Project: 3 lightbulbs, each independently controlled by its own switch (parallel circuit). When all 3 switches are closed, a 4th light bulb should light, but only when all 3 switches are closed. Each switch should be able to close independently and in any order. 120v.

Tried using a relay but it would light when any one switch closed. How can I prevent it from working until all 3 switches are closed?
Thank you from a novice!

 

Is this homework?

 

OK, an interesting challenge here. But first to verify the initial assertions: All three lights are mains powered , each thru it's own single pole switch, which is in the "line" feed to the light. And the other side of each light is tied to the neutral. The lights are #1, #2, and #3.
So the function to be implemented is a triple input AND function. To power light #4 requires #1x#2x#3=#4
If the switches were double pole it would be very simple. But the switches are all single pole.
So the solution requires two relays with 120 volt coils. The coil of relay #1 is connected across light #2, and the coil of relay #2 is connected across light #3. Relay #1 contact terminal "A" is connected to the line side of light #1, the "B"contact terminal connects to the "A" terminal of relay #2, the "B" terminal of relay #2 connects to one side of light #4, the other side of light #4 connects to neutral. So switch #1 provides the power to the contact of relay #1 and when light #2 is powered the relay #1 contacts feed power to the contact of relay #2, which when light #3 is powered connects it to light #4.

 

the simplest relay has coil and one contact. one can certainly find relays with more than one contact but it is also possible to use multiple relays as one - simply parallel their coils and all contacts will operate in unison. to learn logic using relays is great but i would suggest to tinker with low voltage first (such as 5V, 12V or 24V). also getting to know PLC programming using RLL (relay ladder logic) is advised. for that there is bunch of free online simulators like
https://app.plcsimulator.online/

 

if you use DPDT relays, couldn't you send one power supply wire through all 2nd NC contacts (in series) that would bring voltage to the fourth light bulb? So if all relays are NC (no relay is active, 3 light bulbs are off) this voltage would light up the 4th bulb. If at least one of the 3 bulbs is on this serial connection will be interrupted.

 

that is correct... just as MisterBill2 mentioned, one contact is needed to drive bulb (one per channel).
then additional contact (2nd contact) is needed on each channel is needed for AND logic. it does not matter if that additional contact is from the switch or from relay. one could minimize number of contacts since first one in the AND circuit is also one driving relay and independent lightbulb.

 

and they are all eqivalent

 

Consider that the request is functionally for a "THREE INPUT AND" function to light the fourth bulb. Consider also that the switches are each able to correctly light a bulb, meaning that the minimum required number of relays to isolate the three bulbs is two, as I suggested. Certainly it can also be done with three relays, or even four, but two SPST Normally Open relays, as I suggested, is the minimum number required.

 

with relays one can also do the unpopular things like choose how the switches and coils are arranged.
then you only need only single extra contact (either on switch or relay since they do the same logic function)

 

The circuit in post #9 Requires that "SW1" be either change to a DPST switch or for it to be in two places at once. I see that second option as a temporal challenge that I am not able to discover a solution to.

 

my idea, i think it would work :

 

if you use DPDT relays, couldn't you send one power supply wire through all 2nd NC contacts (in series) that would bring voltage to the fourth light bulb? So if all relays are NC (no relay is active, 3 light bulbs are off) this voltage would light up the 4th bulb. If at least one of the 3 bulbs is on this serial connection will be interrupted.

I think you have the logic backwards. #4 comes on when all of the others *on*. Three DPST relays controlled by the three switches: one set of contacts for the #1 - #3 lights and one set of contacts for the AND function. Or, three SPST relays, one relay coil in parallel with each of lights #1 through #3 and the three sets of contacts in series for light #4.

I like the way posts #6 and #7 reduces the number of relays (or double-pole switches) needed to only 2. But #9 is slick, and meets the letter of the original question.

ak

 

You are right. I completely misinterpreted the TS request. Mad paint skills for nothing...

Edit: on the right relays sides, change all connections from NC to NO and it should work.

 

Can you change the switches? 3 DPST or DPDT switches can do it with no relay.

 

if the switches are rated for 120VAC, that is the simplest solution.

 

Everybody has 2¢ to put in. Here's mine, and I am following

that is correct... just as MisterBill2 mentioned, one contact is needed to drive bulb (one per channel).
then additional contact (2nd contact) is needed on each channel is needed for AND logic. it does not matter if that additional contact is from the switch or from relay. one could minimize number of contacts since first one in the AND circuit is also one driving relay and independent lightbulb.

View attachment 346334

So here's my contribution - all based on @panic mode's second diagram. (post #7)

Took a bit but I got PM's solution.

Is this homework?

 

if you use DPDT relays, - - - .

IF the DPDT approach is taken then no relays are necessary.

In the configuration shown, all four lights are lit.

 

If the switches were double pole it would be very simple. But the switches are all single pole.

Actually, Mr. Bill gets credit for the DPDT comment. But nobody said anything about switches being SP or DP Double Throw. But as usual, I got caught up in the mix. Nevertheless, you have my two drawings to work from. But even @crutschow said: IS THIS HOMEWORK?

 

If this is homework, and the answer includes a relay, you get a C at best. Unless, of course, double pile switches were prohibited.

 

you get a C at best.

A " - - - C - - - " ? ? ? It was Panic Mode who first solved the problem with a relay. MY work was just based off his diagram. Maybe that's why I'm getting a C? Because I copied? I DID give credit where it was due. Same with the DPDT approach.