagram

Variable Power Supply with 78XX regulator

This variable power supply is using 7805, 7809, 7812 or 7815 voltage regulators, where the last 2 digits represents the maximum output voltage of the IC.
This circuit offers excellent ripple rejection, eliminates mains hum, and has a design using a pi filtered C-L-C.

A core should be chosen to work within the specific frequency as stated by the manufacturer. L1 is a powder core and has 32 turns of 0.75mm wire.

Variable Power Supply Circuit Diagram

The transformer has a 240V primary and has a secondary rated 24V at 2A. The bridge rectifier contains 4 diodes, their current rating needs to be high with respect to the transformers output current; if not the current may damage the diodes. C1 is the mainfiltering capacitor, the supply is further smoothed by the combination of L1 and C3. C2 and C4 are decoupling capacitors; their action further reduce ripple factor.

The regulator 78xxr,

High Power Ozone Generator

Here is a more powerful ionisator and ozone generator than described in my version-2. It is meant for continuous operation in problem spaces such as cellars with some rot & mould. It can also be operated in normal living rooms when you are not at home. But it is too aggressive for your lungs, if used continuously in inhabited spaces. It is not powerful enough to carry out short disinfection actions, for example after a burning frying pan has put your kitchen under smoke.

The transformer is from an old TV set, which still used a single rectifier instead of a tripler or even a diode-split rectifier. The latter ones you cannot use because they output positive polarity against ground, and it is very difficult to “turn around”. Here the rectifier diode was inverted to generate “Minus” at the output.

Schematic of the Ozone Generator Circuit

I have employed lacquered paper layers between

Numeric Water Level Indicator Circuit

Most water level indicators for water tanks are based upon the number of LEDs that glow to indicate the corresponding level of water in the container. Here we present a digital version of the water level indicator. It uses a 7-segment display to show the water level in numeric form from 0 to 9.

The numeric water indicator circuit works off 5V regulated power supply. It is built around priority encoder IC 74HC147 (IC1), BCD-to-7-segment decoder IC CD4511 (IC2), 7-segment display LTS543 (DIS1) and a few discrete components.

When the water tank is empty, all the inputs of IC1 remain high. As a result, its output also remains high, making all the inputs of IC2 low. Display LTS543 at this stage shows ‘0,’ which means the tank is empty. Similarly, when the water level reaches L-1 position, the display shows ‘1,’ and when the water level reaches L-8 position, the display shows ‘8.’ Finally, when the tank is full, all the inputs of IC1 become low and its output goes low to make all the inputs of IC2 high. Display LTS543 now shows ‘9,’ which means the tank is full.

Water Level Indicator Circuit Schematic

CD4511 datasheet

74147 datasheet

Assemble the water level indicator circuit on a general-purpose PCB and enclose in a box. Mount 7-segment LTS543 on the front panel of the box. For sensors L-1 though L-9 and ground, use corrosion free conductive-metal (stainless-steel) strips.

Source:-http://www.electroschematics.com/5655/numeric-water-level-indicator/

Water Level Indicator Circuit

This simple water level indicator will activate an active buzzer (e.g. BeStar BPT-23CX ) in order to make a noise when a certain level of water is being reached. Because the water sensor and the command circuit are located on the same printed circuit board, indicator, together with its 9 V battery and the buzzer can be mounted in a compact case. Obviously the sensor, that is made by corossion, on the board, must not be mounted directly on iron or steel bathtubs but with a magnet atached on the case.

Schematic of the Water Level Indicator Circuit

BC548 datasheet

Video:-https://youtu.be/pQYDXS7iKV0

Check out the new water level sensor circuit.

In order to

Temperature Controlled DC Fan

This circuit can cool your heat generating electronic devices by operating a DC fan when the temperature in its vicinity increases above the preset level. Its operation is fully automatic and turns off when the temperature returns normal. It uses a small 12V DC brush less fan used in computers.

Schematic of DC Fan Controlled by Temperature Circuit

Note by P Marian: this is an updated version of the old circuit designed by D Mohankumar that didn’t function at

Static Reversing the 3 Phase Induction Motor

This solid-state thyristor (SCR) switch circuit is perhaps the simplest means of reversing a 3 phase motor. “Static” is a catch-all term that essentially means without moving (mechanical) contacts –the traditional means of reversing is via a pair of contactors that swap two of the three AC lines. However, there are disadvantages to contactors as they are expensive, and have a finite life in repetitive reversing applications. Note that this circuit does not provide speed control as the motor runs at its base speed, nor does it provide zero voltage switching.

