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8/8/2019 Submitted by Tom Email

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Submitted by Tom Email: [email protected]

The circuit uses a 555 timer wired as an astable oscillator and powered by the emitter current of theBC109C. Under dry conditions, the transistor will have no bias current and be fully off. As the probes getwet, a small current flows between base and emitter and the transistor switches on. A larger current flowsin the collector circuit enabling the 555 osillator to sound.

An On/Off switch is provided and remember to use a non-reactive metal for the probe contacts. Gold orsilver plated contacts from an old relay may be used, however a cheap alternative is to wire alternate

copper strips from a piece of veroboard. These will eventually oxidize over but as very little current isflowing in the base circuit, the higher impedance caused by oxidization is not important. No base resistoris necessary as the transistor is in emitter follower, current limit being the impedance at the emitter (theoscillator circuit).


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12V Lamp flasher circuit.

By admin

Description.

Here is a simple yet powerful circuit that can be used for flashing 12V lamps especially that is

used on automobiles.The flashing circuit is based on transistor Q1(BC557) and MOSFET Q2(IRF530) where the Q2 provides the necessary drive for the lamp.Any number of bulbs can be

flashed using this circuit provided that the total load must not exceed 42 Watts.

Circuit diagram with Parts list.

Notes.

y Assemble the circuit on a good quality PCB or common board.y The circuit can be powered form the car battery itself.

y The switch S1 can be used as the ON/OFF switch.y All capacitors must be rated 25V.

y Slight variations in the flashing frequency is possible by varying the value of C1.


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Voltage

Comparator

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Circuit : Andy Collinson

Email me

Description

This circuit will provide an indication whenever the input voltage differs from two definedlimits, V1 and V2. The limits are adjustable and the circuit made to trigger from the adjustable

"window".

Notes


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This circuit will provide an indication whenever the input voltage differs from two definedlimits, V1 and V2. The supply voltage, Vcc must be higher than the highest input voltage by at

least 2 volts. One application here is to monitor a 12V car battery. V1 could be set to 14V andV2 to 11V thus giving an indication of over charge or a weak battery.

The op-amps used here are MOSFET CA3140. They are used to advantage as they have verylittle output offset voltage and can switch down to near 0volts. If any other op-amp is used suchas LF351 or CA741 then it will be necessary to have an offset null control. This is just a 10k

preset contacted between pins 1 and 5, the wiper connected to the negative supply rail or op-amppin 4. With this circuit either op-amp will light the LED if the input voltage goes out of limits,

the two 1N4148 diodes forming an "AND"-gate at the output. The input voltage to be monitoredis fed via a series 10k resistor to inputs of both op-amps. If the input voltage is greater than the

limit set by V1 then the CA3140 will swing its output to almost the full supply voltage and lightthe LED. Similarly, if the input voltage is less than the limit defined by V2 then this op-amp will

swing towards Vcc and light the LED.

Circuit Exchange International Return to Switching Circuits http://www.zen22142.zen.co.uk


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VCS using

555 Timer

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Circuit : Miroslav Adzic - Serbia & Montenegro

DescriptionIn this circuit the 555 timer is used in a novel way, as a voltage controlled switch.

Notes:The old and omnipresent NE555 can be very good at something it was not meant for: driving

relays or other loads up to 200 mA. The picture shows an example circuit: if the input level rises


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over 2/3 of the supply voltage - it will turn on the relay, and the relay will stay on until the levelat the input drops below one third of the supply voltage.

If the relay and D1 were connected between pin 3 and ground, the relay would be activated when

the input voltage drops below one third, and deactivated when the input voltage goes over two

thirds of the supply voltage.

It is also a nice advantage that the input requires only about 1 uA, which is something bipolar

transistors can't compete with. (This high impedance input must not be left open.) A largehysteresis makes the circuit immune to noise. The output (pin 3) can only be either high or low

(voltage-wise), and it changes its state almost instantenously, regardless of the input signalshape.

The voltage drop across the NE555's output stage (at 35-100 mA) is 0.3-2.0 V, depending on the

way the relay is connected and the exact current it draws. D1 is absolutely vital to the safety ofthe integrated circuit.

