1/4" TRS With LED?

[LydaRA]

This may be a bit odd, but has anyone seen a 1/4" TRS connector with a built-in LED?  (Or like a three-pin variant of a Dell laptop power adapter with that glowing blue ring on the DC plug hood?)

I'm designing a new model rocket launch controller for the Cub Scouts. Thinking of how to make removable "keys" that also indicate circuit status....and also harder to spoof.

A 1/4" connector seems about the right size, not too small and easy to lose, but not too large and a pain to carry 16 at once.

Using a TRS/stereo type, so as to have both positive and negative sides of the circuit....and an extra contact in-between. If the Scout inserts the valid connector, we'll have the tip and sleeve wired together, and it'll work. But if they try sticking a pocket knife, wire, whatever into the jack; the tip will short to the ring and tell the controller to lock-out!

Don't know that I've seen such a connector, with an LED. But before I take on building one, thought I'd ask the community to share if they know of one. Or are there other ideas for such a removable circuit "key?" Thanks in advance.

[Buriedcode]

You should be aware that TRS connectors could possibly short contacts on insertion.  If your tip and sleeve are wired, then upon insertion depending on the socket, you could possibly short the sleeve and ring before it is fully inserted, causing a false negative/lockout.  One of the reasons TRS isn't suited to any kind of power connection.  I'm sure many sockets will only have solid connections when the plug is fully "in", but you will have to fully test the sockets you're using.

Although it adds complexity, you could always wire some resistors as a voltage divider for the three contacts, with the device using a uC with an ADC or just a comparator to check the resistor ratio. They would easily fit inside a small TRS serso plug, and would mean you're not looking for a short/open type situation, but rather something more specific.  It also means if needs be, you could use different resistor ratios for different keys.  Hardly high-tech security, but simple and not easily spoofed - and can't be fooled with a screwdriver like some kind of dead-short would be.

[oPossum]

Just measure Vf of the LED. Much less chance of a false key detection.

[LydaRA]

Very good point and idea, BuriedCode.  Thanks. 

There is another part of the circuit, a button, that has to be pressed simultaneously to trigger the launch.  So we really don't need to try reading the presence & validity of the "key," until the flip covered button is activated.  But definitely a good consideration to document.