NE555 Timer IC
The NE555 Timer IC is an integrated circuit (chip) used in a variety of timer, delay, pulse generation, and oscillator applications. Derivatives provide two (556) or four (558) timing circuits in one package. It was commercialized in 1972 by Signetics and it was reported to still be in wide use as of 2013.
Numerous companies have made the original bipolar timers and similar low-power CMOS timers too. In 2017, it was said over a billion NE555 Timer ICs are produced annually by some estimates, and “probably the most popular integrated circuit ever made.
The NE555 Timer IC was designed in 1971 by Hans Camenzind under contract to Signetics. In 1968, he was hired by Signetics to develop a phase-locked loop (PLL) IC. He designed an oscillator for PLLs such that the frequency did not depend on the power supply voltage or temperature. Signetics subsequently laid off half of its employees due to the 1970 recession, and development on the PLL was thus frozen.
Camenzind proposed the development of a universal circuit based on the oscillator for PLLs and asked that he develop it alone, borrowing equipment from Signetics instead of having his pay cut in half. Other engineers argued the product could be built from existing parts; however, the marketing manager approved the idea.
The first design for the NE555 Timer IC was reviewed in the summer of 1971. Assessed to be without error, it proceeded to layout design. A few days later, Camenzind got the idea of using a direct resistance instead of a constant current source finding later that it worked.
The change decreased the required 9 pins to 8 so the IC could be fit in an 8-pin package instead of a 14-pin package. This revised design passed a second design review with the prototypes completed in October 1971 as the NE555V (plastic DIP) and SE555T (metal TO-5).
The 9-pin copy had been already released by another company founded by an engineer who attended the first review and retired from Signetics; that firm withdrew its version soon after the 555 was released. The NE555 Timer IC timer was manufactured by 12 companies in 1972 and it became a best selling product.
Several books report the name NE555 Timer IC comes from the three 5 kilo-ohm resistors inside the chip. However, in a recorded interview with an online transistor museum curator, Hans Camenzind said “It was just arbitrarily chosen. It was Art Fury (Marketing Manager) who thought the circuit was gonna sell big who picked the name ‘555’.
These devices are precision timing circuits capable of producing accurate time delays or oscillation. In the time-delay or mono-stable mode of operation, the timed interval is controlled by a single external resistor and capacitor network. In the a-stable mode of operation, the frequency and duty cycle can be controlled independently with two external resistors and a single external capacitor.
The threshold and trigger levels normally are two-thirds and one-third, respectively, of VCC. These levels can be altered by use of the control-voltage terminal. When the trigger input falls below the trigger level, the flip-flop is set, and the output goes high.
If the trigger input is above the trigger level and the threshold input is above the threshold level, the flip-flop is reset and the output is low. The reset (RESET) input can override all other inputs and can be used to initiate a new timing cycle.
When RESET goes low, the flip-flop is reset, and the output goes low. When the output is low, a low-impedance path is provided between discharge (DISCH) and ground.
The output circuit is capable of sinking or sourcing current up to 200 mA. Operation is specified for supplies of 5 V to 15 V. With a 5-V supply, output levels are compatible with TTL inputs.
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