Hi there,

I want to make a 555 siren that switches between 3,000 and 4,000 hz back and forth with around 100 miliseconds in between. I've managed to make the 3,000 and 4,000 sounds using the 555 timer, however im not sure how to switch back and forth between them.

I managed to get a 555 4khz sound to go high and low using the output from a 555 5hz sound going to the control pin on the 4kz.

Do i need a 3rd, slow oscillating 555 timer to switch back and forth between these two?

 

There are several approaches to this.

Are you aware a 555 has a input that can be treated like a VCO (Voltage Controlled Oscillator)? You can feed the 555 a square wave with the two peaks of the square wave at the required voltage. The square wave would be made with a second 555 and modified externally to make the voltage requirements match your needs.

Something your interested in?

I've never done this exactly, though I'm sure someone has, so some experimentation would be required. Personally I think it is gravy.

Your suggested approach would also work.