INTRODUCTION• Introduced by Signetics in 1972, most popular is NE555 by STM electronics and Fairchild Semiconductor.• The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications.• The 555 is used to provide time delays, as an oscillator, and as a flip-flop element.• It gets its name from the three 5k ohm resistors which give the two comparators reference voltage.• Depending on the manufacturer, the standard 555 package includes 25 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-PIN DIP (Dual in-line) package.• It is available in low power CMOS type ICM7555 package and 556 Dual Timer (14 pin) with two timer in one IC and 558 which is Quad timer.
PIN DESCRIPTIONPIN 1-Ground, The ground pin connects the555 timer to the negative (0v) supply rail.PIN 2-Trigger, when < 1/3 Vcc (active low)this makes the output high (+Vcc). Itmonitors the discharging of the timingcapacitor in an astable circuit.PIN 3-Output, The output pin can drive any TTL circuit and iscapable of sourcing or sinking up to 200mA of current at an outputvoltage equal to approximately Vcc - 1.5V so small speakers, LEDs ormotors can be connected directly to the output.PIN 4-Reset, when less than about 0.7V (active low) this makes theoutput low (0V), overriding other inputs. When not required itshould be connected to +Vcc.
PIN DESCRIPTIONPIN 5-Control Voltage, this can be used toadjust the threshold voltage which is setinternally to be 2/3 Vcc. Usually this function isnot required and the control input isconnected to 0V with a 10nF capacitor toeliminate electrical noise.PIN 6-Threshold, when > 2/3 Vcc (active high)this makes the output low (0V)*. It monitorsthe charging of the timing capacitor in astableand monostable circuits.PIN 7-Discharge, The discharge pin is connected directly to the Collectorof an internal NPN transistor which is used to "discharge" the timingcapacitor to ground when the output at pin 3 switches "LOW".PIN 8-Supply +Vcc, This is the power supply pin and for general purposeTTL 555 timers is between 4.5V and 15V (18V Absolute Maximum).
MODES OF OPERATIONThe 555 Timer IC has three modes of operation:MONOSTABLE: in this mode, the 555 functions as a "one-shot"pulse generator. Applications include timers, bounce free switches,touch switches, frequency divider and so on.BISTABLE: the 555 can operate as a flip-flop, if the DIS pin is notconnected and no capacitor is used. Uses include bounce-freelatched switches.ASTABLE, the free running mode: the 555 can operate as anoscillator. Uses include LED and lamp flashers, pulse generation,logic clocks, tone generation, security alarms, pulse positionmodulation and so on. The 555 can be used as a simple ADC,converting an analog value to a pulse length.
MONOSTABLE MODEIn the monostable mode, the 555 timeracts as a “single" pulse generator. The pulse begins when the 555 timer receives a signal at the trigger input that falls below a third of the voltage supply.The width of the output pulse is determined bythe time constant of an RC network, whichconsists of a capacitor (C) and a resistor (R). The output pulse ends when the voltage on the capacitor equals 2/3 of the supply voltage. The output pulse width can be lengthened or shortened to the need of the specific application by adjusting the values of R and C.
MONOSTABLE MODE CIRCUIT DI The output pulse width of time t, which is the time it takes to charge C to 2/3 of the supply voltage, is given by t=RC ln(3) where t is in seconds, R is in ohms and C is in farads. While using the timer IC in monostable mode, themain disadvantage is that the time span between any two triggeringpulses must be greater than the RC time constant.
BISTABLE MODEIn bistable mode, the 555 timer acts as aflip-flop. The trigger and resetInputs are held high via Pull-up resistorswhile the threshold input is simplygrounded. Thus configured, pullingthe trigger momentarily to ground acts asa set and transitions the output pin toVcc (high state). Pulling the reset input toground acts as a reset‘ and transitionsthe output pin to ground (low state). Nocapacitors are required in a bistableconfiguration. Pin 5 (control) isconnected to ground via a small-valuecapacitor (usually 0.01 to 0.1 uF); pin 7(discharge) is left floating.
ASTABLE MODEIn astable mode, the 555 timer puts out acontinuous stream of rectangular pulseshaving a specified frequency. the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low Impedance to ground during output low intervals of the cycle, therefore discharging the capacitor.In the astable mode, the frequency of thepulse stream depends on the valuesof R1, R2 and C: High time: Low time:
APPLICATIONS OF 555 TIMERThe 555 Timer IC being easy to use, low price, and good stability isused widely in many applications.Some of which we are going to show you now, namely:1. Blinking LED lights2. Police Car Siren3. Light Detector4. Mosquito Repellant5. Railway Lights
BLINKING LED LightsThe 555 timer is used to blink an array of LEDs’, The timer is used inASTABLE mode which gives us pulsed output. According to the above formula the frequency of the blink is approximately 2.2Hz. The frequency can be changed by either changing the value of resistance or the capacitor.
POLICE CAR SIRENPolice siren circuit given here is a circuit based on 555 timer IC. Thiscircuit uses two 555 ICs and which are the core working parts of thiscircuit. Here two timer ICs are wired to work as astablemultivibrators.
WORKING OF POLICE CAR SBut these two timer ICs are working at different frequencies. First 555Timer is slow astable multivibrator operating at a frequency of 20Hzand a duty cycle of 50% whereas second 555 timer works at 600Hz asfast astable multivibrator. The output of first timer is fed to thecontrol pin (pin5) of second 555 timer. By this connection, the outputfrequency of second 555 timer will be modulated with the outputfrequency of first 555 timer.The circuit can be powered from a DC supply voltage ranging between6V and 15V. Power supply used here is 12V DC supply. The loudness ofoutput can be further increased by inserting a power amplifier circuitat the output stage of this circuit.
LIGHT DETECTORIn this circuit the 555 timer runs in ASTABLE mode, where theoutput is controlled by varying the potential of the RESET (pin 4). Tocontrol the voltage an LDR is used. When light is shown in the LDRits resistance decreases and its potential is almost equal to Vcc. Hence the internal flip flop is not reset, which in turn produces output pulses of desired frequency which makes the buzzer beep. The sensitivity can be controlled by the connected potentiometer.
MOSQUITO REPELLANTThis circuit produces a tone above human audible range and this issupposed to keep the mosquitoes away. You need a piezo diaphragmthat will respond to 15kHz. The 555 timer works in ASTABLE mode in this circuit.
RAILWAY LIGHTSIn this circuit, the 555 timer works in ASTABLE mode. One LED glowsdue to sinking of current and the other due to sourcing of current.This happens simultaneously because the Duty Cycle of this circuit isapproximately 50%. Frequency=3Hz