To clarify the question I draw the following illustration:

enter image description here

Inside an enclosure I basically need a control such that when the SW switch above is closed, the mains relay latches if the momentary push button PB is pressed and released. And the mains relay stays latched unless the SW is opened.

Both relays above are normally open non-latching type. And as you see, I use a DC relay to achieve this. If SW is closed and PB is pressed and released, both relays will stay activated since the output of the DC relay is fed back to both relay trigger inputs. So after PB is released the Bulb will remain ON and can be turned off only by opening the SW.

So the operation required is:

SW is closed & PB is pressed and released --> Bulb is ON (unless SW is opened again)

SW is open --> Bulb is always OFF

My questions are:

1-) Is there a type of relay where one can achieve this operation with a single relay?

2-) What IC gate type would replace the Relay_DC?


2 Answers 2


A single DPDT relay should do.

However, two relays may be used for greater isolation between the low voltage and high voltage contacts.

Here's the schematic.

enter image description here


It sounds like you need what is known as a start-stop circuit (or run-stop circuit): -

enter image description here

Image from here. The above uses a 24 volt DC relay but, it's the same circuit when used with an AC power line.

To control your load, choose a relay with an extra contact to those shown above. Here's a version that uses one contact to do latching and output switching: -

enter image description here

Taken and modified from here.

What IC gate type would replace the Relay_DC?

You would probably use a flip-flop. Maybe a D flip-flop type that has a clear input so, once the flip-flop latches it can be unlatched by the clear (or reset) line.

  • \$\begingroup\$ In my case the control side should be low voltage because it will be exposed to an interlock where a person might touch. The only part AC should be the relay contact for the load. \$\endgroup\$
    – ty_1917
    Commented Sep 16, 2022 at 8:45
  • \$\begingroup\$ @ty_1917 then what would you change on the circuit diagram to get that isolation? a small change only. \$\endgroup\$
    – Solar Mike
    Commented Sep 16, 2022 at 8:56
  • \$\begingroup\$ @ty_1917 imagine the top circuit used a 5 volt relay instead and, an extra contact was brought out for the load (as mentioned). Clearly, you need to have contact-contact isolation ratings in accordance with your safety requirements. \$\endgroup\$
    – Andy aka
    Commented Sep 16, 2022 at 9:07
  • \$\begingroup\$ @Andyaka " a relay with an extra contact " Is there any jargon to search/google such relay like SPDT ect? \$\endgroup\$
    – ty_1917
    Commented Sep 16, 2022 at 9:21
  • 1
    \$\begingroup\$ +1 from me. I note also a -1. Not obvuious why - but there has been a lot of hard to understand downvoting of late :-) \$\endgroup\$
    – Russell McMahon
    Commented Sep 20, 2022 at 10:33

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