Circuit Exchange International Return to Switching Circuits http://www.zen22142.zen.co.uk


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Pulse Width Modulation DC Motor Control

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251,761 155 87

Often, people attempt to control DC motors with a variable resistor or variable resistor connected to a

transistor. While the latter approach works well, it generates heat and hence wastes power. This simple

pulse width modulation DC motor control eliminates these problems. It controls the motor speed by

driving the motor with short pulses. These pulses vary in duration to change the speed of the motor. The

longer the pulses, the faster the motor turns, and vice versa.

Schematic

Parts

Part Total Qty. Description Substitutions

R1 1 1 Meg 1/4W Resistor

R2 1 100K Pot

C1 1 0.1uF 25V Ceramic Disc Capacitor

C2 1 0.01uF 25V Ceramic Disc Capacitor

Q1 1 IRF511 MOSFET IRF620

U1 1 4011 CMOS NAND Gate


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S1 1 DPDT Switch

M1 1 Motor (See Notes)

MISC1 Case, Board, Heatsink, Knob For R2, Socket For U1

Notes

1. R2 adjusts the speed of the oscillator and therefore the speed of M1.

2. M1 can be any DC motor that operates from 6V and does not draw more than the maximum

current of Q1. The voltage can be increased by connecting the higher voltage to the switch

instead of the 6V that powers the oscillator. Be sure not to exceed the power rating of Q1 if you

do this.

3. Q1 will need a heatsink.

4. Q1 in the parts list can handle a maximum of 5A. Use the IRF620 for 6A, if you need any higher.

5. This circuit is not a true pulse width modulation control. Because only the frequency of pulses

6. varies, it is really pulse frequencymodulation. This works, though not as well as true PWM.


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Touch Switch

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114,660 45 47

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.

Schematic

Parts

Part Total Qty. Description Substitutions

C1 1 10uF 16V Electrolytic Capacitor

R1, R22 100K 1/4 Watt Resistor

R3 1 10 Meg 1/4 Watt Resistor

U1 1 4011 CMOS NAND Gate IC

MISC 1 Board, Wire, Socket For U1


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Notes

1. The contacts an 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 usedto drive a relay, transistor, other logic, etc.

3. You can vary the length of the output pulse by using a smaller or larger capacitor for C1.


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Ramp Generator Circuit-using 555 Timer IC

We know that if a capacitor is charged from a voltage source through a resistor, an exponentialwaveform is produced while charging of a capacitor from a constant current source produces a

ramp. This is the idea behind the circuit. The circuit of a ramp generator using timer 555 is

shown in figure. Here the resistor of previous circuits is replaced by a PNP transistor thatproduces a constant charging current.

Ramp Generator Circuit

Charging current produced by PNP constant current source is

iC = Vcc-VE / RE

where VE = R2 / (R1 + R2) * VCC + VBE

When a trigger starts the monostable multivibrator timer 555 as shown in figure, the PNP

current source forces a constant charging into the capacitor C. The voltage across the capacitoris, therefore, a ramp as illustrated in the figure. The slope of the ramp is given as

Slope, s = I/C


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This electronic schematic diagram is simple, you can use this schematic diagram as yoursimple electronic project. Its easy and fun.

The mainpart of this doorbell circuit are two NE555 timer ICs. When some one presses switch

S1 momentarily ,the loud speaker sounds a bell tone as long as the time period of the monostablemultivibrator built around IC1.

When the switch S1 pressed, IC1 is triggered at its pin 2 and output pin 3 goes high for a time

period previously set by the values of POT R4 and POT R5.When the output ofIC1 goes high itresets IC2 and it starts to oscillate to make a bell sound through the speaker.The IC2 is

configured as an astable multivibrator whose oscillation frequency can be varied with the help ofPOT R5.By adjusting the values of R4 & R5, modifications on the tone are possible.

Notes.

y The circuit has to assembled on a good quality PCB or common board.y The IC1 & IC2 has to be mounted on IC holders.

y Powerthe circuit from a 9V battery or 9V DC power supply.y Switch S1 is push button switch.


