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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:

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ To add a schematic, save it as an image file (JPG, for example) and the click the "picture" icon when editing your question. And follow the prompts to upload your image. \$\endgroup\$ – The Photon Oct 30 '19 at 21:45
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    \$\begingroup\$ Or, if they're still enforcing a minimum rep to include an image, share a link to the image (on imgur, for example) and someone with more rep can edit it in to the question for you. \$\endgroup\$ – The Photon Oct 30 '19 at 21:46
  • \$\begingroup\$ Photon, thanks for the tip.. I noticed the "schematic" option is available while editing..and was able to enter this draft (above) \$\endgroup\$ – JimS Oct 30 '19 at 22:47
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Try this.

schematic

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.
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  • \$\begingroup\$ This Open Collector alternative looks like an excellent way to solve it and save needing the relay too. I will try it. But I do worry a bit about doing it this way with an Open collector since I'm supposed to provide a contact closure across J3 pins T and U and I don't really know the circuit in the equipment behind J3.. is there any significant risk that this alternative could damage it? I do appreciate the time and effort you put into this reply VERY much. Thank you.. \$\endgroup\$ – JimS Oct 31 '19 at 17:44
  • \$\begingroup\$ Well I looked into it and I really need a contact closure, so can't use or even try to use the Open Collector driver option since any voltage at all might damage the unit behind J3 and can't risk it. Thanks.. I've updated the draft schematic to Rev 2.. will it work and drive the NO relay into a closed state? \$\endgroup\$ – JimS Oct 31 '19 at 20:25

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