AG,
Wondered how that circuit would sound.

 

AG,
Wondered how that circuit would sound.

A harsh buzzer that has its pitch going up and down. I never heard an old mechanical air raid siren.

 

Late to the party, but still confused. The multivibrator timing components have a one second time constant, so approximately 1 second in each of two states. I think the intent is for either the siren oscillator pitch **or** volume to increase for one second, decrease for one second, repeat. The problem here is that the words "raise" and "high level" can apply to either characteristic, frequency or amplitude, with two very different consequences.

TS: What do you mean by "high level": high frequency or high volume?

ak

 

I never heard an old mechanical air raid siren.

A mechanical air horn that rotates once every 5-10 seconds. After an initial ramp-up in volume due to the air compressor coming up to speed, there is a slight doppler change in pitch, and a much more noticeable sinusoidal variation in loudness with each rotation cycle.

ak

 

Here is the simulation resulting from Kmoffett's suggestion, including the collector voltage at Q2.;

and the resulting Vout.:

 

Late to the party, but still confused. The multivibrator timing components have a one second time constant, so approximately 1 second in each of two states. I think the intent is for either the siren oscillator pitch **or** volume to increase for one second, decrease for one second, repeat. The problem here is that the words "raise" and "high level" can apply to either characteristic, frequency or amplitude, with two very different consequences.

TS: What do you mean by "high level": high frequency or high volume?

ak

The simulation clearly shows the audio frequency (pitch) slowly going up and down.
But since the pulses are narrow I think the high frequencies will be louder than the low frequencies (PWM).

 

... a longer duration view at the collector of Q2:

... and the resulting Vout:

 

The higher frequencies on the old WWII air raid siren were louder than the lower frequencies. The faster the horn was cranked, it got higher pitched and louder. I still remember hearing it in England in my childhood, during the blitz.
Keith

 

Increasing the resistor values in the multivibrator but not changing the RC that makes the triangle waveform for the voltage controlled oscillator causes the oscillator to stop at low frequencies.

 

Change R3 and R4 to 47K, C3 to 200uF, and C4 to 0.2uF. I have no idea what it will sound like, but it does go up and down within the astable's cycle.
LSpice is fun!