I designed the below circuit.
The relay open above 13V and close below 13V.
How can I make relay open above 14V and close below 12V?
Tell me if the circuit have major problem.

Thank you.


  • \$\begingroup\$ Where is the battery connected? How is the 'charger' powered? \$\endgroup\$ Feb 13, 2022 at 5:23
  • \$\begingroup\$ Try a high value resister like 470k from C of Q2 to B of Q1 and use a 10 volt zener for R2 \$\endgroup\$
    – Autistic
    Feb 13, 2022 at 5:55
  • \$\begingroup\$ Explore windowed comparators using opamps. \$\endgroup\$
    – Syed
    Feb 13, 2022 at 5:57

1 Answer 1


tell me if the circuit have major problem.

Simulation for some positions of pot, dependence to BJT beta and Temperature.

There is some "dispersion". Can "work", but not precisely.

Use comparators (as @Syed suggestion) or certainly differential BJT for comparison ... with, obviously, also a "zener" for "reference" (as Autistic suggestion).

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This could be better. Pot2 used for "centering" on usable voltages. R9 scales.

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For the last point, one can add hysteresis with another resistor.

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