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Simple LED Flasher circuit

Here the simple LED flashercircuit that you can use for yoursimple project.

schematic diagram:

componentpart list:

How the circuit works:

This LED flasher uses a common 555 timer IC for its operation. It is configured as an astablemode which means that its output is a square wave oscillator. Two LEDs are connected to its


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output in such a way that when one LED is ON, the other LED will turn OFF. It uses only 10simple parts that are easily available at any electronic shops.

Capacitor C2 charges exponentially through resistors R1, R2 and the resistance of the trimpot.

When C2 has charged to about 2/3 VCC it stops charging and it discharges to about 1/3 VCC

through R2 and the trimpot resistance via pin 7. This is the standard operation of a 555 timer.When a Vcc of 5 V to 15 V DC is applied to the circuit, the LED will start to flash. Thefrequency of the flashing can be changed by varying the resistance of the potentiometer or

trimpot.


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Digital code lock

By john

Description.This is a simple but effective code lock circuit that has an automatic reset facility. The circuit is

made around the dual flip-flop IC CD4013.Two CD 4013 ICs are used here. Push buttonswitches are used for entering the code number. One side of all the push button switches are

connected to +12V DC. The remaining end of push buttons 2,3,6,8 is connected to clock inputpins of the filp-flops. The remaining end of other push button switches are shorted and connected

to the set pin of the filp-flops.The relay coil will be activated only if the code is entered in correct sequence and if there is any

variation, the lock will be resetted. Here is correct code is 2368.When you press 2 the first flipflop(IC1a) will be triggered and the value at the data in (pin9) will be transferred to the Q output

(pin13).Since pin 9 is grounded the value is 0 and so the pin 13 becomes low. For thesubsequent pressing of the remaining code digits in the correct sequence the 0 will reach the Q

output (pin1) of the last flip flop (IC2b).This makes the transistor ON and the relay isenergised.The automatic reset facility is achieved by the resistor R11 and capacitor C2.The

positive end of capacitor C2 is connected to the set pin of the filp-flops.When the transistor isswitched ON, the capacitor C2 begins to charge and when the voltage across it becomes

sufficient the flip-flops are resetted. This makes the lock open for a fixed amount of time andthen it locks automatically. The time delay can be adjusted by varying the values of R11 and C2.

Circuit diagram with Parts list.


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Notes.

y Assemble the circuit on a good quality PCB.

y The circuit can be powered from 12V DC.y

Mount the ICs on holders.y The L1 can be a 12V, 200 Ohm SPDT relay.y Capacitor C1 should be tantalum type.y The C1 and C2 must be rated at least 25V.


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555 based Reset Generator - del50008

You must have read all the circuits and explanation on the other pages at delabs to understandthis, as i cannot repeat the same thing as i have done it more than once already.

The circuit is a 555 monostable, The push-switch is to trigger and generate a reset pulse for uC.

The diode is for OR' ing later. The High will go thru the diode but the Low of 555 cannot drinkany current as diode blocks.


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The bottom part of the Circuit is the CD4093 Schmitt nand based flipflop. This is my favorite

because of studying designs in Elektor Electronics.

What is the Schmitt then ?, It is better to visit the links below to learn. It can make a sine or

triangle or any shape waveform to square. It can help square a very messy waveform.

y Schmitt Trigger Applications

y Schmitt Trigger 1

y Schmitt Trigger 2


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y Schmitt trigger - Wikipediy German Schmitt.

Two nand gates are connected to form a flip-flop toggle switch. When 555 gives out a pulse, the

pulse is delayed by a R-C which results in a ramp, the third nand-Schmitt gives out delayed low

trigger to toggle nand-flipflop-switch. The Nand-toggle-switch is rest at any time with the lowerpushbutton. The fourth nand output gate is not really required. But the 4093 is quad nand, sounused nand inputs should not float , pull-up or pull-down. So it has just been added in circuit so

that he can fit some role, or else he will be bored.

Now you can figure out how such blocks can be used like Lego blocks in your own amazingdesigns.

