![]() There's really not much point in going through the circuit in detail, but one thing that needs to be pointed out is the voltage divider that creates the reference voltages used internally. Schematics from other manufacturers may differ slightly, but the operation is identical. A block diagram is useful, and Figure 1A shows the essential parts of the IC's innards.įigure 1A - Internal Diagram Of 555 Timerįigure 1B shows a complete circuit diagram for a 555 timer, based on the schematic shown in the ST Microelectronics datasheet. A great many of the functions are pre-programmed, but a control input allows the comparator threshold voltages to be changed, and many different circuit implementations are possible. The 555 uses two comparators, a set-reset flip-flop (which includes a 'master' reset), an output buffer and a capacitor discharge transistor. Note that the quad version has only the bare minimum of pins, reset and control voltage are shared by all four timers, and it has no separate threshold and discharge pins (they are tied together internally, and called 'timing'). All pin numbers refer to the 8-pin version, and will need to be changed if you use the dual or quad types, or choose one of the SMD versions that has a different pinout. I don't intend to even attempt to cover all the variations because there are too many, but the following material is all based on the standard 8 pin package, single timer. The 555 and its derivatives come in DIP (dual in-line package) and SMD (surface mount device) packages. There are many different manufacturers and many different part number prefixes and suffixes, and they are available in a dual version (556). The CMOS version (7555) has lower output current and also draws less supply current, and can run from 2V up to 15V. The IC can provide up to 200mA output current (source or sink) and operates from a supply voltage from 4.5V up to 18V. It can function as an oscillator, a timer, and even as an inverting or non-inverting buffer. The use of a microprocessor-based circuit can then convert the pulse period to temperature, linearize it and even provide calibration means.The 555 timer has been with us since 1972 - that's a long time for any IC, and the fact that it's still used in thousands of designs is testament to its usefulness in a wide variety of equipment, both professional and hobbyist. The 555 can be used as a simple ADC, converting an analog value to a pulse length (e.g., selecting a thermistor as timing resistor allows the use of the 555 in a temperature sensor and the period of the output pulse is determined by the temperature). Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation and so on.
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