Summary of 555 Timer Teardown
The article explains the history, function, and internal structure of the 555 timer IC, describing how its resistor divider, comparators, flip-flop, and external capacitor create a stable oscillator, and how the silicon die and metal interconnects are arranged inside the chip.
Parts used in the 555 Timer Teardown:
- 555 timer integrated circuit (IC) package (plastic or metal can)
- Silicon die
- Metal interconnect layer on die
- Silicon dioxide insulating layer
- Contact holes between metal and silicon
- Thin bond wires connecting die pads to external pins
- Three internal resistors forming a resistor divider
- Two comparators (upper/threshold and lower/trigger)
- Flip-flop (bistable latch)
- Discharge transistor
- External resistor network (for timing configurations)
- External capacitor (timing element)
- Supply voltage pin
- Reset pin
- Control voltage pin
If you’ve played around with electronic circuits, you probably know[1] the 555 timer integrated circuit, said to be the world’s best-selling integrated circuit with billions sold. Designed by analog IC wizard Hans Camenzind[2] in 1970, the 555 has been called one of the greatest chips of all time with whole books devoted to 555 timer circuits.
Given the popularity of the 555 timer, I thought it would be interesting to find out what’s inside the 555 timer and how it works. While the 555 timer is usually sold as a black plastic IC, it is also available in a metal can, which can be cut open with a hacksaw[3] revealing the tiny die inside.
A brief explanation of the 555 timer
The 555 timer has hundreds of applications, operating as anything from a timer or latch to a voltage-controlled oscillator or modulator. The diagram below illustrates how the 555 timer operates as a simple oscillator. Inside the 555 chip, three resistors form a divider generating references voltages of 1/3 and 2/3 of the supply voltage. The external capacitor will charge and discharge between these limits, producing an oscillation. In more detail, the capacitor will slowly charge (A) through the external resistors until its voltage hits the 2/3 reference. At that point (B), the upper (threshold) comparator switches the flip flop off and the output off. This turns on the discharge transistor, slowly discharging the capacitor (C). When the voltage on the capacitor hits the 1/3 reference (D), the lower (trigger) comparator turns on, setting the flip flop and the output, and the cycle repeats. The values of the resistors and capacitor control the timing, from microseconds to hours.[4]
To summarize, the key components of the 555 timer are the comparators to detect the upper and lower voltage limits, the three-resistor divider to set these limits, and the flip flop to keep track of whether the circuit is charging or discharging. The 555 timer has two other pins (reset and control voltage) that I haven’t covered above; they can be used for more complex circuits.
The structure of the integrated circuit
The photo below shows the silicon die of the 555 through a microscope. On top of the silicon, a thin layer of metal connects different parts of the chip. This metal is clearly visible in the photo as yellowish-white traces and regions. Under the metal, a thin, glassy silicon dioxide layer provides insulation between the metal and the silicon, except where contact holes in the silicon dioxide allow the metal to connect to the silicon. At the edge of the chip, thin wires connect the metal pads to the chip’s external pins.
Read more: 555 Timer Teardown
- What is the basic function of the 555 timer?
The 555 operates as timers, latches, oscillators, or modulators by charging and discharging an external capacitor between 1/3 and 2/3 of the supply voltage using internal comparators, a resistor divider, and a flip-flop. - How does the 555 generate reference voltages?
Three internal resistors form a voltage divider that provides reference voltages at 1/3 and 2/3 of the supply voltage. - How is the oscillation produced in the 555 timer?
The external capacitor charges through resistors until it reaches 2/3 Vcc triggering the threshold comparator to turn the output off and enable discharge; it discharges until 1/3 Vcc triggering the trigger comparator to set the output, repeating the cycle. - What internal components determine the timing interval?
The resistor values and the external capacitor control the timing interval from microseconds to hours. - What additional pins does the 555 have besides the timing pins?
The 555 includes reset and control voltage pins for more complex configurations. - What does the die of a 555 timer look like under a microscope?
The silicon die shows a metal interconnect layer as yellowish-white traces, with silicon dioxide insulation and contact holes, plus bond wires at the chip edge connecting to external pins. - What role do the comparators play inside the 555?
The upper comparator detects when the capacitor reaches 2/3 Vcc to reset the flip-flop, and the lower comparator detects 1/3 Vcc to set the flip-flop. - Can the metal can package be opened to see the die?
Yes, the metal can package can be cut open to reveal the die inside, as described in the teardown.
