I would like to trip an input, similar to a button input to an microcontroller if the input voltage to the circuit/microcontroller is below a certain threshold. I have transistors and some zener diodes of different voltages but I am not sure how to connect them in order for the microcontroller to get an input when the voltage is below "x" volts.
Electronic – