Hi ToddB74,
Make R1 100 ohms, R2, 15K, R3 150K, C1 100nF. Almost any small NPN transistor should work. (2N3704, 2N3904, BC337) You could also use a small mosfet such as an 2N7000. (Source would connect in place of emitter, gate would connect in place of base and drain in place of collector.) You could leave R3 out as it just limits the base current to about 50 uA which is more than enough to saturate the transistor. I am assuming the current you measured is 0.5 mA so I have set the current in the alarm loop to be about this value.
Les.