555 dual timer

555 IC Tutorial & Circuits

The 555 timer IC has been around now for quite some time and the list of potential uses for this device appears to be endless. This article contains a few examples of circuits that you might incorporate in your next design.

555 pinout


Before we review the circuits though, lets take a look at the basic anatomy of the timer. A diagram shows that the circuit consists of two comparators, which share a common voltage divider. Voltage derived from the divider sets two reference points. A low voltage, equal to 1/3 the supply voltage, is applied to comparator 2 and establishes the turn on voltage, which is anything below 1/3 of the supply voltage. Comparator 1 is biased at 2/3 the supply voltage and will respond only to a signal that exceeds this level. This enables the 555 to operate as a window detector.

555 pin chart

The conventional 8 pin DIP package is the most common, but a T metal can type is also available. Supply voltage requirements range from 3 to 18 volts, making the 555 one of the most versatile integrated circuits available.
Another mode of operation requires only a few millivolts to trigger the device on or off, making it ideally suited to touch plate circuits.


An example of a dual plate controller can be seen in the first circuit illustration. Resistor R1 acts as a pull-up resistor to keep pin 2 biased off. Touching the on plate applies the AC voltage present on the surface of your skin, to the circuit and the LED glows. Power can be taken from pin 3 to operate a low current device. To turn the circuit off, touch the off plate and the power at pin three goes to zero and the LED as well as anything connected to pin three, turns off.

555 touch plates

A second touch plate design automatically turns off after a period of time which is determined by the value of capacitor C1 and is adjustable through potentiometer R3. This type of circuit is well suited for car alarms if you substitute a piezo buzzer for the LED.

555 touch plate


Alarms are very popular circuits today and the 555 timer is ideally suited to the task, so I would be remiss if I did include at least one in this article. Light provides the perfect trigger source for alarms and the circuit shown here is among the easiest to construct.

555 light alarm

A cadmium sulfide photo-resistor is used to trigger the 555 and a potentiometer, R2, adjusts sensitivity. Switch S1 resets the alarm and a piezo-buzzer serves as the sound-producing element.

All of this and a 9-volt battery can be enclosed in a small project box. Small units like this can be used by a business man as a brief case alarm or hotel room alarm.

scr relay


The 555 with the use of SCRs or relays can control robotic systems that require more voltage or current. An example of this kind of interface is shown the next illustration. Resistor R1 provides biasing to stabilize the input which is sensitive to low level triggering. Triggering requires a negative pulse and the output stays high until the reset is engaged. Coupling capacitor C1 passes the quick positive pulse required by the SCR. Once the SCR is turned on, it provides power to the load until the reset input is triggered.

If you need to control a heavy-duty power source, you might consider using a relay to substitute for the SCR. The relay requires a constant DC current to remain on so the capacitor is omitted and two diodes are used to prevent false triggering.


As you can see, the 555 needs a negative trigger to turn on. If you need a positive trigger response, take a look at the positive trigger circuit. Resistor R1 serves as a pull-up resistor while transistor Q1 takes positive pulse input and presents a negative pulse to pin 2.

555 positive trigger

The input is in phase with the output and resistor R2 and capacitor C1 determine the pulse width.


555 midi processor

Pulse processing is an important function that the 555 is uniquely qualified to perform. Missing pulse detectors for production line monitoring is a very popular circuit. We have here though, a circuit that responds only to pre-selected pulses. Electronic music synthesizers produce a pulse train that triggers percussive events. This pulse is useless without a way to select the pulses you need for a given rhythmic pattern. The MIDI pulse processor enables you to select the pulses you want while ignoring all others.

A MIDI pulse is taken from a digital synthesizer through J1, a 5-pin DIN connector and fed to an optocoupler, IC1. All rhythm pulses appear at the out of IC1 and passed to input pin 2 of the 555 timer. When triggered, the timer stays on until capacitor C2 charges to 2/3 the supply voltage through resistor R3 and two potentiometers, R4 and R5. One of the potentiometers is tune and the other is fine tune. As they are manipulated, the time between pulses is changed from original frequency to about one beat every 15 seconds. The output can be fed to either analog or MIDI inputs.


Wave generating is another function that the 555 can perform with ease. In the oscillator schematic we see resistor R1 and potentiometer R2 provide an adjustable charging voltage to capacitor C1. When the voltage rises to 2/3 of the supply voltage, pin 6 is triggered and enables pin seven to ground the capacitor and pin two, starting the charge all over again. The output at pin 3 is a short duration pulse fed to Jack 2. This shape is not particularly useful as a test shape or a musical shape.

555 oscillator circuit

A ramp wave is an ideal shape for both test purposes and music production because it contains a full complement of harmonics and can be converted to any other wave shape. The high input impedance of the FET also eliminates any circuit loading problems. Resistor R3 isolates a sensitive FET from the heavy 10-volt wave differential while resistor R4 limits the current through the transistor. Volume of both outputs is controlled by potentiometers R5 and R6, while R7 limits current through capacitor C3 and on to output jack J2.


The output pin is taken from a buffer to prevent interaction with other functions of the IC. An output of full power or dead ground is the only option at pin 3, so an appropriate protection resistor should chosen for interface.


In almost all circuits with the 555 timer, pin 4 is connected to positive end of the power supply. This is because it resets the internal flip-flop when an operating cycle is complete. You may choose to use pin 4 as an enable control. You can see what circuit modification is necessary to use pin 4 as an enable control in the pin treatment illustration.


In all of the circuits you have seen in this article, not one of them has a use for pin five; however, pin five should not be overlooked. Access to the voltage divider high reference point is found at pin 5 and a capacitor from pin 5 to ground can stabilize a battery-operated circuit.

555 pin treatment

Another use for pin 5 is to change the reference voltage window from 2/3 to ½ of the supply voltage. This will effectively change the operating frequency of an oscillator and serve as a modulating control. An illustration of both techniques can be seen under the heading, pin treatments.


If you have ever seen a circuit with pin seven connected to the power supplies positive terminal through a potentiometer. This is not a good idea, for if the pot is ever turned to its point of minimum resistance, the pot will be damaged or the timer. The purpose of pin seven is to ground the charging capacitor at pin 6. This completes the cycle and pin 7 will remain at ground potential until pin 2 is triggered It is therefore imperative that at least one fixed resistor put in series with any potentiometer connected to pin seven, to prevent accidental damage.


If you need timing circuit for your next project, you might try the 555 timer. Considering the voltage requirements, the various modes of operation, and the cost of the device, it would be difficult to find anything that would compare to the 555 timer.

555 dual timer

A dual version of the timer is also available. The 556 is two independent timers on a single 14-pin DIP chip. Pin placement is shown in the illustration.


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