I am a beginner studying electronics and I have recently learned about the 555 timer. My course includes the following circuit diagram: enter image description here

The goal of this circuit is to have the speaker produce a siren sound, however, I don't understand how the second 555 accomplishes this.

I understand that the output of the first 555 will be fed into the Resistor/Capacitor pair. When the pulse goes HIGH, the capacitor will be charged and once the timer flips, the capacitor will be discharged, which will cause the pulses to look like this. enter image description here

This output is then fed into pin 5 on the second 555. This is the part that I don't understand. What exactly does the second 555 do, that makes the speaker produce a siren sound?

Thank you in advance.

  • 1
    \$\begingroup\$ Have you tried to simulate it? \$\endgroup\$
    – winny
    Mar 23, 2023 at 21:22
  • 2
    \$\begingroup\$ There are several/dozens websites on 555 circuits that cover the 2nd 555 circuit and the modulation input (pin 5). \$\endgroup\$
    – Andy aka
    Mar 23, 2023 at 21:24
  • \$\begingroup\$ draw the 555 timer internal circuit in your schematic diagram ... build-electronic-circuits.com/555-timer \$\endgroup\$
    – jsotola
    Mar 24, 2023 at 1:29

3 Answers 3


Pin 5 of the 555 is the CONTROL pin and is normally held at 2/3 VCC with a resistor divider network internal to the chip. The timing capacitors C1 and C4 charge through R1 and R2 or R4 and R5 respectively, which makes the voltage at pin 6 (THRESHOLD) and pin 2 (TRIGGER) rise. When THRESHOLD rises above the voltage on CONTROL, pin 7 (DISCHARGE) shorts the timing capacitor to ground through R1 or R5, respectively and pulls pin 3 (OUTPUT) low. When TRIGGER falls below 1/3 VCC, OUTPUT is pulled high and DISCHARGE stops shorting the timing capacitor, allowing the cycle to continue.

On the left-hand 555, OUTPUT is used to slowly charge and discharge C2 through R3. This voltage is applied to the CONTROL pin of the right-hand 555 which overrides the internally generated CONTROL voltage and changes the voltages at which TRIGGER and THRESHOLD cause the timer to start and stop its timing cycle. And, since R4/R5/C4 are still being supplied from +V, this changes the frequency of the output.


Since the output of the first one goes into pin 5, the "control" pin of the second timer, it alters the timing response of the second 555. That is to say, it changes the threshold that pins 6 and 2 will switch at. This turns the second 555 into a sort of voltage-to-frequency converter.

This will make the pitch of the siren go up and down.


Looks like the 2nd 555 is a trailing edge cutoff circuit as the output from your initial 555 would sound the siren consistantly "WoooOOOOooooOOOOoooo" with a degree of variable 'loudness'. the 2nd 555 would allow the wave to peak then instantly cut off sounding more like "WooOOp WooOOp"still with rising 'loudness'


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