Timing Basic timer IC - 555 description of operation

Chapter-3a Page 12

This simplest of timers will flash the LED at a constant rate. It could be used anywhere if powered by a DCC derived Utility Power

Construction:

The components are mounted on a 9 by 5 row piece of strip board. On the copper side mark the holes to be cut and after double checking them use a #31 drill twisted in the hole to remove the copper so the strip continuity is broken. On the top side install the two jumpers. Note one is under the IC, use an uninsulated wire here to reduce height.

Insert the resistors and capacitor. Turn the board over and solder the leads to the copper strip. Check the relationship to the cut holes before you solder. It is easy to make a mistake. Solder everything, install the power wires and wires to the LED. That is about it. Check carefully that there are no solder bridges between strips, use a magnifier or meter. Apply power, 9 to 12 volts, and the LED should start flashing. For two LEDs alternating, install an LED and 680 Ohm resistor between pin 3 and ground.(0 v)

The Basic Timer

The Schematic:

When first powered on the capacitor is at 0 volts and this enables the trigger, pin 2. The output at pin 3 will go positive but is not used. The capacitor then charges through the 1 meg Ohm resistor and LED. When pin 6 is at 2/3V voltage point, the output and pin 7 will go to 0 volts (ground). This turns the LED on. The capacitor is now discharged to the 0 volts at pin 7 until the 1/3V point is reached and it retriggers.

The 555 Timer

This timer was first made in 1971 and has been produced in billions since. It is a very cheap, popular and useful precision timing device that can be used either as a single pulse generator, long time delay or an oscillator. The 555 timer chip is an extremely robust and stable 8-pin device that can operate in many different ways. For timing, driving or as a bi-stable memory. It is available in standard, low power or dual forms. Perhaps the most used Integrated Circuit of all times by the general experimenter, it forms the basis of many of these projects.

The heart of the device is the three resistors that form a voltage divider chain. The 1/3 V comparator will produce a low output when the Trigger is below the 1/3 V level to set the flip flop. This makes the output positive at close to the +ve voltage potential. The 2/3 V comparator will produce a low output when the threshold pin is above the 2/3 V level. This will reset the flip flop. The discharge pin is a grounded level when the flip flop is not set and a floating condition while the flip flop is set. It can also be used to drive external components. When reset pin 4 is at ground it holds the flip flop in a reset condition which over rides the set input.