555 Oscillator Applications

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555 Oscillator Applications

The maximum output to either sink or source the load current via pin 3of 555 is about 200mA and this value is more than enough to drive orswitch other logic IC's, a few LED's or a small lamp etc and that we

would need to use a bipolar transistor or MOSFET to amplify the 555'soutput to drive larger current loads such as motor or relays. But the555 Oscillator can be used in a wide range of waveform generatorcircuits and applications that require very little output current such asin electronic test equipment for producing a whole range of differentoutput test frequencies from very accurate sine, square and pulsewaveforms or as LED or lamp flashers and dimmers to simple noisemaking circuits such as metronomes, tone and sound effectsgenerators and even musical toys for Christmas.We could very easily build a simple 555 oscillator circuit to flash a fewLED's "ON" and "OFF", but one very nice and simple to build projectusing an astable based 555 oscillator is that of an ElectronicMetronome. Metronomes are devices used to mark time in pieces ofmusic by producing a regular and recurring musical beat or click. Asimple electronic metronome can be made using a 555 oscillator as themain timing device and by adjusting the output frequency of theoscillator the tempo or "Beats per Minute" can be set. A tempo of 60beats per minute means that one beat will occur every second and inelectronics terms that equates to 1Hz. So by using some very commonmusical definitions we can easily build a table of the differentfrequencies required for our metronome circuit as shown below.

Metronome Frequency Table

MusicalDefinition

RateBeats perMinute

CycleTime (T)

Frequency

Larghetto Very Slow 60 1sec 1.0Hz

Andante Slow 90 666ms 1.5Hz

Moderato Medium 120 500ms 2.0Hz

Allegro Fast 150 400ms 2.5Hz

Presto Very Fast 180 333ms 3.0Hz

The output frequency range of the metronome was simply calculatedas the reciprocal of 1 minute or 60 seconds divided by the number ofbeats per minute required, for example (1/(60 secs / 90 bpm) = 1.5Hz)and 120bpm is equivalent to 2Hz, and so on. So by using our nowfamiliar equation above for calculating the output frequency of anastable 555 oscillator circuit the individual values of R1, R2 and C canbe found.


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The time period of the output waveform for an astable 555 Oscillator isgiven as:

For our electronic metronome circuit, the value of the timing resistor

R1 can be found by rearranging the equation above to give.

Assuming a value for resistor R2 = 1k and capacitor C = 10uF thevalue of the timing resistor R1 for our frequency range is given as71k6 at 60 beats per minute to 23k5 at 180 beats per minute, so avariable resistor (potentiometer) of 100k would be more than enoughfor the metronome circuit to produce the full range of beats requiredand some more. Then the final circuit for our electronic metronomeexample would be given as:

555 Electronic Metronome


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This metronome circuit demonstrates just one simple way of using a555 oscillator to produce an audible sound or note. It uses a 100kpotentiometer to control the full range of output pulses or beats, andas it has a 100k value it can be easily calibrated to give an equivalentpercentage value corresponding to the position of the potentiometer.For example, 60 beats per minute equals 71.6k or 72% rotation.Likewise, 120 beats per minute equals 35.6k or 35% rotation, etc.Additional resistors or trimmer's can be connected in series with the

potentiometer to pre-set the outputs upper and lower limits topredefined values, but these additional components will need to betaken into account when calculating the output frequency or timeperiod.While the above circuit is a very simple and amusing example of soundgeneration, it is possible to use the 555 Oscillator as a noisegenerator/synthesizer or to make musical sounds, tones and alarms byconstructing a variable-frequency, variable-mark/space ratio waveformgenerator. In this tutorial we have used just a single 555 oscillatorcircuit to produce a sound but by cascading together two or more 555oscillator chips, various circuits can be constructed to produce a whole

range of musical effects such as the police car "Dee-Dah" siren givenin the example below.

555 Oscillator Police "Dee-Dah" Siren


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The circuit simulates a warble-tone alarm signal that simulates thesound of a police siren. IC1 is connected as a 2Hz non-symmetricalastable multivibrator which is used to frequency modulate IC2 via the10k resistor. The output of IC2 alternates symmetrically between300Hz and 660Hz taking 0.5 seconds to complete each alternatingcycle.


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AUTOMATIC CURTAIN CLOSER Circuit


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This circuit uses a mixture oftransistors, an IC and a relay and is

used to automatically open and close apair of curtains. Using switch S3 alsoallows manual control, allowingcurtains to be left only partially openor closed. The circuit controls a motor


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that is attached to a simple pulleymechanism, to move the curtains.

Automatic OperationThe circuit can be broken into threemain parts; a bi-stable latch, a timerand a reversing circuit. Toggle switchS3 determines manual or automaticmode. The circuit as shown above isdrawn in the automatic position andoperation is as follows. The bi-stable isbuilt around Q1 and Q2 and associatedcircuitry and controls relay A/2. S1 isused to open the curtains and S2 toclose the curtains. At power on, a briefpositive pulse is applied to the base ofQ2 via C2. Q2 will be on, and activaterelay A/2.

