Bill_M wrote:

0.001 0.001 uF @400 V*
|| || |
----||-----||---------------------|------------
|| || | | |
| | 9V
120 VAC --- --- to smoke detector
^ _
| |
10K | |
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My smoke detector draws 7 uA = 60Hzx2xsqrt(2)x120VxC, which makes C = 344 pF,
eg 2 0.001 uF 400 V caps in series, Digikey's P1058-ND, $11.43/100. They also
sell 12 V zeners suitable for 9 V batteries, 1N5242BDICT-ND, $13.65/100.

Of course you don't have enough current for the audible device so your smoke
detector should sit quietly by while you toast!

No. That current would come from the 9 V trickle-charged battery.

We might replace it every 6 years or so, if the alarm
fails to shriek with the test button.

Nick