I'm fairly new to electronics and am stuck at how to get a circuit working. I have 3 components connected in the circuit:- a card reader, relay and an electronic door lock release, and all operates at 12v DC. The card reader is continually powered on and is a NC type, meaning when the reader has power an internal switch is always closed and it outputs a short to ground. When a registered card is presented the card reader's internal switch closes for a set time (approx. 15 secs) and the short to ground becomes an open circuit, and after the set time the internal switch returns to a NC position and the short to ground is restored.
The reason this card reader operates as a NC type is because it is primarily used for magnetic locks which are always energized to lock the door (hence a constant short to ground to power a component) and de-energized when the door is unlocked (short to ground becomes an open circuit and power removed from a component).
I will be using a conventional electronic lock release which requires the relay to be powered on only when the door is required to be unlocked and no power to the relay when the door is required to be locked (basically the opposite way a magnetic lock works). So when the door is required to be locked I need the relay to be powered off so the lock release (on the load of the relay) is not operating. When a registered card is presented to the card reader I need the relay to be powered on and in turn the load circuit will be complete and the door lock release will operate and unlock the door. After approx. 15 seconds the card reader will return back to the NC position and I need the power to the relay to be removed and hence the door lock release will not be operating and the door will be in the locked position again.
I have looked at NPN / PNP transistors but they require the output to be in the “on” position to operate the circuit, but my card reader had no output when I want the circuit to operate. I think I need a MOSFET transistor but can’t figure out how to make it work.
NOTE: I cannot do this with a relay with NC outputs, i.e when the relay isn’t powered to use the NC outputs of the relay to complete the load circuit because of fail safe. By using a NC relay if the card reader was to lose power, so will the relay and it will default to the NC position (as it won’t be powered on) and the door will open when it shouldn’t be.
Thanks in advance for any help.