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Latching hall effect sensor

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  • Brad Levy

    Hi Jennifer,

    Yes, current matters. If you are powering the LEDs off of the 3V source, then they are going to need to be in parallel. (If they were in series, the voltage driving them would need to be at least the sum of the Vf specs of the LEDs.)
    To drive them in parallel, the driver has to be able to supply the sum of the currents through each of the LEDs.
    If you are planning on 5ma per LED, you would need to provide 30ma to drive 6 of them.

    The hall effect sensors are mostly available in two output types: push-pull or open collector/open drain.
    Push-pull outputs are usually designed to drive other logic, with the output being a voltage that swings between ground and the supply voltage of the sensor. Push-pull outputs are usually used to drive other logic circuitry or processor inputs, which draw very little current. So most push-pull outputs on sensors aren't designed with high output current capability - usually 5 to 10ma is common, with the pull-up side of the driver frequently having less drive capability than the low side.

    Open drain or open collector type of output has a transistor that provides a path to ground when turned on, and leaves the output high impedance when turned off. These output transistors frequently have a higher current capability than the ones used in push-pull outputs. Some (for example, https://uk.rs-online.com/web/p/hall-effect-sensor-ics/7512869/) are capable of sinking 50 to 100 ma, though that particular one won't run on a 3V supply.

    Some sensors are only designed for low voltage (eg, 5V), others are designed for voltages up to 24V or more.

    Hall sensors also vary in power consumption. Many take 4 to 10 ma of current. Some have a built-in timer sample the field periodically instead of continuously, putting most of the detection circuitry in a very low power mode in between samples, while still supplying the output circuitry. This can reduce the average power consumption down to microwatts, useful for battery operation. On the other hand, if you are using the sensor as a tachometer for a high speed shaft rotating at a nominal 10000 rpm, a low-power sensor that only samples 1000 times a second would under count the shaft rotations. But if you are just trying to detect a magnet held by a human hand or attached to a laptop cover, sampling ten to a hundred times per second is plenty sufficient, and worth the power savings. Note that the power consumption specs don't include power drawn by loads attached to the output. The data sheets will usually include a leakage specification for amount of current that can still pass through an open collector or open drain output when the output is in the high impedance state. That leakage current is usually far below what would be enough to cause the LEDs to be visible, but will still be added to the power drain on your battery or supply.

    If you are wanting battery operation without a separate on-off switch, and can't find one of the very-low-power hall sensors with a high enough output current capability, you could use an external transistor to boost the current capability.

    Also - you'll notice there are hall effect switches and hall effect latches. The difference is the switch is activated by a threshold - field strong enough -> output turns on, field weakens enough -> output turns off. The latch is activated by a strong enough field of a specific polarity, and the output stays on until a strong enough field of the opposite polarity is detected. (i.e., the switch turns off as soon as the magnet is moved away, but the latch stays on until the magnet is turned around.)

     

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  • Jennifer Smith

    Hi Brad,

    Thanks for your reply.  That helps a lot.  

    I actually have two projects I'm working on - a larger up to 10 LED  microprocessor board and a basic LED board with 5-6 LEDs but they have similarities in design requirements.  

    Yes, they will be in parallel and usual current I use is 10-15mA per LED for the correct brightness.

    The board currently is designed to use a physical switch but it has to be external so it can be accessible.  On the microprocessor board it can potentially use the hall sensor to change modes as well as turn on/off.  

    Both are going to run off a 3v coin cell in some circumstances (or external AAA's in larger models) so power drain is a big consideration and I would test one that seemed suitable.  So I was looking for one with <1mA power consumption.

    I thought a hall latch would be better as it wouldn't require the magnet to be left on the model but I couldn't find any with a higher current capacity and also 3v or less minimum voltage.  

    What kind of transistor would you use?  I was thinking of a MOSFET as I've used those before to boost current off an IC I/O pin.

    So I think you're saying I'd want an open drain/collector type 

    Is there any difference between one required for the noddy board and one required for the microprocessor board?  I see there are analogue and digital versions.  I did buy one a while ago that's a basic switch version but it's now discontinued.  