Static Reversing 3 Phase SSS Schematic

Gate Driver Control Schematic

How it works

The SCRs that are to be conducting are given a repetitive gate pulse train that both turns them on and keeps them conducting. The gate pulses are generated via a 555 oscillator and isolated via a 4secondary pulse transformer. There

20W FM RF Amplifier Circuit

This FM RF power amplifier has 2 transistors from Philips: BLV10 and BLW87. The 2 rf transistors work in class C and this fm amplifier has a total gain of 21dB (100x) and 55 – 65% efficiency. One low pass filter with 9 components ensures 60dB rejection of the the second harmonic. There is no need for tune-up to cover the whole 88-108 MHz FM band.

20W RF FM amplifier circuit diagram

Source:- http://www.electroschematics.com/18/rf-fm-88-108-mhz-20w-amplifier-blv10-blv87/

FM Radio Receiver

This simple fm radio receiver circuit consists of a regenerative rf stage, TR1, followed by a two of three-stage audio amplifier, TR2 to TR4. In some areas 3 stages of audio amplification may not be necessary, in which case TR3 and its associated components can be omitted and the free end of capacitor C5 connected to the collector of TR2.

Radio Receiver Circuit Diagram

The critical part of the fm radio receiver is the first stage, TR1/VC1, where the wirings must be kept as short as possible. Coil L1 is formed by winding 8 turns of 1mm (20 swg) enamelled copper wire on a 6 mm diameter former, which is then removed. After that L1 should be stretched carefully and evenly to a length of about 13mm.

Transistors List
TR1 = BF199
TR2 = TR3 = TR4 = BC547

Video presentation and photos of the working radio receiver

The tunning capacitor VC1 is one of the two fm sections of a miniature fm transistor radio with built-in trimmers (VC2). The “earthy” end (moving vanes and spindle) is connected to the 22pF capacitor C1. The value of the rf choke L2 is not critical, anything from 1µH to 10µH being suitable.

The output is suitable for ordinary earphones connected in series to provide an impedance of 64Ω.

Tuning-in the fm radio receiver

To operate the radio receiver, potentiometer VR1 must first be advanced slowly (towards the end of the track connected to battery positive) until, at about the half-way point, a sudden slight increase in background noise will be heard, indicating the onset of oscillation. It then should be backed off, very slowly, until oscillation just stops; it then should be possible to tune in some stations.

The correct frequency range of 87 MHz to 108 MHz can be obtained by adjusting VC2 at the high frequency (108 MHz) and slightly stretching or squeezing together the turns of coil L1 at the end (87 MHz).

RF Based Wireless Remote Control System (Tested)

It is often required to switch electrical appliances from a distance without being a direct line of sight between the transmitter and receiver. As you may well know, an RF based wireless remote control system (RF Transmitter & RF Receiver) can be used to control an output load from a remote place. RF transmitter, as the name suggests, uses radio frequency to send the signals at a particular frequency and a baud rate.

The RF receiver can receive these signals only if it is configured for the pre-defined signal/data pattern. An ideal solution for this application is provided by compact transmitter and receiver modules, which operate at a frequency of 434 MHz and are available ready-made. Here, the radio frequency (RF) transmission system employs Amplitude Shift Keying (ASK) with transmitter (and receiver) operating at 434 MHz. The use of the ready-made RF module simplifies the construction of a wireless remote control system and also makes it more reliable.

RF Transmitter

This simple RF transmitter, consisting of a 434MHz license-exempt Transmitter module and an encoder IC , was designed to remotely switch simple appliances on and off. The RF part consists of a standard 434MHz transmitter module, which works at a frequency of 433.92 MHz and has a range of about 400m according to the manufacture. The transmitter module has four pins. Apart from “Data” and the “Vcc” pin, there is a common ground (GND) for data and supply. Last is the RF output (ANT) pin.

Pin Assignment of the 434MHz Transmitter module

Note that, for the transmission of a unique signal, an encoder is crucial. For this, I have used the renowned encoder IC HT12E from Holtek. HT12E is capable of encoding information which consists of N address bits and 12N data bits. Each address/ data input can be set to one of the two logic states. The programmed addresses/data are transmitted together with the header bits via an RF transmission medium upon receipt of a trigger signal. Solder bridges TJ1 and TJ2 are used to set the address and data bits.

