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Question about my halleffect sensor> transistor> LED c

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idtkid

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Quick question, hoping someone can point me in the right direction.

I need to light up 3 x 3.3V 20mA LEDs using a 5V power source. I need to turn these LEDs (in series) on and off via a small hall effect sensor THIS ONE. That sensor says it can only output 10ma which my LED circuit draws 60. I have THIS transistor which I was hoping to use to supply the 60 mA of current. When I apply 5V to my circuit the LEDs glow dimly as about 1.5-2.5V "sneaks" through my transistor and halleffect sensor before I use a magnet to "open up" the full amount of power.

Hopefully this makes sense? Its one of my first circuits, any advice would be greatly appreciated! Thanks!
 

jumper2high

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Re: Question about my halleffect sensor> transistor> L

If you're connecting the LEDs in series, a total of 20mA is flowing through all three of them, with the voltage being distributed among them.

If you, however, connect them in paralell, you'd have all three of them getting 20mA (given that you calculate the correct resistor to limit the current properly) through each of the tree LEDs, so it'll be 60mA all together.

I didn't quite understand what the question was, but if you're trying to make the LEDs NOT light up before the magnet is introduced, you should probably start by trying to connect them in parallel (and add a resistor). I believe the Hall Sensor acts as a switch and gives a 'TTL compatible' output, so it'll be around 0V when the magnet is away and around 5V when the magnet is close to it.

Since it's never an ideal circuit, the output voltage won't exactly be 0, it'll be something around that...which may cause the transistor to start conducting before the magnet gets close. One thing you wanna do to prevent this is to put the LEDs in parallel, because if you remember that the "output" current of the transistor is a function of the "input" current (base current), and the LED light intensity depends on the current, by distributing that 'leak' current across three leds you will have each individual LED shine 1/3 of the brightness it would shine if it was connected in series. Also, add a current limiting resistor on the base of the transistor, since - the lower the Base current, the lower the output current will be.



I hope all of this makes sense.
 

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