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What Is a 555 Chip?

Developed in the early 1970s, the 555 timer chip remains a popular component; at time of publication, chipmakers still produce a billion of them a year. Its modular, building-block design makes it useful for a wide variety of electronic timing and signal generation applications. Casual hobbyists appreciate the 555 for its simplicity; professional engineers find it useful and economical.

Description
- The 555 chip is an eight-pin integrated circuit with a standard dual-inline package. It requires a handful of external resistors and capacitors to function. Depending on how those components connect to the chip, the 555 operates in astable, monostable or bistable modes, producing a single timing pulse or steady stream of them. Its timing cycle ranges from microseconds to hours.
Building Blocks
- The 555 design configures its 28 transistors into four main blocks, namely: a flip-flop, two comparators and an output stage. A flip-flop is a type of electronic switch, with two stable states, on and off. An incoming current pulse changes the switch̵7;s state and it remains that way until it receives another pulse. The comparators compare two incoming voltages and switch to an on or off state depending on which voltage is greater. The output stage protects the 555̵7;s inner circuitry and gives it enough power to drive modest loads.
Power
- The 555 chip has a relatively low-power design; on its own, it uses about 15 milliamps of current. The output stage can drive light emitting diodes and low-power devices such as small speakers and relays. It can produce a maximum of 1180 milliwatts of power. For more demanding loads, such as motors and incandescent lamps, an external transistor easily extends the 555̵7;s power capabilities.
Uses
- A 555 timer can turn a light-emitting diode on and off at a specified rate, such as twice per second. You can vary the percentage of on time by changing the values of the timing chip̵7;s external resistors, so the LED can flash briefly or stay on for longer periods. A speaker connected to the 555̵7;s output will produce a set of clicks like a metronome. By using smaller values for the resistors and capacitors, the 555 will run faster, producing an audio tone. The timer also produces single pulses; an input trigger from a momentary switch turns the 555 on for 15 seconds, for example, after which time it turns off and waits for another trigger. This can drive a motor to open a gate.

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