12V LED, Buzzer, and 4 Switched grounds enter a Bar
My project is 12 volts DC and to have a buzzer activate when any of 4 LEDs are illuminated.


We'll assume that we have full access to this part of the circuit.


  I do not have access to the buzzer ground.


We'll assume that you mean that it's grounded by its mounting or buried in the dashboard and you can only switch the positive.


  My signal is a switched ground coming out of a Transmission Control Unit


I'll assume that this is a vehicle transmission.


  I can not have all LEDs illuminate when one signal has continuity to ground.


It seems that you are saying that the controller switches the LED negative.


  I have access to 280 SPDT Micro relays (like this) and an assortment of diodes, resistors, and capacitors.


280 relays should be plenty.


  The LEDs have a 1/4 watt 6.2K OHM resister +- 1%.


The tolerance is irrelevant.


  My research seems to point me to a combination of diodes (like the LEDs), maybe a bridge rectifier, and a 10 contact NAND gate?


A bridge rectifier is used to convert AC to DC. Further research required. Diodes are useful to prevent back-feed from one circuit to another.


  I think what I want to do is have a diode tapped off each LED ground that all attach to one side of the bridge rectifier. The other side of the rectifier  kicks over (POSN. 86) of a 1.2 watt relay that can supply more power to the buzzer.








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Diode-relay logic solution.

How it works:


With all TCU switches open there is no negative for the relay so it switches off and the buzzer stops.
If any of the TCU switches close then one of the diodes D5 to D8 will provide a ground path for the relay. The other three diodes will be "reverse biased" and prevent the other three LEDs from turning on.
D9 is called a "free-wheeling" or snubber diode and provides a safe path for the relay induced current to flow at the instant of switch off. It eliminates any risk of damage to the other diodes and LEDs.
Any small rectifier diodes will suit. e.g., 1N4001 diodes are rated at 50 V, 1 A which is well above anything they will see in this application.