Switching on a light with a separate, low-voltage circuit

I’ve got an old light switch, the kind you twist on and off (think of the one in Clara Bow’s apartment in It). “June '98” is stamped on the interior of it, and I presume it isn’t speaking of 19–. Anyway, I’d like to use it to toggle a couple sconces in new library I’m building in the basement. My first thought was to just wire it up and use it as-is, but it isn’t like the push-button switches I’ve used elsewhere that look safe as any modern switch, it really looks rather scary.

Is there any way (I’m sure there is, but any good way) of using the switch to control a low-voltage circuit of some sort that in turn controls the main household circuit with the lamps on it using modern parts to handle the latter?

A “relay” is a device that does exactly what you are looking for.

You will also need a power supply for the pilot side. ( low voltage control)

To elaborate on Kevbo’s reply -

You will need a low voltage power supply of some type and a relay whose coil voltage matches that of your power supply. The relay contacts will need to have a current and voltage rating capable of handling your lighting load.
Your low voltage will pass through the switch to the relay coil which will close the relay contacts. The high voltage will then pass through these contacts to your lights or what ever.
I don’t know how available they are to the general public, but I would recommend a Functional Devices RIB (Relay in a Box) type. They are set up to accept a wide range of voltages (usually 10-30 VAC or VDC and 120 VAC) and their contacts are rated at 10 A, 250 VAC. Other coil voltages and relay ratings are available. This type of relay is also available from a umber of other suppliers. The beauty of it is that whatever low voltage source you have around, it will probably be adequate to drive this relay.

You can get a doorbell transformer at Radio Shack. You should also be able to buy a 24 V. a.c. relay at the same place. Use your switch to connect and disconnect the relay coil from the output of the bell transformer. This will open and close the relay contacts which will connect and disconnect other house circuits from the 120 V. You have to make sure the relay contacts are rated for the current that your are intending to switch. If you are intending to switch a lot of current you might need to use a relatively low current 24 V. relay to switch a heavy duty relay with contacts rated on the order of 50 Amp. which would be more along the lines of a motor controller. In that case the low voltage relay would only have to provide the current to operate the heavy duty one.

I like DS’s idea of using a transformer. You don’t want 120 VAC to show up on that old switch due to arcing and isolation concerns. In addition, I’d go ahead and stick an RC snubber network across the switch contacts, or across the relay coil. This will further help reduce arcing across the switch contacts.

You can also buy varactors at Radio Shack. These are back to back zener diodes that break down and quench the voltage when it exceeds about 200 V. This reduces contact arcing considerably.

On the RIB:
Because of the wide “coil” voltage range, and the fact that only AC load radings are provided, it is almost certain that these are solid state relays (SSR) but I can’t find where it actually says that.

SSRs don’t need a snubber or freewheel components, as they don’t really have a coil. The input circuit is an optoisolator, which has negligable inductance.

Also, doorbell circuits are typically 8-12VAC. 24VAC transformers are widely available for HVAC pilot duty. Lots cheaper to buy them at the home center than ratshack.

DS, I can’t believe I’ve never heard of the RIB. Dang, I’ve been solving that problem harder ways for like 20 yrs. :smack: Now if they’d put the pilot connections outside the NPT boss, I could keep the pilot circuit physically isolated from the line side.

I should have been thanking Ruhbarb.

Also, I downloaded the data sheets, and the RIBs are NOT SSRs, so a snubber network would be a good idea.

Our house has low-voltage switching circuits everywhere. The light switches are momentary rocker switches.

The guts of the system is a little transformer-relay unit located in the ceiling box. Household current comes to the ceiling box and is wired to the transformer-relay unit; the unit puts out approx 12 volts (DC, I think), which goes to the switches; and a switched 110V output that the light is wired to.

This was apparently a fad at the time the house was built (1958ish). I’m not quite sure what advantages the system has that impelled the builder of the house to install it; the only one I can think of is that operating a light with several switches is simple; you wire the switches in parallel.

God help me if I ever have to find a replacement switch or transformer-relay.

For the OP’s situation, I don’t think it would work. My switches are momentary, and I gather that the OP’s switch is detented.