NC Opening Contact detector to drive a NO relay Contact circuit

Question

I'm looking for a simple +5.5vdc powered circuit (perhaps a PNP transistor driving a NO Reed Relay coil?) - I need a circuit that can detect when a pair of Normally Closed contacts with no voltage across them at its input, then Open, and then a transistor? drives a NO relay coil to close a pair of contacts at the output.

I have a +5.5vdc source and 5.5 return pin available at the output relay side of the detector (so don't want to use a battery) and I plan to add the flywheel-diode across the coil and bias resistors but am not sure what wiring/ resistor values, or transistor type to use.

I have small a SPST-NO Digikey PN: 725-1354-ND 5V Reed Relay in mind but am open to substituting it with something else. If I could eliminate the transistor or the relay I would, but assume I need something similar to detect and convert the OPEN contact state I'm monitoring into the closing pair of contacts that I need at the output to carry a very small current back into the equipment connected at the output. See this draft Rev2 schematic:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Try this.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A simple open-collector inverter.

• With the switch open Q1 is off and T is allowed to float high.
• When the switch closes Q1 is turned on and T is connected to U with, perhaps, a 0.2 V drop between the collector and emitter.