After have locked myself out of my apartment one too many times, I decided that running to make a keyless entry system. I’ve been looking for a project to use a Raspberry Pi in for awhile, and this seemed like a good fit.
The basic set up was pretty straight forward. My apartment had a buzzer, so I wanted to be able to switch on the Buzzer through my LAN.
Since the buzzer is medium voltage AC, I decided to use a relay as the switch. I went with a g5le-1-g dc5 . The important stats were that it supported the voltage and current my buzzer used, and that the coil was 5V and draws about 80 mA. Unfortunately, the GPIO pins on the Raspberry Pi are 3.3V and can only output a max of about 16mA. This meant that I needed to use a transistor to drive the relay. I had a bunch of 2N4401 lying around, so that’s what I used. I used this circuit:
The basic theory of this circuit is that a small current will go from the input through the base of the transistor and out the emitter to ground. This current allows a much larger current to flow from the transistors collector to ground, driving the coil of the relay. Rb should be chosen so that the current coming from the input is within the Raspberry Pi’s GPIO spec, but the amplified current going through the coil can still close the relay.
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