How to stop an electro magnet from sticking
I am using an electro magnetic door lock which I am powering with 12 volts. It is switched on and off using a transistor array (uln2003.) I have also included a flyback diode. My problem is this: after I have switched off the magnet, there appears to be a lingering field that holds the door shut until it is pushed open. It is not a strong field, and it goes away after the door is pressed open. I need to build a circuit that can neutralize this field. How?


(1) A small airgap when at closest approach will (should) make a substantial difference. This can easily be achieved with a small layer of non magnetic material if desired. Try tape, paper etc initially. A more permaent solution can follow. This applies even with a non-magnetic material for the door as the plate the magnet attracts may suffice.

(2) A low force spring should be able to be made to work. Magnet-on attracts plate and compresses spring. Magnet off - spring is designed to exceed remaining attraction. 

Again, start informally - you can eg use bubble wrap air bubble with suitable mechanical placement and sellotape to hold it.