These two circuits will function as a broken loop warning device. Can think of this as a circuit that sets off an alarm when a loop of wire is cut. This cut wire could also represent a sensor that is normally closed and then goes open circuit when some parameter is exceeded. This kind of sensor is normally used because it automatically sets off the alarm if the wires to the sensor get damaged.

There are two circuits shown in the figure that can do this, one based on a BJT and one based on a MOSFET. They both have the same kind of operating principle.

The function of the circuit is to react and let current flow through the alarm when the wire loop is broken. The breaking of the loop is a switching event and the reaction of the transistors is also needs to be a switching event. Therefore, The transistors are modelled as switches that turn on and off.


If these two circuits were to operate under identical conditions and needed to be battery powered, Which transistor circuit would be better to use over the other for the longest battery life.

My Thoughts

I would choose a MOSFET as a MOSFET is a lot faster than BJT and therefore, it will switch a lot faster than BJT. Also, aren't MOSFETS more efficient and BJTs consume more power?

Please let me know if I have said anything incorrect or if I could add anything, thaks!

  • \$\begingroup\$ Do you know how much current the alarm uses? I feel like you need to figure out the ideal value of R1 for each of the circuits first to determine how much current they draw. There's no particular reason why R1 should be the same between the two. \$\endgroup\$
    – Justin
    Commented May 18, 2020 at 18:00
  • \$\begingroup\$ There is not a particular reason why R1 is the same. For this design it is the same cause the question is that the two transistors are under identical conditions. So the question is: which transistor would drain less power from the battery causing it to last longer? \$\endgroup\$
    – John
    Commented May 18, 2020 at 20:34

2 Answers 2


You are only going to be switching ONCE.

The MOSFet gate will be 10,000 picoFarad. You are in NO HURRY to charge that up.

If the gate connection is exposed to the elements, and to corrosion and pollution out in the elements, then both choices need to tolerate the stray leakage paths ---- outside the circuit.

If you can maintain a very clean gate wire, then you can use a 22,000,000 ohm resistor as pullup.

The time constant of 22,000,000 ohms and 0.000,000,010,000 farads is 2.2e+7 * 1e-8 = 2.2e-1 = 0.22 seconds.

  • 1
    \$\begingroup\$ Thank you but which transistor would drain less power from the battery causing it to last longer? \$\endgroup\$
    – John
    Commented May 18, 2020 at 20:32

In its normal state (the loop closed) it is not the transistor that drains the battery, it is R1 / R2 .

Your choice of R value depends on the loop properties first.

  • You have to allow for some resistance to not activate the alarm, e.g. the resistance of certain length of wire.
  • You have to allow for another (higher) resistance that will not prevent the alarm from activating, e.g. some amount of current passing even with the loop broken, because of moisture or other factors.

And then, you choose a transistor.

The transistor itself is irrelevant for the power consumption, with a small exception - when the alarm is activated, BJT will still sink current from R. MOSFET won't.

In short, the circuit will be marginally more effective with a MOSFET, but only in activated state.


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