Simple electronic circuit

I need a simple timer that whenever is given current, it lets it pass for a second or two and then switches off. Basically all it needs is send an impulse to an electric lock for it to open and then the circuit should close to avoid to damage/heat the lock. Can be 12 Volts DC or 110 Volts AC as could be placed before or after the transformer. The idea is to take the current from the Swing arms whenever they are activated and trigger the electronic lock to open. When closing the gate, the arms are activated as well and the lock would get the impulse, but that doesn't really matter as long as there is no current when the gate locks. Attached is a possible solution someone sent me. Does anyone know where I could buy this or could someone build it for me for a reasonable price? Any help is appreciated.