[brian_]

I need to figure out if I have a 120Vac in or a 240Vac on the pin2 on the J1772 connector to enable my L1 charge circuit or my L2 charge circuit.

Why do you have two charge circuits? Assuming that we're talking about a normal North American style SAE J1772 "J plug" (IEC 62196 Type 1) the same two conductors are used regardless of voltage so no switching is required; the same terminal (labelled "N") is "AC Neutral" for 120 V Level 1 charging or "AC Line 2" for 208–240 V Level 2 charging. I assume that normal practice, as catphish suggested, is to use a single onboard charger which accepts the whole range (from somewhat less than 120 V to somewhat more than 240 V, usually 100 to 250 V) without a configuration change.

I'm not trying to be difficult or disregard the original question, but the best answer to "how do I detect voltage and switch charging circuits" may actually be "don't detect voltage and always use the same single charging circuit".

 

[brian_]

Public Level 1 chargers are relatively uncommon, so having only Level 2 work through the J1772 plug seems like a reasonable compromise for now. The best long-term solution is likely a new charger to handle the whole AC voltage range, rather than a custom switching circuit.

 

[brian_]

The requirement is to be able to charge using both J1772 Level 1 Charger and Level 2 charger.

I appreciate all the other comments, but I'm more interested in the technical question. Not all my friends have 240V accessible at their apartments and I don't want a connector that anyone can accidentally pull out. I think using my locking plug is a good idea. So I still need to solve my technical problem.

Okay, but then my solution would be this one:

The best long-term solution is likely a new charger to handle the whole AC voltage range, rather than a custom switching circuit. One charger can fit all of your stated scenarios (voltage and power level).

If you want to build something that no one else has, just to prove that you can, there might be something along the lines of a voltage-sensing relay. Those are usually used in 12 V or 24 V DC systems, but they do exist for 240 VAC; here's an example (although it's only good for 15A... I only searched for two minutes):
Voltage Sensing Relay, 240V AC, 15A @ 240V, 5 Pins, Mounting: DIN Rail, SPDT

 

[brian_]

I agree that trying to dynamically switch this without a carefully designed bespoke solution would be extremely dangerous, as a 240V input could easily be detected as 120V for a brief period during plug-in. In my opinion you need to sample the voltage for at least 1 second before making a decision which circuit to enable.

I don't think you need custom electronics - especially built by someone with little clue what they're doing - but I agree that the right switch logic (e.g. normally closed contacts power the higher-voltage charger, which is the reverse of the example that I showed) with suitable settling time is required. There are catalogs full of 250 VAC switching devices, and one is likely suitable - it would be worth a look for anyone seriously interested in an automatic switching solution.

I wouldn't dream of doing anything but buying a new charger with a wide voltage input though.

Same here, and probably with everyone except the original poster.