nick pine wrote:

0.022 uF @400 V*
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| | 3V
120 VAC --- --- to clock
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10K | |
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*Digikey's $29.95 M400-KIT-ND 400 V capacitor assortment contains
10 of these, along with 10 of 14 other values from 0.001 to 0.1 uF.

The rest of this trickle charger is cheap...

$13.65 for 100 1N5231BD1CTND 5.1V zeners,
$15.86 for 1000 P10KBACTND 10K resistors,
and a line cord cut off an old appliance.

My smoke detector draws 7 uA = 60x370C, which makes C = 315 puffs, 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.

Nick