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A HIGH RESOLUTION / PRECISION THERMOMETER USING ST7 AND NE555

1 DESCRIPTION OF NE555

The NE555 monolithic timing circuit is a highly stable controller capable of producing accurate

time delays or oscillation. In the time delay mode of operation, the time is precisely controlled

by one external resistor and capacitor. For a stable operation as an oscillator, the free running

frequency and the duty cycle are both accurately controlled with two external resistors and

one capacitor. For more details see NE555 datasheet.

2 ASTABLE OPERATION

The circuit is shown in Figure 1. (pin 2 and 6 connected). It triggers itself and operates as a

the self-triggered mode, the charge and discharge times and therefore frequency are inde-

pendent of the supply voltage.

Figure 1. Circuit Diagram of NE555 in a-stable mode

The charge time (output HIGH) is given by

(1)

and the discharge time (output LOW) by

(2)

Thus the total period T is given by

(3)

Output

Control

Voltage

0.01µF

4

3

5

1

2

6

7

8

NE555

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