How Does a 555 Timer Work?

What is it?

The 555 timer implements many timer and multivibrator applications in the form of an integrated circuit or chip developed in 1971 by Signetics. The name may come from the 5 ohm resistors used in early implementations even though the number was arbitrary. A typical design for the 555 timer consists of more than 20 transistors, two diodes and 15 resistors in a silicon chip. It utilizes eight-pins for ground, trigger input, output, reset input, control voltage, threshold input, discharge and supply voltage or +Vcc (+5 to +15 volts in normal use). The integrated chip remains in production due to its ease of use, low price and good stability. An estimated one billion units had been produced as of 2003. The 555 timer operates in three ways: monostable, astable and bistable modes, depending on the connections and arrangement of external components.

Operation

The 555 timer centers on the three resistors that create a voltage divider across the power supply with the two comparators connected to it. The integrated circuit's three resistors in the voltage divide have the similar value so the comparator notes the voltage at 1/3 and 2/3 of the supply voltage which are affected by the control voltage input. For example, changes in state inside the integrated chip happen when the trigger input goes below +Vcc/3 and makes the output change to +Vcc so the discharge transistor turns off. If the threshold input increases above (2/3)+Vcc, then, the output is at ground and the discharge transistor turns on again. In order to rise the threshold voltage to (2/3)+Vcc, the capacitor (C) is connected to the pin and charged by a resistor (R) since adjusting the R and C values creates close to any timing interval desired.

Applications

The 555 timer works in many different electronics in everyday use. The quartz watch uses the device to keep track of time. Regular AM radio requires carrier waves for the station and, in turn, the AM receiver helps to tune into the station through a resonator. Microprocessors register by clock speed in megahertz (MHz) and determines how fast computers process instructions. Cell phones, pagers and GPS need wireless receivers and transmitters. Audio-frequency equipment utilize the part also for music synthesizers.

References

• The 555 Timer