That way battery banks can be charged, electroplating can be done, current and voltage can be

controlled with opamps, thyristors are very rugged compared to transistors and MOSFETS inthat order.

y 555 and 556 Timer Circuits


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LM555 Voltage Doubler - del50007

This circuit shows the voltage doubler working with a 555. LM555 has good drive 200mA, bothVcc and Gnd.

"A voltage multiplier is an electrical circuit that converts AC electrical power from a lowervoltage to a higher DC voltage by means of capacitors and diodes combined into a network.

Voltage multipliers can be used to generate bias voltages of a few volts or tens of volts or

millions of volts for purposes such as high-energy physics experiments and lightning safetytesting." - Wikipedia

I drew this for a person who makes toys.


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Person Counter 9

The integrated circuit is a 4516 up/down counter. Diode 2 determines the count direction. The value of theresistors R depends on the type of diode used. (You will probably need resistors in the range 100k to1M.) When entering the room, the beam illuminating diode 1 must be cut first. This causes the counter tocount UP one. If (when a person leaves the room) the beam illuminating diode 2 is cut and while thisbeam is still cutthe beam illuminating diode 1 is cut, the counter will count down one. The output can beused (with a relay and driver) to switch on a l ight when a person first enters the room and then to switchthe light off when the last person leaves the room (maximum number of people, 15).

Pins 1,3,4,5,12, and 13 of the 4516 must be connected to battery negative (see vero diagram below). The "switch-on

reset" network (R and C) can have almost any values; try 100k and 1.5nF.

Use I.R. receiver diodes with black insulating tape (or something similar) around them. Alternatively, put the diodes

in a small plastic box, as shown below.


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One Digit Event Counter

This is a simple one digit counter using CMOS instead of the traditional TTL ICs . UsingCMOS makes it possible to use a power supply from 5 to 15 volts as long as we use thecorrect value for the current limiting resistors (Rs) which can be calculated with the

simple following formula " Rs = Supply voltage - 1.7 volts divided by current ( 10 mA) " .

In designing this circuit , I had in mind a design as simple as can possibly be made withminimum readily available components . For the counter a single chip is used , Of thetwo counters available on the CMOS 4518 only one is used . The LEDs are notmultiplexed and each segments must be connected to it own current limiting resistor .Each segments are single LED which can withstand current of up to 20 mA each but it

is recommended that a 10mA maximum current be used to ensure long life withadequate illumination . Thus with an anticipation of all segments when lit showing "8"would amount to a total 7 segments times 10mA would require a total current of 70 mAfor the one digit and this should be the minimum supply current plus a 25% for safety fora total of 87.5 mA , so lets say 100 mA power supply at whichever voltage you chose touse from 5 to 15 volts .

Using the Counter

In order to use the counters the following conditions must be set .

" Enable " pins 2 and 10 must be HIGHT (1)

" Reset " pins 7 and 15 must be LOW (0)" Count " pins 1 and 9 must be HIGH (1)

Normally an IC like a CMOS 40106 is used to debounce a push button switch , a NPN2N2222 transistor and a 1K resistor with a .1uF capacitor is parallel from base to groundare used to debounced the trigger switch . Most any small signal NPN transistors canbe used and a capacitor from .05uF to 1.uF can be used. The lower the capacitor valuethe faster and noisier the trigger signal will be .


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Using a simple push button without the conditioning is not recommended as it will giveerror counts .See Switch debouncing for alternate circuits that can be used .

LED Display

The system can accomodate Common Anode ( CA ) or Common Cathode ( CC ) LEDdisplays with single LED segments . This circuit uses a Common Cathode Led Display .

Construction

The PCB is actual size and a graphic representation shows how the display is

connected to the limiting resistors ( Rs ) . It is a wise move to used sockets for the ICs .Construction can be made using the PCB layout or hand wired whichever you feel at themoment .For intermittent operation a 9 volts battery can be used otherwise , several " D " cells inseries should be used or a wall transformer type with good rectification can also beused .


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If you do not have any experience with this type of project , I would strongly suggest thatyou start with this one . In order to acquire more knowledge about counters feel free toread the following pages describing in more details more complicated counter projects .

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