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  • Brad Levy

    That data sheet for the SiLabs parts is a bit confusing, because it actually covers a range of part numbers, with somewhat different characteristics. It also mentions programmability, but that is done at the factory. The different part numbers reflect the same basic die factory-programmed for different features and options.

    Assuming you want minimum power consumption, the Si7202 would be the most appropriate of those. It samples every 200 ms (five times a second), and its average power consumption is less than a microamp, so by itself, it could run for thousands of hours on a CR2025 coin cell. (Obviously, you won't get thousands of hours with the LEDs on!)
    The Si7202 has a push-pull output, with relatively limited drive current, so would need an external transistor or driver IC to provide enough current sinking capability for the LEDs. A transistor like RE1C002UNTCL would probably work well. It has a leakage current of 1uA at 20V, which you'd add to the Si7202's avg supply current and output leakage current when calculating the LEDs-off battery life.

    Another characteristic that varies significantly between different available sensors is the sensitivity, or strength of field needed to turn the output on. These are listed on the data sheets as BOP or BRP for operating point and release point, respectively, in units of G (Gauss), T (tesla), or mT (milliTesla). 1mT = 10 Gauss.  The higher the number, the more powerful the magnetic field has to be to activate the sensor. The strength of the magnetic field depends on the magnet and the distance between the magnet and sensor (plus to some degree other elements nearby).



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  • Brad Levy

    I posted my last comment before seeing your earlier reply.

    On the microprocessor version, you'd feed the push-pull sensor output to a processor input, no additional parts needed. Microchip and others offer many choices of microprocessors with very low power idle/sleep modes. You could use a switch-type sensor instead of a latch-type sensor for that application, and let the processor toggle between LEDs off / LEDs on with successive detections of a magnet, without require opposite poles for on vs off.

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  • Jennifer Smith

    Thanks Brad, I really appreciate your input.  There was just so many options I had no idea how to choose one.

    The transistor you mentioned has a statement in its datasheet: "This product might cause chip aging and breakdown under the large electrified environment. Please consider to design ESD protection circuit."

    Do you know what that means and is it relevant to my basic design?  I'm assuming perhaps not if it's only driving LEDs!  

    I have an IC picked out - one of the microchips PIC16F1503 and on testing it seemed pretty reasonable off a coin cell.  In most cases where there are more than 5 LEDs I'd probably be using the external AAA battery box anyway.  

    A good point about not needing the latch for the microprocessor option.  I'll look for a similar non latching option.  

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  • Brad Levy

    DMN65D8L (https://uk.rs-online.com/web/p/mosfets/7705140/) would be an alternative transistor that has ESD protection for the gate built in, and still pretty low leakage characteristics.

    Keep in mind with the transistor that the transistor will act as an inverter (switching the load "on" to ground when the input goes higher than the Vgs threshold voltage). When used with the hall latch, that has the effect of flipping which polarity of magnetic field turns the load on.


    Meanwhile, for your processor-based version, hall switches are available in polarity-sensitive and polarity-insensitive versions. You may want to give thought to which will be most appropriate if they user has both versions of your product.

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  • Jennifer Smith

    Thanks Brad,  okay, will get some to test out.  

    I didn't think out the polarity being switched but as long as the magnet has both north and south I guess it won't matter.

    Yes, a good point.  Thanks.

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  • Jennifer Smith

    Would this one be okay for the microprocessor option? https://uk.rs-online.com/web/p/hall-effect-sensor-ics/1446543/

    I think it fits the specs required.

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  • Brad Levy

    That Si7201 looks like it should do.


    Note. For the processor-based application, ultra-low current drain by the hall sensor itself may not be needed, because the power consumption of many of the hall sensors is low enough that the sensor can be powered by a processor output pin, and the processor can then provide power to the sensor on a timed basis to reduce the average power consumption. This won't be satisfactory in every situation (for example, if the on/off magnetic threshold and hysteresis needs to be precise, since the hysteresis will be lost while the sensor is powered down) but I think it would be okay for your app.

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  • Jennifer Smith

    Okay, thanks Brad!

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