Please excuse my schematic.


This is a Dim-bulb Tester I am building. This is supposed to protect the device under test (DUT) that I will plugin into "outlet", by limiting the current, "forwarding" it to the 60W bulb in case of a short within the DUT.

Wall socket is at 220 VAC.

"main neon" is rated 220 VAC / 20mA, and is supposed to light red when I turn on the "main switch". If the DUT seems fixed (that is to say if the bulb does not glow at full brightness), I intend to turn on the "bypass switch", in order to bypass the bulb.

My problem is the following : I want to wire an other neon lamp that would light green only when the "bypass switch" is turned on.

  • 1
    \$\begingroup\$ use a 2 pole switch \$\endgroup\$
    – Neil_UK
    Sep 29, 2021 at 4:15

1 Answer 1


This is one possible wiring, shown at the top. When the main switch is on, the bypass neon will glow when the bypass switch is closed.


simulate this circuit – Schematic created using CircuitLab

It may be safer to the device under test (DUT) to have the alternative wiring shown at the bottom. The bypass neon will be alight when the bypass switch is closed, whether the main switch is on or not. Then you will know before you close the main switch whether the DUT is going to get a protected supply or not. It might be better to have the bypass neon red rather than green, as it shows a danger to the DUT.

  • \$\begingroup\$ Would a "DPST 2NO rotary switch with 2 positions" do the trick? \$\endgroup\$
    – Musa
    Sep 29, 2021 at 21:22

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