The current consumption with a supply voltage of near 5.4V is about 10 mA. Since the current consumption is very little,the power can also be provided by standard button cells. Recommended antenna length is 17 cm for 433.92 MHz, and a stiff wire can be used as the antenna. Remember to mount the antenna (aerial) as close as possible to pin 4 (ANT) of the transmitter module.

RF Transmitter – Schematic Diagram

RF Receiver

This circuit complements the RF transmitter built aorund the small 434MHz transmitter module. The receiver picks up the transmitted signals using the 434Mhz receiver module. This integrated RF receiver module has been tuned to a frequency of 433.92MHz,exactly same as for the RF transmitter.

434MHz receiver module

The miniature 434MHz RF receiver module receives On-Off Keyed (OOK) modulation signal and demodulates it to digital signal for the next decoder stage. Local oscillator is made of Phase Locked Loop (PLL) structure. Technically, this is an Amplitude Shift Keying (ASK) receiver module based on a single-conversion, super-heterodyne receiver architecture and incorporates an entire Phase-Locked Loop (PLL) for precise local oscillator (LO) generation. It can use in OOK / HCS / PWM modulation signal and demodulate to digital signal.

The receiver module has eight (4+4) pins. Apart from three “ground (GND) ” and two “Vcc” pins, there are two pins (one for Digital Data & other for Linear Data) for data output. Last is the RF input (ANT) pin.

Pin Assignment of the 434MHz Receiver module

Pin Connections

• 1 Antenna
• 2 Ground
• 3 Ground
• 4 Vcc
• 5 Vcc
• 6 Linear Data (Normally NOT used)
• 7 Digital Data (Normally Used)
• 8 Ground

The “coded” signal transmitted by the transmitter is processed at the receiver side by the decoder IC HT12F from Holtek. VR1 and R1 are used to tweak the oscillator frequency of the decoder to that of the transmitter. Any possible variations due to component tolerences and/or a different supply voltage can be compensated by this arrangement. HT12F is capable of decoding informations that consist of N bits of address and 12N bits of data. HT12F decoder IC receives serial addresses and data from the HT12E encoder that are transmitted by the RF transmitter module. HT12D compare the serial input data three times continuously with the local addresses.

If no error or unmatched codes are found, the input data codes are decoded and then transferred to the output pins. The “Valid Transmission” (VT) pin also goes high to indicate a valid transmission.

For proper operation, a pair of HT12E/HT12F ICs with the same number of addresses and data format should be chosen. The data bits are set up using solder bridges RJ1 and RJ2. Output of the decoder is brought out on a pinheader K1 , making the logical signal available to circuits that need it. This output is also fed to the relay driver transitor T1. The RF Receiver circuit can be powered from a standard 5VDC supply. Just as for the RF Transmiitter, the aerial (17 cm for 433.92 MHz) has to be mounted as close as possible to the RF IN (ANT) pin of the 434MHz RF receiver module.

RF Receiver – Schematic Diagram

Notes

• RF transmitter circuit can be safely powered from DC 4.5V to 6V power supply. Here, diode D1 is added to introduce a 0.65V drop, but this is not very crucial
• The 434 MHz RF module (Tx &Rx) is available from many sources. Connection terminals are usually labelled on the PCB. In case of any doubt, refer datasheets of the RF modules
• In practice, the transmitter is usually powered by batteries. But you can power the receiver from an onboard /external dc supply too. In this case, make an optional “noise filter” arrangement , as shown here, to “clean” the 5V power rails of the 434MHz RF receiver module

optional “noise filter” circuit

Source:-

Simple Sound Processor Circuit

Simple Sound Processor presented here is an ultra-simple circuit module suitable for almost all basic electronics and microcontroller projects. For example, this module can be used as a front-end in an electronics project to detect and process ambient sound in an efficient manner. Even less-experienced hobbyists will find the construction of the circuit on a general purpose printed circuit board a fairly easy affair, since there is no perplexed wiring and all components are housed on the board. As the circuit has no critical adjustment points it is immediately ready for real-world applications.

