Booster for relay, Relay Booster, non-latching

This relay driver boosts the input impedance with a regular 2N3904 transistor (or equivalent). Very common driver. It can drive a variety of relays, including a reed-relay, and is non-latching.
Transistor Q1 is a simple common-emitter amplifier that increases the effective sensitivity of the 12 volt relay coil about a 100 times, or in other words, the current gain for this circuit is 100.
Using this setup reduces the relay sensitivity to a few volts. R1 restricts the input current to Q1 to a safe limit (the impedance is equal to the value of R1 plus 1K). Diodes D1 and D2 are EMF dampers and filter off any sparking when the relay de-energizes.