555 IC Project - 'Red Alert' Siren query.

[Yorkie]

I wonder if anyone might like to comment on the attached circuit please.

In looking at the circuit, I'm unsure about the role of the 1N4148 diode - 'D1'.

I'm puzzled by the polarity of that diode as to my simple mind it seems to be back to front, in that as shown, it will surely block the output from pin 3 of IC1 from getting to IC2.

From my limited understanding on 555s, in an astable multivibrator configuration, (using the circuit below as an example), the combination of R1, R2 and C1 (10k, 75k, 10uF)  set the frequency of IC1, which (using an online calculator I must stress!) I believe will be just under 1 Hz. Maybe that is intended to be the repetition rate of the 'red alert' sound? The combination of R4, R5 & C3 (10k, 100k, 0.01uF) should I think produce a frequency of 687Hz from IC2 - the audible 'alarm sound'.

I might of course be quite mistaken in my understanding of the operation of the circuit. If so, no surprises there then!  

I haven't built it - someone else has (on strip-board)  and it doesn't work!

Maybe he's made an error or maybe there's an error in the circuit?

I've designed a PCB for it, (which - if the circuit is wrong will only replicate such errors) so might built it to see what sound (if anything!) it produces.

PCB & circuit attached.

Any thoughts please anyone?