The network of C3 and R4 form a lowcurrent holding circuit for the relay.Relay A/2 is a 12V relay with a 500 ohmcoil. It requires slightly less current tokeep it energized than it does tooperate it. Once the relay hasoperated, the current through the coil

is reduced by R4, saving powerconsumption. When Q2 is off, C3 willbe discharged, but when Q2 becomesactive (either at switch-on or bypressing S1) capacitor C3 will charge


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very quickly via the relay coil. Theinitial charging current is sufficient toenergize the relay and current flow

through R4 sufficient to keep itenergized. CAR TACHOMETER Circuit

A 555 is configured as a monostable or one shot in this project. The period of the 555 isdetermined by the 47k and the capacitor from pin 6 to ground (100n). Time "T" = 1.1 RCor 1.1 X 50,000 X 0.1 X10 -6 = 0.0055 or 5.5 mS (milli-seconds).

The 555 receives trigger pulses from the distributor points. These are limited by the 1kand 5v zener diode. These are AC coupled to the trigger input through the 100n couplingcapacitor. The 50mA meter receives pulses of current through the 200k pot to show areading.

Integration of the current pulses produces a visible indication of the cars engine speed onthe 0-1mA meter.Supply is taken from the cars 12v system and for the 555 it is reduced to a regulated 9vby the 15 ohm resistor in conjunction with the 9v zener diode. Note: the 10u electrolyticmust be placed physically as close as possible to supply pin 8. The 555 is capable ofsinking and sourcing up to 200mA, but it gets very hot when doing this on a 12v supply.The following circuit shows the maximum number of white LEDs that can be realisticallydriven from a 555 and we have limited the total current to about 130mA as each LED is

designed to pass about 17mA to 22mA maximum. A white LED drops a characteristic3.2v to 3.6v and this means only 3 LEDs can be placed in series.


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KNIGHT RIDER Circuit

This circuit mimics the lights in knight rider's car. They flash one at a time chasing eachother.


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In the Knight Rider circuit, the 555 is wired as an oscillator (Astable mode). The outputof the 555 is directly connected to the input of a 4017 decade counter.

The input of the 4017 counter is called the CLOCK line. The 10 outputs Q0 to Q9 becomeactive, one at a time, on the rising edge of the waveform from the 555. Each output can

deliver about 20mA but a LED should not be connected to the output without a current-limiting resistor (100R or 220R).

Using six 3mm LEDs, the display can be placed in the front of a model car to give a veryrealistic effect. The same outputs can be taken to driver transistors to produce a largerversion of the display.

Schematic

This circuit consumes 22mA while only delivering 7mA to each LED. The outputs arefighting each other via the 100R resistors (except outputs Q0 and Q5).

Parts

1x NE555 Bipolar Timer

6x LED (Red)8x 100 Resistor (1/4W)2x 220 Resistor (1/4W)1x 1K Resistor (1/4W)1x 68K Resistor (1/4W)1x 3.3 F Electrolytic Capacitor (16V)1x 4017 Decoded Decade Counter1x 9V Voltage battery
http://www.555-timer-circuits.com/operating-modes.htmlhttp://www.555-timer-circuits.com/operating-modes.html


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MACHINE GUN Circuit

This circuit produces a sound very similar to a machine gun:

METAL DETECTOR Circuit

This circuit detects metal and also magnets. When a magnet is brought close to the 10mHchoke, the output frequency changes.


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MUSIC BOX Circuit

This circuit produces 10 different tones and by selecting suitable values to change thevoltage on pin 5, the result can be quite pleasing. Note: the two unused outputs of the4017 produce a tone equal to that produced by the 555 when pin 5 has no external controlvoltage.

POLICE LIGHTS Circuit

This circuit flashes the left LEDs 3 times then the right LEDs 3 times, then repeats.

Overview


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This circuit uses a 555 timer which is setup to both runn in an Astable operating mode.This generates a continuous output via Pin 3 in the form of a square wave. When thetimer's output changes to a high state this triggers the a cycle on the 4017 4017 decadecounter telling it to output the next sequential output high. The outputs of the 4017 are

connected to the LEDs turning them on and off.

Schematic

SERVO TESTER Circuit

This circuit can be used to manually turn a servo clockwise and anti-clockwise. Bypushing the forward or reverse button for a short period of time you can control therotation of the servo. It will also test a servo.Here is a photo of a kit from Cana Kit for $10.00 plus postage (it is a slightly differentcircuit) and a motor and gearbox, commonly called a "servo." The output shaft has a diskor wheel containing holes. A linkage or push-rod is fitted to a hole and when the disk

rotates, the shaft is pushed and pulled. The shaft only rotates about 180 to actuate flaps

or ailerons etc.


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A pot can be used to control the position of the servo by using the following circuit. Itproduces a positive pulse between about 0.9 milliseconds and 2.1 milliseconds. The offperiod between pulses is about 40 milliseconds. This can be shortened by reducing thevalue of the 3M3 resistor.

ZENER DIODE TESTER Circuit

This circuit will test zener diodes up to56v. See Talking Electronics website,left index, 200 Transistor Circuits(circuits 1-100) and go to Zener Diode


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(making) to see how to make a zenerdiode and how to create a zenervoltage from a combination of zeners.

Place the zener across the terminals inthe circuit below and read the valueacross it with a multimeter set to 50vrange.

VOLTAGEDOUBLER Circuit

A voltage higher than the supply can be created by a "Charge-Pump" circuit created witha 555, diodes and capacitors as shown in the following circuit. The output will deliverabout 50mA


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ROULETTE Circuit

This circuit creates a rotating LED that starts very fast when a finger touches the TOUCHWIRES. When the finger is removed, the rotation slows down and finally stops.

555 Oscillator Applications

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