Lm317 voltage regulator works with 30 vDC wall adopter:
Use 240 ohms resistor for R1 or plug in 1k POT (I used fixed 240 ohm). I used 5k pot for R2.
(i tried this circuit with 9VDC adopter and it did not work for me. The pot might do the trick here. The schematic in datasheet did called for 28 VDC in) It works well with 30 VDC 500mA unregulated wall plug.
Many highly recommend heat sink on the LM317, I touched it and it barely gets warm at all (since there is very little current flow)
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Switching polarity circuit works from 1-30 minutes.
http://www.zen22142.zen.co.uk/ronj/rt2s.html
In this one I used 1M pots + 100k fixed - when I set total resistance to 650k it yields about 5.5 minutes intervals.
Also I got 5VDC DPDT none-latching relay
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auto shut off circuit:
http://silversol.net63.net/systems/csoff.htm
I have not tested this circuit yet.
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To your health,
Gill