The acoustic sensor in the circuit is a standard omni-directional foil electret microphone (MIC). Sound signals picked up by this microphone is processed by two BC547B transistors (T1 and T2). Pre-processed signals from this section is linked to the monoshot brick realized using the popular NE555N chip (IC1). The whole module can be powered from any standard 5 volt dc supply. Pins 1 and 2 of the 3-pin JST connector (J1) denotes VCC (+5V) and GND (0V) connections respectively, while the final 3rd pin works as the digital signal output (DO) pin.

555 datasheet

BU4S584G2 datasheet

Final component in the circuit – IC2 – is a single chip encapsulated in an SSOP5 package BU4S584G2 which is a single Schmitt trigger inverter, added here to ensure high degree of noise tolerance. However, this is not a critical component, so you can replace it with direct equivalents, if available. Pin description and input/ouput table of BU4S584G2 is shown below:

Working of the sound processor circuit is very simple and straight forward. The circuit captures incoming sound signals through the microphone. When the circuit detects a sound activity it generates a digital/logic-level (active-low state) output for a pre- settled duration fixed by the 10K pot (P1). This output can be given to almost all micro controllers for further processing. Although this little circuit module is not intelligent to separate different sounds, it is very useful in making general/microcontroller/robotic projects integrated with sound activation features. As an example, consider the possibility of building a security camera system which takes pictures only when triggered by an audible acoustic wave frequency.

Lab Note:

If you are looking for some other simple solution, you can replace the microphone electronics with a single-transistor circuit as shown below:

Prototype tested with both front-end circuits, but a drop-off in sound detection sensitivity was noticed with the second option!

Source:- http://www.electroschematics.com/12327/simple-sound-processor-circuit/

Night Security Light with Hacked PIR sensor

This circuit is copied from:- http://www.electroschematics.com/12414/night-security-light-with-hacked-pir-sensor/

Security lighting is widely used as a preventive measure against intrusions on a physical piece of property. Since adequate lighting around a physical structure reduces the risk of an intrusion, security lighting can be used in residential settings to increase the feeling of safety. Presented here is a trusty security light activated by a passive infrared sensor, turning on only when a person (or other mammal) approaches. This battery operated security light provides rich illumination without harsh brightness great for kids rooms, the elderly, and for comforting your pets. An important feature of this security light is that it wakes up only at nighttime!

Described security light circuit design is centered around an inexpensive passive infrared (PIR) motion detector module, type HC-SR501. The PIR motion detector module is used here with an added ambient light detection (day/night) feature, supported by a generic photodiode. Following part is prepared to help you get started with your new HC-SR501 passive infrared motion detector module, so let’s start.

Note that imparting the ambient light detection feature involves skillfully combining one photosensitive component and the motion detector electronics with a great deal of dexterity. First of all remove the fresnel lens, and search for the “LDR” pads located near the PIR sensor element. Next, attach a 5mm/3mm (generic) silicon photodiode with its anode (A) connected to pad-A, and cathode (K) connected to pad-K. Finally, add a

10 OUTPUT LED SEQUENCER

Here is 10 output LED sequencer. After the last LED is illuminated, the circuit is reset. This circuit is build around readily available, low cost components - a 555 and decade counter CD4O1 7. The timer IC NE555 is wired as an astable multivibrator that produces 6Hz clock at its output pin 3. The 4017 is a CMOS decade counter with 10 outputs. Inputs include a CLOCK (Pin 1 4), a RESET (Pin 15), and a CLOCK INHIBIT (Pin 13). The clock input connects to a Schmitt trigger for pulse shaping and allows slow clock rise and fall times (not needed in our case). The counter advances one output at the rising edge of the clock signal if the CLOCK INHIBIT line is low. A high RESET signal resets the counter to the zero output. The circuit may be configured for counts less than 10 by connecting RESET to an output pin (one after the desired count). Thus, a five stage sequencer can be made by connecting pin 15 to pin 1. A CARRY-OUT signal (pin 12) can be used to clock subsequent stages in a multi-device counting chain. The output from 1C2 pin 3 is connected to clock pin (pin 14) of the IC3 for sequencing operations. NPN transistors Q1- Q10 are used to increase the output current for the LEDs which is set by the common 150 ohm resistor. In the circuit, only one of the outputs is HIGH at any one time and the output advances by one count with every clock pulse. But the circuit above is poorly designed. It does not need the voltage regulator as both chips can work up to 15v. The 4017 can supply 10mA to a LED on a 12v supply so that none of the transistors are needed. The circuit below shows the necessary components. The secret to designing a circuit is to look at the final design and ask: "is this component necessary?" Try removing a component and see if the circuit still works. Keep doing this with all the components. The circuit above was published in an Indian magazine with over 1,000,000 readers. The faults were obvious. How these faults passed an editorial committee is beyond me.  They are showing very poor design-leadership in allowing this oversight to be published. The faults are technical but are obvious to anyone who has constructed the circuit and experimented with it. Obviously the circuit has never been assembled with anyone with technical expertise.

