Uploaded July 2012 | Updated September 2026, 1 week ago

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The 555 Timer is a famous and versatile timing chip. Thanks to the excellent book _Practical Electronics for Inventors_ I can finally explain what's going on.

Here it's running in astable (oscillating) mode. Refer to the schematic or similar here: circuits-diy.com/simple-tone-generator-circuit-using-ne555-timer-ic
The actual timing is driven by an RC circuit external to the IC that charges and discharges. The 555 timer consists of a voltage divider (to 1/3 and 2/3 Vcc), two comparators, an RS latch, and discharge transistor.
At the start, the transistor is off and the capacitor charges through R1 and R2. When Threshold reaches 2/3 Vcc, the first comparator outputs high, resetting the RS latch. Inverted Q turns on which turns on the transistor. This shorts Discharge (which is wired to Threshold) through Ground so the capacitor discharges through R2. When Trigger goes below 1/3 Vcc, the other comparator outputs high, setting the RS latch and turning off the transistor. Thus the capacitor starts charging again and the cycle repeats.

In this circuit, R1 is a 1 MΩ potentiometer used as a rheostat (variable resistor), R2 is 1 kΩ resistor, and C is a .01 μF capacitor.
t_lo = 0.693 R2 C1
t_hi = 0.693 (R1 + R2) C1
T (cycle period) = t_lo + t_hi
Frequency = 1 / T, approx. 1.44 / (R1 + 2 R2) C

You can use a log-log plot to estimate the frequency based on the capacitance and resistor values: 555-timer-circuits.com/astable-multivibrator.html

Original description:
Fun!
Questions?
Schematic reconnsworld.com/audio_tonegenerate.html