The 555 timer can be used in monostable mode to divide a known frequency. This is due to the fact that the 555 cannot be retriggered during its timing cycle.
The simple frequency divider circuit is actually the basic circuit of a 555 in monostable (one shot) mode. The frequency to be divided is applied to trigger input (pin 2). The negative edge of the applied signal triggers the timer and the C1 capacitor starts charging. During the charging process, further incoming pulses have no effect. The capacitor charges to its threshold value. Then, C1 discharges rapidly and the circuit waits in this state to be triggered by the next incoming pulse.
Simple frequency divider circuit
If R1 and C1 are chosen properly, the circuit can be made to trigger on second, third, fourth or fifth or more pulses and the output will be a frequency equal to the input frequency divided by that number.
