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I am looking for a circuit that would be switching a relay coil and few small leds (max 3A) with a low voltage input. Now the input is variable voltage 1.1v to 5v produced by a hall sensor pot. I want the circuit to be switched on when my input reaches 1.2 or 1.3v to switch a relay with 12vdc. Also the 1.1v to 5v values can not be affected by this additional circuit. Any help greatly appreciated .

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    \$\begingroup\$ Use a comparator. \$\endgroup\$ – Andy aka Jul 16 '18 at 10:32
  • \$\begingroup\$ Any suggestion on it , diagram ? Im coming from auto elec. field and not too familiar with IC chips. thanks \$\endgroup\$ – Marius Ruzveltas Jul 16 '18 at 11:02
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Use a comparator to sense whether the signal is above or below your 1.25 V threshold. Use the output of the comparator to turn on a transistor, which then turns on a relay.

Driving a relay from a digital signal has been discussed many times here before, so I won't repeat it. Here is one example.

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Any suggestion on it , diagram ? Im coming from auto elec. field and not too familiar with IC chips. thanks

There are plenty of examples on the web. Here is one: -

enter image description here

You'll need a 1.25 volt reference to feed one input of the LM339 and the other input is from your hall sensor. The voltage reference is common-place and there are many options such as this link from Mouser.

Q1 is virtually any PNP transistor. The 470 ohm and LED are optional.

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  • \$\begingroup\$ So using a stepdown dc/dc as ref v to comparator , input from hall sensor to comparator and a P=channel Mosfet to drive the coil of a relay with a flywheel diode ? \$\endgroup\$ – Marius Ruzveltas Jul 16 '18 at 12:03
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    \$\begingroup\$ That would work but make sure you filter the stepped-down voltage to remove ripple. You might also consider adding a bit of hysteresis when Vin is very close to Vref - this would prevent relay chattering. \$\endgroup\$ – Andy aka Jul 16 '18 at 12:57
  • \$\begingroup\$ Im still not sure where I will be feeding the stepdown from as the 12vdc is also variable ie. vehicle 12vdc that ranges anywhere from 9 to 14. I also have the steady 5v that supplies the hall sensor but i would prefer having that circuit as little interrupted as possible. \$\endgroup\$ – Marius Ruzveltas Jul 16 '18 at 13:03
  • \$\begingroup\$ That is why I recommended a 1.25 volt voltage reference and gave you links to Mouser. Having said that I doubt very much that using a high impedance potential divider from 5 volts is going to upset anything. \$\endgroup\$ – Andy aka Jul 16 '18 at 13:05
  • \$\begingroup\$ 1.25 would be acceptable but having an adjustable v ref would be much preferred for my application ie. 1.1 to 1.6. but having main components figured out I can play about with the variables a bit and see what suites me best. also could you just throw me the part number of mouser part , I dont seem to be able to access the link. thank you. \$\endgroup\$ – Marius Ruzveltas Jul 16 '18 at 13:16

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