Electronics, programming & other stuff

Timers using 555

555 as Monostable

Given the value of the resistor and capacitor, this page calculates the high time of the output signal, in a 555 IC used as monostable.

R: Ohms

C: Farads


555 as monostable schematic

555 temporal diagram

555 as Astable

Given the value of the resistors and the capacitor, this page calculates the period, frequency and duty cycle, of the 555 IC used in an astable configuration

R1: Ohms

R2: Ohms

C: Farads






555 as astable schematic

555 temporal diagram

Generally, the power supply needed for the 555 should be between 3 and 15 volts. Regarding the input voltage (in the monostable configuration), it shouldn't exceed VCC, and the detection threshold is 0.67*VCC (for the high side) and 0.33*VCC (for the low side)

To use relatively big times (in the order of tens or hundreds of seconds), the usage of a variation of the IC is needed. The 7555 IC is built using CMOS technology, and has less input and parasitic currents, so it allows the usage of bigger resistors and capacitors, allowing to get more accurate timings and better efficiency.

The capacitor in pin 5 helps to stabilize the circuit and attenuates the effect of external noise. For small prototypes it might be removed without harm, but it should be used when possible.