Introduced in 1972 by Signetics,the 555 is still in widespread use due to its low price, ease of use, and stability. It is now made by many companies in the original bipolar and in low-power CMOS. As of 2003[update], it was estimated that 1 billion units were manufactured every year. The 555 is the most popular integrated circuit ever manufactured.
History
In 1962, Camenzind joined PR Mallory's Laboratory for Physical Science in Burlington, Massachusetts. He designed a pulse-width modulation (PWM) amplifier for audio applications,but it was not successful in the market because there was no power transistor included. He became interested in tuners such as a gyrator and a phase-locked loop (PLL). He was hired by Signetics to develop a PLL IC in 1968. He designed an oscillator for PLLs such that the frequency did not depend on the power supply voltage or temperature. However, Signetics laid off half of its employees, and the development was frozen due to a recession.
Camenzind proposed the development of a universal circuit based on the oscillator for PLLs, and asked that he would develop it alone, borrowing their equipment instead of having his pay cut in half. Other engineers argued the product could be built from existing parts, but the marketing manager bought the idea. Among 5xx numbers that were assigned for analogue ICs, the special number "555" was chosen.
Camenzind also taught circuit design at his nearby university in the morning, and went to the Northeastern University to get the master's degree at night. The first design was reviewed in the summer of 1971. There was no problem, so it had gone to the layout design. A few days later, he got the idea of using a direct resistance instead of a constant current source, and found 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 design passed the second design review, and the prototype was completed in October 1971. Its 9-pin copy had been already released by another company founded by an engineer who attended the first review and retired from Signetics, but they withdrew it soon after the 555 was released. The 555 timer was manufactured by 12 companies in 1972 and it became the best selling product.
Design
The NE555 parts were commercial temperature range, 0 °C to +70 °C, and the SE555 part number designated the military temperature range, −55 °C to +125 °C. These were available in both high-reliability metal can (T package) and inexpensive epoxy plastic (V package) packages. Thus the full part numbers were NE555V, NE555T, SE555V, and SE555T. It has been hypothesized that the 555 got its name from the three 5 kΩ resistors used within,but Hans Camenzind has stated that the number was arbitrary.Low-power versions of the 555 are also available, such as the 7555 and CMOS TLC555.The 7555 is designed to cause less supply noise than the classic 555 and the manufacturer claims that it usually does not require a "control" capacitor and in many cases does not require a decoupling capacitor on the power supply. Those parts should generally be included, however, because noise produced by the timer or variation in power supply voltage might interfere with other parts of a circuit or influence its threshold voltages.
Pins
Pin | Name | Purpose |
---|---|---|
1 | GND | Ground reference voltage, low level (0 V) |
2 | TRIG | The OUT pin goes high and a timing interval starts when this input falls below 1/2 of CTRL voltage (which is typically 1/3 VCC, CTRL being 2/3 VCC by default if CTRL is left open). More simply we can say that OUT will be high as long as the trigger is kept at low voltage. Output of the timer totally depends upon the amplitude of the external trigger voltage applied to this pin. |
3 | OUT | This output is driven to approximately 1.7 V below +VCC, or to GND. |
4 | RESET | A timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides THR. |
5 | CTRL | Provides "control" access to the internal voltage divider (by default, 2/3 VCC). |
6 | THR | The timing (OUT high) interval ends when the voltage at THR ("threshold") is greater than that at CTRL (2/3 VCC if CTRL is open). |
7 | DIS | Open collector output which may discharge a capacitor between intervals. In phase with output. |
8 | VCC | Positive supply voltage, which is usually between 3 and 15 V depending on the variation. |
look for 555circuits in future Circuit Mondays
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