BENCH POWER SUPPLY

Here is a regulated power supply for your bench. The 100n capacitors are needed across the input and output of the regulator IC's to prevent high-frequency instability.   The transformer is only 500mA so the maximum you can deliver from the power supply is 300mA TOTAL.

 

HOME ALARM-1

Here is a Home Alarm using 555 IC's.The Home Alarm-1 circuit above can be simplified by using a single 74C14 IC. This IC is also known by the following numbers: 40106, 40014, and 74HC14.  These are CMOS chips and are characterised by low current consumption, high input impedance and a supply voltage from 5v to 15v. (Do not substitute 7414 or  74LS14. They are TTL chips and operate on 4.5v to 5.5v and have low impedance inputs.)
The 74C14 contains 6 Schmitt Trigger gates and

MOVING LEDs

Here is another disastrous circuit. As each output goes high it pulls the previous output high to turn on two, three, four LEDs etc.
But any output that is not high is PULLED LOW by the chip and this circuit is pulling the outputs HIGH against the drivers inside the chip. This could lead to failure and certainly will heat up the chip.
This circuit is a bad design and is not recommended. 

ULTRASONIC REMOTE CONTROL

Here is a low cost, wireless switch controller. It uses ultrasonic sound waves for remote control of a switch.
As with any other remote control, the system basically comprises a transmitter and a receiver circuit. Frequencies up to 20kHz are audible. Frequencies above 20kHz are  not audible. The transmitter circuit generates an ultrasonic frequency between 40-50kHz. The receiver senses the ultrasonic sound and switches on a relay.
The transmitter uses a 555 astable multivibrator. It oscillates at a frequency of 40-50kHz. An ultrasonic transducer is used to transmit the frequency. The transmitter runs on a 9v battery. The ultrasonic receiver uses

LIGHTENING DETECTOR

Clouds can carry such huge electric charges that may to cause lightning flashes of thousands of volts. It is really a fascinating phenomenon.
When a lightning flash takes place a broad spectrum of radio-frequencies is generated. In this broad spectrum there is special intense emissions of the VLF (Very Low Frequency) band. This project will allow you to build a receiver to pick up a band near 300 KHz. An LED will flash to indicate the lightning flashes.THE CIRCUIT
The radio-signal generated by the lightning flash is picked up by the telescopic antenna with the help of a 10mH choke. The choke L1 resonates with the antenna and allows current to flow into the receiver circuit.
The L2 of 330uH in parallel with the 680pF capacitor C1 forms a tuned circuit for 300KHz. This parallel-tuned tank circuit is coupled to the base of Q1 via D2. The amplified radio signal is again coupled into the base of Q2. Transistors Q2 and Q3 form an LED flasher circuit. Transistor Q4 is the LED driver.
The flasher is biased so that when VR1 is carefully adjusted the LED flashes only when a radio burst appears at the input due to a lightning flash.
Positive feedback ensures the LED to be full on. The circuit quickly resets by charging C4 capacitor through diode D1.
The circuit draws only about 100uA in idle state. Therefore it can run on two cells for many hours.
 

ELECTRONIC SIREN

The 741 is a versatile chip and it can be used in the design of a wide variety of sound-effect generators. This circuit produces a siren that can be used in conjunction with other circuits. You can also use an LM358 dual op-amp chip.
The operation of the op-amp was not discussed correctly in the original article, so a full explanation has been provided:
The principle of an op-amp is to provide a very high gain. This means a small change in either input produces an almost full rail swing on the output.
The circuit starts to work like this.
As soon as you put a slight voltage on the "+" input, the output goes full HIGH.
The two 100k resistors on the "+" makes the output go full HIGH.
Now we connect a resistor from the output to "+" and this makes no difference. The output remains full HIGH.
Now we put a resistor from output to "-."
If the "-" input is slightly higher than "+" the output goes LOW. This is what happens. The output voltage drops until the "-" input is slightly lower than the "+" input and that's why the output falls until its voltage is equal to the "+" input.
Now we connect a capacitor to the "-" input.
It does not matter if we add the capacitor later or turn the circuit on with the capacitor fitted.
The voltage on the "-" input will be lower than the "+" input and this will start the circuit oscillating.
This is how it oscillates:
Because the "-" input is lower than the "+" input, the output rises towards the positive rail and this begins to charge the capacitor.
The voltage on the "-" input can rise higher than the "+" input and when it is about 15mV higher, the output drops towards the 0v rail.
This reduces the voltage on the "+"input and the capacitor has to discharge a considerable amount before it is lower than the "+" rail. (Actually before the "+" input is higher than the "-" input).
The voltage on the "+" input is rising and falling by about 30% of rail voltage and this is the amount the capacitor has to charge and discharge for the circuit to work. 

