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Digital 555 Timer IC Schematic Circuit Diagram

Digital Evolution of the 555 Timer IC: Introducing the HCT5555

A digital rendition of the widely recognized Type 555 timer IC has emerged, known as the HCT5555. In its traditional form, the 555 timer can function as either astable or monostable, with timing dictated by an RC network. However, this conventional version faces limitations; crystal control is unattainable, and when the time constant extends, precision tends to diminish.

HCT5555: Tailored for Monostable Operation with Expanded Capabilities

The HCT5555 is specifically designed for monostable operation; if astable functionality is desired, several external components must be incorporated (refer to the diagram). Notably, the timing mechanism is orchestrated through a distinct integral oscillator and a programmable divider. Unlike its predecessor, this advanced device offers a broader array of features. Consequently, it is accommodated in a 16-pin package, deviating from the 8-pin configuration of the classic 555 timer.

Digital 555 Schematic diagram

The chip has two trigger inputs: one for first transitions (A) and one for last transitions (B). These inputs can be interlinked: the mono time, determined by the oscilla-tor and the set scaling factor, then starts at each and every transition of the input When one of the inputs is actuated, output Q goes high. There is a complementary output, Q. Some of the new terminals are:

  • MR (pin 15) is the Master Reset;
  • retriggerable (pin 16), which determines whether the IC reacts to trigger pulses when the mono time has not yet lapsed;
  • OSC (oscillator) CON (control) (pin 14), via which the integral oscillator can be stopped
  • Pins 1, 2, and 3 are used for the oscillator, while pins 10–13 determine the scaling factor (see table).

New Device Application: Precision Timer for NiCd Battery Charging

One potential use of the innovative device is demonstrated in the diagram, showcasing its role as a timer. In this application, it signals when, for example, NiCd batteries have been charging for precisely 14 hours. At this point, the LED either turns off or illuminates (if the Q output is utilized). The oscillator’s frequency is fixed at 333 Hz, established by P1 (with R = 1.84 kΩ), and no switches are engaged. They are either left open or omitted altogether.

Variable Supply Voltage and Low Power Consumption

The suitable supply voltage for the device hinges on the specific IC variant employed. The HCT version operates within the range of 4.5–5.5 V, while the HC version functions effectively with a broader voltage span of 2–6 V. Regardless of the IC version chosen, the device draws a minimal current of approximately 0.5 mA, ensuring energy efficiency in its operation.

Digital 555 Schematic diagram

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