Created by
Created February 04, 2012
Last modified June 02, 2013
Tags 555   oscillator   time-constant  

Summary

A 555 timer can be used with a few resistors and a capacitor to make an oscillator, switching states as the capacitor charges and discharges.


Description

The 555 timer chip has been a staple of electronics tinkerers for decades as it can be configured into a wide range of different modes with just a handful of external components. Here, the capacitor C1 holds charge. When in the "charging" mode, the capacitor charges through R1 and R2, and its voltage approaches +5V. When the voltage reaches above 2/3rds of VCC = 5 volts, the "threshold" input fires, toggling the output pin, and turning on the discharge transistor. The discharge pin becomes grounded via this transistor, so the capacitor discharges via R2. When the capacitor voltage reaches below 1/3rd of VCC, the "trigger" setpoint is hit, and the output is again toggled and the discharge transistor is disabled. The cycle repeats indefinitely!

A few things to try in CircuitLab

Load the simulation by clicking "Open in editor" above, and then click "Simulate" at the bottom. Run a time domain simulation.

Change the frequency

Changing R2 to 2K from its original 1K will make the capacitor charge more slowly, and in this case we'll end up with an oscillator output at about half the original frequency.

Change the duty cycle

Changing R1 allows for a significantly non-50%-duty cycle, meaning that the oscillator output might be high a bigger or smaller fraction of the time than it spends low. That's because the capacitor charges through resistance (R1+R2), but discharges only through R2. Try making R1's resistance 3K ohms, and look at how much the duty cycle of the output changes.

Be careful if you make R2 too small (below about 100 ohms) -- this may prevent the 555's discharge transistor from successfully draining the charge from the capacitor!

See also

555 Timer as Pulse Width Modulation (PWM) Generator:


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