 

LIGHT CONTROLLED LAMP

Here is a circuit of light controlled lamp. This is basically a Schmitt Trigger which receives input from a cadmium sulfide photo cell and controls a relay that can be used to switch a lamp on and off at dawn and dusk. The photo cell should be shielded from the lamp to prevent feedback so the lamp light does not strike the photo cell and switch off the lamp.
The photo cell is wired in series with a potentiometer VR1, so the voltage at the base of transistor Q1 can be adjusted to about half the supply, at the desired ambient light level. The two PNP transistors are connected with a common emitter resistor to produce a gap between the on and off voltages - called the HYSTERESIS GAP.
Under dark conditions, the photo cell resistance will be high producing a voltage on the base of Q1 that is higher than the base voltage on Q2. This causes Q2 to conduct and activate the relay.
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 R3. A value of 3k3 would move the levels to about 3.5v and 5.5v. 

CAPACITANCE BEEPER

Here is a circuit of capacitance beeper which uses a two-transistor flasher in conjunction with a Darlington transistor. When the probe is touched to a capacitor, the project beeps at a frequency that varies with capacitance. The frequency change is so noticeable that small capacitors can be precisely matched or an exact fixed value can be selected to replace a trimmer in a prototype.
When the beeper is properly adjusted it draws only 10uA with nothing touching the probe excluding the LED current. This design is optimized for capacitors less than about 100n. Large capacitors give a low frequency clicking sound and small capacitors sound a tone that increases as the capacitance decreases. Many decades of frequency change occur over the beepers range.
The probe should be built into a metal box so that one hand makes good
contact to 0v.
The resistor values are selected to barely turn on the transistors to conserve battery power. The transistors must have very high gain and good low current properties. The MPS-A18 is a very high gain transistor with excellent gain at very low currents. The capacitors are not particularly critical but the trimmer might require a little care. The trimmer is adjusted until the beeping just stops and only a very weak squeal is heard when a 2.2p is touched to the probe.

BATTERY MONITOR

This circuit makes it possible to monitor the charging process of a battery. After constructing the circuit, final adjustments are simple and the only thing needed is a digital voltmeter for the necessary accuracy. Connect an input voltage of 12.65 volt between the positive and negative connections of the circuit and adjust the VR1 (10K trimmer) until Led 10 lights up. Lower the voltage and in sequence all other LEDs will light up. Check that Led 1 lights up at approximately 11.85 volts. At 12.65 volt and higher the battery is fully charged, and at 11.85 is considered to be at its lowest state. LED 8, 9 and 10 indicate the battery capacity is more than 50%, LED 4 to LED 7 indicate a capacity of 30% - 50% and LED 1 to LED 3 indicate less that 30%. This circuit, with the components shown, uses less than 10mA. Of course you can adapt it to your own needs by making small modifications. This circuit is set for DOT mode, meaning only one LED at a time will be lit. If you wish to use the BAR mode,  connect pin 9 to the positive supply rail, but obviously with increased current consumption. The LED brightness can be adjusted by choosing a different value for the 5k6 resistor connected at pin 6 and 7. The diode 1N4007 was included to protect the circuit from a wrong polarity connection. 

INCREASING THE INPUT IMPEDANCE

The input impedance of a low cost analogue multimeter can be improved using this circuit. The approximate impedance increase will be about 250. 
The LED provides a fixed reference voltage for zero setting of the multimeter via VR1.

