Question

Draw an electrical door warning control circuit for the aircraft toilet "Vacant" (red light) and "In-use" (green light) sign.

The circuit components must include (but are not limited to) the following:
1. a 24 VDC supply,

2. switch(es),

3. relay(s),

4. 2 x 1 kΩ resistors,

5. 2 LEDs (a RED and a GREEN), and

6. earth/ground.

Circuit operation:
The main voltage source is supplied from a 115 VAC generator. A resistor is individually connected to each LED lights that limits the current flow. The turning ON/OFF of the "Vacant" and "In-use" sign is controlled by a SPDT micro switch which allows only one LED to be lighted.

Answer 1

Let us use the relay as the sensor that detects the toilet is vacant or not. Consider, when a person goes in to use the toilet and locks the door, the relay provides an output of +5V, and when the person releases the toilet, that means opens the door, the relay cuts and provides 0 V as output. With this signal, we can construct our ON-OFF controlling Circuit easily.

Consider we already have the relay output as +5V or 0 V for simulation purposes.

The circuit is designed and simulated in Multisim 12.0.1 Platform for better understanding:

When the Toilet is vacant:

When the relay output is 0V, the Voltage controlled SPDT activates line 2 and the RED LED finds a closed path from the 24 V Voltage Source to the ground through 1k ohm resistor.

When the Toilet is In-Use:

When the relay output is +5 V, the Voltage controlled SPDT activates line 1 and the GREEN LED finds a closed path from the 24 V Voltage Source to the ground through 1k ohm resistor.