If you scroll down
this page about half way you will find a variety of photo cell circuits. They use transistor as well as SCR latching designs.
Circuits like this are commonly called "Light Activated" or "Dark Activated" photo cell circuits. You mentioned that your LDR had a resistance of about 500 Ohms with light striking it. Your LDR along with another resistor in series with it will form a voltage divider circuit. Generally you want the series resistor to be about 3X the value of the LDR light resistance. I would look to place a small 2K Ohm pot (ten turn preferred) in series with the LDR. For circuits like this the placement of the LDR determines if the circuit is light or dark activated. LDR on top of divider "Light Activated", LDR on bottom "Dark Activated". The link shows light activated and you want dark activated. Also, in the SCR examples you want a small normally closed push button switch in series with the load (your buzzer, bell, relay or whatever). That is the reset breaking the load connection. Once a SCR is fired ON it remains ON so to turn it off we use a PB N/C switch.
If the circuit(s) start in an ON or set state they simply require a reset.
Ron