An interesting circuit to build with the 555 timer IC is a siren that uses two 555 timer ICs. We will be using the control voltage pin, pin 5 to create the siren effect.
The siren we will be generating has two distinct frequencies or rising and falling sections. IC1 is configured in the usual astable mode with a frequency of about 1Hz and the output of this controls the rising and falling sections. IC2 is also wired in a similar manner with a base frequency of about 480Hz. The output of this is passed through a capacitor and is then fed to a speaker. The capacitor blocks the DC component of the output voltage which is needed for proper operation of the speaker.
Now, to create the actual siren effect, we connect the output from IC1 to the control voltage pin on IC2 which controls the threshold voltages for the internal comparators. This means that the threshold voltages for IC2 will be dependent on the output state of IC1. When IC1 is HIGH, it increases the threshold voltages of IC2 and this decreases the output frequency, giving us the LOW phase of the siren. When IC1 is LOW, it lowers the threshold voltages of IC2 and this increases the output frequency. The resulting sound is very similar to a siren.
This is what the assembled circuit looks like and we will be connecting it to the 5V output of the power supply board. The siren output is not perfect, but it is very good considering that we only used two 555 timer ICs and some few passive components. Let’s move on to the next project.