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Digital 555

There is a digital version of the 200 well-known Type 555 timer IC: the HCT5555. The traditional 555 can astable or monostable, within, be used a the timing determined by an RC network. Crystal control is not possible and when the time constant is long, accuracy goes by the way.

The HCT5555 is intended purely as a monostable: if operation as an astable is required, a number of external components must be added—see diagram. The timing is arranged, however, by a separate integral oscillator and the programmable divider. Owing to the greater number of facilities, the new device is not housed in an 8-pin case like the 555, but in a 16-pin package.

The chip has two trigger inputs: one for first transitions (A) and one for last transi-tions (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.

A possible application of the new device is the timer as shown in the diagram that indicates when, for instance, NiCd batteries have been in charge for 14 hours. The LED then goes out (or just comes on if the Q output is used). The oscillator is set to 333 Hz with P1 (R =1.84 Id2). Leave all switches open or omit them altogether.

The supply voltage depends on the type of IC used: the HCT version needs 4.5-5.5 V. whereas the HC version can operate from 2-6 V. The IC draws about 0.5 mA.
Digital 555 Schematic diagram

Digital 555 Schematic diagram

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