The 555 timer is used to make time delays in electronic devices as an oscillator. It is an Integrated circuit (IC) chip that is easy to use in many timer applications. It is also used as pulse generator or oscillator. It is highly stable to produce accurate time delays in sensitive appliances. The derivatives provide up to four timing circuits in one package.
It was introduced in 1971 by Signetics Corporation. It was initially name as SE555/NE555. It helped in making user friendly circuits. It was normally called “The IC Time Machine. The IC is known particularly for its cheap rates as compared to other similar devices. The chip was later introduced in many devices and was widely manufactured by different companies. These days CMOS version of the IC is mainly used.
The nature of device is basically digital (specially the output) though the functioning relies on both analogue and digital techniques. A dual version of 555 timers is known as 556, in which two free 555 ICs are placed in a packed container.
The output produced by the device can either be low state or high state. In low state the voltage 0V and in high state the voltage of device is nearly equal to the supplied voltage. It ranges from 4.5 Volts to 18 Volts.
The 555 timer has three types of working modes. All of them have a different circuit that results in a different output. Following are the three operating modes:
In this mode of operation, the 555 timer functions as a one-shot. Initially the external capacitor is discharged with help of a transistor placed inside the IC. When the negative trigger is applied pulse of less than 1/3 VCC to pin 2, the flip-flop discharges short circuit across the capacitor and makes the output voltage higher. The potential across the capacitor is then increased exponentially. After the end of that time period the voltage is 2/3 VCC. The flip-flop is then reset and the output is of low state. This is done by the comparator. The time interval is independent of the supplied voltage because the threshold level and the charge of comparator are directly proportional to the supplied voltage. It is used in timers, frequency drivers, pulse width modulation (PWM), pulse detection, touch switches or capacitance calculation etc.
The 555 timer works as an oscillator. The pins 2 and 6 are connected; the circuit will trigger and run as a multi-vibrator. The external capacitor is first charged and then discharged through two different resistors. In such a circuit capacitor charges and discharges between 1/3 VCC and 2/3 VCC. The charge and discharge timing is observed and the frequency is free of the supplied voltage. These circuits are used in pulse position modulation, pulse generators, lamp flashers, logic clocks, tone generators, security alarms and light emitting diodes.
Bistable Mode or Schmitt Trigger
In bi-stable mode the IC works as a flip-flop. It is sometimes called Schmitt Trigger. It has both of the stable states that are high and low. Pin 2 that is trigger and pin 4 that is the reset, are held high with the help of resistors while pin 6 that is threshold is floating. Holding the trigger to the ground sets the output pin that is 3 to high state VCC. There are no capacitors in this circuit. The output would control the delay which would be mounted by a wire as a reversing switch. This reverses the direction of the current to the original track. It is used in bounce-free and latched switches etc.
There are eight pins in every 555 timer. Each pin has a different function and name.
Pin 1 (Ground)
It’s a ground pin so simply provide make it grounded i.e. provide 0V to this pin.
Pin 2 (Trigger)
This pin makes output go high as it is input to the lower comparator. This pin sets the latch. It detects 1/3 rail voltage. It controls pin 6, both have same states. If both the pins are high the output turns low and if the later is high and former low the output turns low. The pins triggers when the pin is held at a voltage of 1/3V or more. The triggering helps in changing the rate of waveforms and pulses. This pin has very high impedance (about 10M) and will trigger with about 1uA.
Pin 3 (Output)
Pin 3 is related to pin 7 as both are adjacent to each other. The voltage of the pin ranges from 0.5 volts to 2 volts and it delivers a current of 200mA. The output is obtained from a high current stage made of transistors. The state of output pin is always inverse to the state of latch. Latch itself is not directly accessible.
The state of the output pin will always reflect the inverse of the logic state of the latch, and this fact may be seen by examining Fig. 3. Since the latch itself is not directly accessible. The output voltage available at this pin is approximately equal to the Vcc applied to pin 8 minus 1.7V.
