6.2  The 555 Timer Chip

The 555 timer is a ubiquitous integrated circuit that can be used to output periodic signals loaded up to around 200 mA. The pinout diagram is shown in figure 1. Note the "dot," which can be used to orient the circuit.

A functional block diagram is shown in figure 2. The VCC pin 8 has its voltage divided by three 5 kΩ resistors in green before being grounded by GND pin 1. Pin 2 (TRIG) and pin 6 (THRES) are compared by comparators to the one-third and two-thirds of VCC. Comparators are digital electronic devices that output boolean TRUE when the upper terminal has voltage greater than that of the lower terminal—and output FALSE, otherwise. Their outputs are connected to an SR flip-flop (purple), which gets set to TRUE by terminal S and reset to FALSE by R. This flip-flop has a memory. Once S sets it, even if S goes FALSE, it will remain TRUE. Only if R goes TRUE will the flip-flop return to FALSE. The RESET pin 4 can reset the flip-flop state at any time.

If the flip-flop is set to TRUE, two things happen. First, the output is inverted (the symbol ○ means inversion), and it shuts off the gate for a transistor (blue) to which we will return in a moment. The flip-flop output is inverted again and switches on an output stage (pink), supplying power to the OUT pin 3.

Once the THRES pin resets the flip-flop, the gate transistor is turned on, allowing current to flow from pin 7 (DISCH) to pin 1 (GND).