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.
Copyright © 2003 - Tony van Roon
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