Pin 4 (Reset)
The reset pin is internally connected high. It must be taken under 0.8 volts to reset the entire chip. It returns the output to a low state. The reset threshold level is 0.7 volt and a current of 0.1mA. It is use to gate the oscillations on and off. The pin activation voltage is from 0 to 4 volts. Irrespective to other pins it forces the output to go low.
Pin 5 (Control):
In control the internal voltage is divided by 2/3 VCC. A voltage applied to this pin will help in changing timing of the RC network. It also provides direct access to the lower comparator. The change in control voltage may range from 45 to 90% of the VCC in the monostable mode. This helps in controlling the width of output pulse.
Pin 6 (Threshold):
This pin is connected to the functioning of trigger. It detects 2/3 of rail voltage to make the output low if the trigger is high. This impedance of this pin is nearly 10M and trigger with the current of 0.2uA. The timing interval ends when the voltage at control is lesser than threshold.
Pin 7 (Discharge):
This pin opens collector output that causes a capacitor to discharge. This pin is in phase with the output. This pins goes low when pin 6 detects a 2/3 rail voltage, only if pin 2 is high. Pin 7 constantly remains low in pin 2 is high, irrespective of pin 6. It goes high when pin 2 detects a 1/3 rail voltage and pin 6 is low as well. It is in phase with pin 3.
Pin 8 (Supply):
It is a positive voltage supply and connects to the positive terminal. The voltage at this pin ranges from 4.5 volts to 15 volts DC. The voltage depends on variation.
Sample Project – TV Remote Control Jammer Circuit
There are many applications of 555 timers. It is used in LED Dimmer, Wailing Siren, Zener Diode Tester, Servo Controller etc. Here as an example we will discuss TV remote control jammer circuit in which 555 timers is used.
If you are irritated with your family members who keep changing channels during your favorite show time or are you annoyed with the loud volume in which your kids watch cartoons then this circuit is all what you need. This circuit baffles infra-red receiver in a TV. The signal that it produces obstructs the signal with the remote control’s signal and inhibits TV from perceiving a channel-change or any other order. 38 kHz is the signal produced by the circuit. We call IR diode as Infra-red transmitting Diode or IR emitter diode to differentiate it from receiving diode which is called as IR receiving diode or IR receiver (the receiving diode is a photo diode). We do not put generic number on the circuit to denote the type of diode because of many IR emitters present but some types are CQY37N, CY85G, INF3940, INF3850, INF3880, and LD271. We include two 1N4148 diodes to limit the current to 100mA flowing through the IR LED, a constant-current arrangement will be created by these diodes when joined with the 5R6 resistor and transistor.
The circuit usually include circuit board, one 10k variable resistor, one 555 Timer, four Resistors of 470ohm, 1k, 560ohm, two High speed switching diodes 1N4148, one Transistor BC557, two Infrared LEDs, two 10nf capacitors, one battery.
The working of this circuit is very easy, when you push the button on IR jammer, it will emit frequency for 30 seconds and time duration can be increased by pressing button again and again. Like if you want jamming for 2 minutes then press button for four times.
The function is that when you press button on normal remote control for TV, it produce series of pulses which the TV receives and decodes into resultant function at the frequency around 3-40kHz and what the jammer do is it sends out steady stream of binary code (1’s an 0’s) at same frequency but with no instruction to decode. In this way receiver will get nothing and will not be able to obtain the real information till you stop jammer. If you turn off TV and turn it on again then TV will work normally as before.
Working Of 555 Timer
NE555 timer IC is used to generate frequency. Also from this IC output is taken and is send to PNP transistor which works as switching network, whenever output of IC drives into negative going cycle transistor will go in conduction. IR led will switch on by conduction of transistor and will emit frequency in form of IR rays which will disable TV from getting information.
The 555 timer IC is most important IC whose CMOS type is commonly used these days. It is basically used to produce time delays and works as an oscillator. It has mainly 8 pins. The reason why this IC is being used widely is the cheap price at which it’s available compared with high coat ICs used for same purpose.