Forum Diagram

Enter values for R1, R2, and C and press the calculate button to solve for positive time interval (T1) and negative time interval (T2). For example, a 10K resistor (R1) and 100K (R2) and 0.1 uF capacitor will produce output time intervals of 7.62 mS positive (T1) and 6.93 mS negative (T2). The frequency will be about 70 Hz. R1 should be greater than 1K and C should be greater than .0005 uF. Scroll down page for basic 555 information (pinout and two basic diagram).

First introduced by the Signetics Corporation as the SE555/NE555 about 1971.Pin connections and functions: (See schematic below for basic diagram)

Pin 1 (Ground)    - The ground (or common) pin is the most-negative supply potential of the device, which is normally connected to schema common when operated from positive supply voltages.

Pin 2 (Trigger)   - This pin is the input which causes the output to go high and beginthe timing cycle. Triggering occurs when the trigger input movesfrom a voltage above 2/3 of the supply voltage to a voltage below1/3 of the supply. For example using a 12 volt supply, the triggerinput voltage must start from above 8 volts and move down to avoltage below 4 volts to begin the timing cycle. The action islevel sensitive and the trigger voltage may move very slowly. Toavoid retriggering, the trigger voltage must return to a voltageabove 1/3 of the supply before the end of the timing cycle in themonostable mode. Trigger input current is about 0.5 microamps.

Pin 3 (Output)    - The output pin of the 555 moves to a high level of 1.7 volts less than the supply voltage when the timing cycle begins. The output returns to a low level near 0 at the end of the cycle. Maximum current from the output at either low or high levels is approximately 200 mA.

Pin 4 (Reset):    - A low logic level on this pin resets the timer and returns the output to a low state. It is normally connected to the + supply line if not used.

Pin 5 (Control)   - This pin allows changing the triggering and threshold voltages by applying an external voltage. When the timer is operating in the a stable or oscillating mode, this input could be used to alter or frequency modulate the output. If not in use, it is recommended installing a small capacitor from pin 5 to ground to avoid possible false or erratic triggering from noise effects.

Pin 6 (Threshold) - Pin 6 is used to reset the latch and cause the output to go low.Reset occurs when the voltage on this pin moves from a voltage below 1/3 of the supply to a voltage above 2/3 of the supply.The action is level sensitive and can move slowly similar to the trigger voltage.

Pin 7 (Discharge) - This pin is an open collector output which is in phase with the main output on pin 3 and has similar current sinking capability.

Pin 8 (V +)       - This is the positive supply voltage terminal of the 555 timer IC.Supply-voltage operating range  is +4.5 volts (minimum) to +16volts (maximum).

The pin connections for the 556 which is a dual 555 timer (2 in one package) areshown in table below. For example, the two outputs for the two timers of the 556 areon pins 5 and 9 which correspond to the output pin 3 of the 555. 

555      556 timer #1        timer #2

Ground         1                  7               7 

Trigger        2                  6               8

Output         3                  5               9

Reset          4                  4               10

Control        5                  3               11

Threshold      6                  2               12

Discharge      7                  1               13+ 

Power Vcc    8                  14              14

Below is a pictorial view of the 555 timer wired as a LED flasher and powered with a 9 volt battery. The LED will turn on during time T1 and off during time T2.

The 555 schema below is a flashing bicycle light powered with three C or D cells (4.5 volts). The two flashlight lamps will alternately flash at a approximate 1.5 second cycle rate. Using a 4.7K resistor for R1 and a 100K resistor for R2 and a 4.7uF capacitor, the time intervals for the two lamps are 341 milliseconds (T1, upper lamp) and 326 milliseconds (T2 lower lamp). The lamps are driven by transistors to provide additional current beyond the 200 mA limit of the 555 timer. A 2N2905 PNP and a 2N3053 NPN could be used for lamps requiring 500 mA or less. For additional current, a TIP29 NPN and TIP30 PNP could be used up to 1 amp. A PR3 is a 4.5 volt, 500 mA flashlight bulb. Two diodes are placed in series with the PNP transistor base so that the lower lamp turns off when the 555 output goes high during the T1 time interval. The high output level of the 555 timer is 1.7 volts less than the supply voltage. Adding the two diodes increases the forward voltage required for the PNP transistor to about 2.1 volts so that the 1.7 volt difference from supply to the output is not enough to turn on the transistor. You can also use an LED in place of the two diodes as shown in the lower schematic.