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HOW TO USE THE CALCULATOR


Enter values for R1, R2, and C and press Calculate to determine HIGH interval and LOW interval. For example, a 10k (R1), 100k (R2) and 100n will produce output time intervals of 7.62mS HIGH and 6.93mS LOW. The frequency will be about 70Hz and a Delay of 0.01sec. R1 should be greater than 1k and C should be greater than 1n.

SUBSTITUTING A 555

A 555 (and the 556 varieties) can be replaced by low-power 555's such as TLC555, LMC555, ICM7555. This has been covered above.

In this section we will show how to replace any of the 555 or 7555 devices with a building block called a SCHMITT TRIGGER. The Schmitt Trigger chip we suggest is a 74c14 (40106 -CD 40106). This chip contains six Schmitt Triggers. It allows up to 6 building blocks to be created, similar to the capabilities of a 555.

This is a much-more economical and professional way to designing a circuit and two other very important features are also provided.

The Schmitt Trigger consumes less current and battery designs can be created.
A Schmitt Trigger does not put noise on the power rails of a project and it can be used with other digital blocks without creating interference problems.

Six gates in a single hex Schmitt trigger chip allows the designer to produce 6 different building blocks and quite complex circuits can be produced.
The type of 555 circuit we are suggesting be replaced with a Schmitt design is one that meets one of more of the following criteria:
1. A design that needs to be upgraded and improved in "professionalism."
2. A design that needs to be reduced in quiescent current,
3. A design that uses more than one 555
4. A design that employs 555 IC's with digital IC's.
A simple 555 design for a car, for example, does not need to be converted.

THE 555 OSCILLATOR
The following diagrams show a free-running 555 oscillator and its Schmitt Trigger equivalent.
The circuit can be called an OSCILLATOR, SQUARE-WAVE OSCILLATOR or FREE-RUNNING OSCILLATOR.
The 555 can sink or source 200mA and the two diagrams show this:

The only difference between the two circuits is the Schmitt version will draw about 10mA -15mA less.
The 555 draws about 10mA for its internal operation and about 1mA - 5mA will be "wasted" through R2.
If the load is less than 25mA, the following circuits can be used:

The output of a single Schmitt Oscillator will drive a load up to 25mA, depending on the frequency of oscillation and the voltage of the supply. As the voltage decreases, the load current reduces. At 5v, the load will be a maximum of 10mA.
As the load current increases, the output will not rise to 66% of rail voltage and the oscillator will "freeze."