In some digital circuits, you must press a reset button right after you have switched on the power in order to bring the circuit to an stabilized condition.
This resetting can be done automatically by this circuit. It generates a resetting pulse after the power is turned on. The reset pulse is also generated when the power has dropped to
a certain low level for a certain period of time. The reset pulse is about 30 mS long.
The circuit features a very high level of immunity from short voltage disturbances appearing at the power supply lines.
Parts Placement Layout
The circuit functions in a very simple way: Right after the power supply is turned
on, the voltage drop at the capacitor C1 remains at 0 volt until the power supply level reaches 4.5 volts. When this level is reached, transistor T1 begins to conduct while T2 shuts off.
At this moment, the capacitor C1 charges up via R5 at a certain time constant. At this very same moment, the junction of R4,R5, and R6 drops down to zero volt.
The output of U1 will become a logical 1 when a certain voltage level appears at the junction of R4,R5, and R6. Another advantage of this circuit is that
it generates a reset pulse when a short power supply interruption appears. This automatic resetting
is highly advantageous for microcomputer circuits which need to automatically reset themselves and load their boot or initialization program after a short power supply interruption.
Electronic Circuits volume 1.0 - Circuit Nr. 89
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.