EMERGENCY LIGHT

Here is a circuit of an emergency light. As long as the power supply is present, transistor Q1 conducts. Since the base of the transistor Q2 is connected to the collector of Q1, transistor Q2 and Q3 do not conduct and hence the lamp remains off. LED glows as long as the supply is present.
When the power supply fails, the base drive to Q1 disappears. Thus Q1 stops conducting and its collector voltage jumps to battery voltage and starts conducting, switching on the lamp instantly. The load current is supplied by the battery. Whenever the power supply is restored, Q1 starts conducting turning Q2 & Q3 off and the lamp is switched off. Transistor Q2 conducts and provides sufficient base drive to transistor Q3.
The circuit above is too complex. The first diode is not needed and the rest of the circuit can be re-arranged. The 2R2 will overcharge the battery and dry it out in a few months.
It can be simplified to this:

The 100R gives 40mA charging with a 12v battery and 12v DC plug pack.

DOOR WATCHER

Three reed switches are at the heart of the circuit, one fitted to each door. They close when a door is opened. An associated LED lights when a door is opened.
The remainder of the circuit is powered by either D1, D2 or D3. However the 555 is not enabled until pin 4 goes high and this requires the output of either IC1a or IC1b to go high. In turn, this requires either pin 1 ,2 or 4 go high and this happens when a door opens. Because the high on each pin is only momentary (i.e. about 1/3 second, while C1 ,C2 or C3 is charging) there is only a short burst of buzzer activity (two brief beeps) at each door opening, after which it goes mute again.
So the beep calls attention to the fact that a door has opened and the LED indicates which door, staying lit until [i door is closed. If another door opens before the first door is closed, there is another beep and another LED lights.

The circuit above is too complex. It is very poorly designed. The 3 signals diodes are doing NOTHING. The are simply across each other!! One diode could be placed in the supply line to the 555 if it is needed for the reset line to work correctly. R7 is not needed as the output is taken to the pins 2&6 and the 74LS32 chip can be replaced by 3 x 470u electrolytics.
It can be simplified to this:

AUTO CUTOUT

A 12v relay is connected across the 12v supply. When the output is shorted, the 12v falls to 0v and the relay drops-out. The contacts open the 12v is reapplied to the relay and it will "chatter" if the short is not removed.
This circuit will simply not work and the relay will simply become a "Buzzer."


In the following circuit, the transistor will only turn on if the output voltage is above 0.6v.

AC DETECTOR

This circuit will detect AC line currents of about 250mA or more without making any electrical connections to the line. Current is detected by passing on of the AC lines through an inductive pickup (L1) made with a 1 inch diameter U-bolt wound with 800 turns of #35 magnet wire. The pickup can be made from other iron type rings or transformer cores that allows enough space to pass one of the AC lines through the center. Only one of the current carrying lines, either the line or the neutral should be put through the center of the pickup to avoid the fields cancelling.
This is most important is very difficult to achieve. The best method is to make a short extension cord with the three conductors separated from each other.
If you make a 3-turn loop with say the active line, and pass a straight rod such as a metal bolt, containing 400 or more turns through the centre of the 3-turns, you will produce a very sensitive pick-up.
The magnetic pickup produces about 4 millivolts for AC line current of 250mA, or AC load of around 30 watts. The signal from the pickup is increased about 200 times at the output of the op-amp pin 7 which is then peak detected by the capacitor and diode connected to pin 7. The second op-amp is used as a comparator which detects a voltage rise greater than the diode drop. The minimum signal
needed to cause the comparator stage output to switch positive is around 800mV which corresponds to about a 30 watt load on the AC line. The output of the 1458 op-amp will only swing within a couple volts of ground so a voltage divider (1k/470) is used to reduce the no signal voltage to about 0.7 volts. An additional diode is added in series with the transistor base to ensure it turns off when the op-amp voltage is 2 volts. You may get a little bit of relay chatter if the AC load is close to the switching point so a larger load of 50 watts or more is recommended. The sensitivity can be increased by adding more turns to the pickup.

CONSTANT CURRENT SOURCE

In the following circuit an LED is used to give a fixed reference voltage to a transistor. The output constant current I out is given by:
The LED lights up only when a load is connected at the output. Thus it indicates when the circuit is operating.

The operation of the circuit can be made clearer by re-arranging the components as follows:

The output will be limited to 100mA by using a red LED and 10R for Re.
The output will be limited to 500mA by using a red LED and 2R2 for Re.
The output will be limited to 1A by using a red LED and 1R0 for Re.