I would like to strobe an LED strip at about 16 Hz using an A3144 Hall effect sensor. The strip will draw about 300 mA, but the sensor output is only rated for 25 mA. My plan is to switch the power to the LEDs using a transistor so the sensor output only has to supply the control signal. I’m a bit confused about what type of transistor I should be using here.

My very basic understanding is that I could use a BJT transistor that is switched by current, or a FET style transistor switched by voltage. If I go with a BJT transistor, I know only a small current is needed, but how do I know that the base terminal won’t pull more than my 25 mA maximum? What type of transistor would be recommended for this use case?


1 Answer 1


Assume you're going to drop 700mV across the B-E junction of the transistor. Lets say you hall-effect sensor puts out 5V (maximum/worst-case). You're going to need a resistor that satisfies ohm's law: (5V - 0.7V)/25mA = 172 ohms. Or 180 ohm's rounding up to nearest standard value. Almost any transistor will have enough gain for this application - it just has to handle the 300mA load.

Or you can use a logic-level MOSFET with almost any resistor (or possibly no gate resistor at all - depends if the A3144 is stable driving capacitive loads).

  • \$\begingroup\$ Thanks. Any hints as to what I should be looking for to see if the sensor is stable driving capacitive loads? \$\endgroup\$ Commented Jan 2 at 15:28
  • \$\begingroup\$ @JoshuaDotson I just briefly checked the datasheet (this part is discontinued BTW) and there seems to be no limit for capacitive loads - it's an open-collector output so you're going to need a pullup resistor in either case. And TBH, at 16Hz, you won't have to worry about the higher-order capacitive effects I was mentioning. 16Hz is super slow - even poorly controlled output will settle relatively fast compared to 16Hz switching period. \$\endgroup\$
    – MOSFET
    Commented Jan 2 at 15:36
  • \$\begingroup\$ Thanks again! Odd that the sensor is discontinued. These seem to be ubiquitous on Amazon. \$\endgroup\$ Commented Jan 2 at 17:44

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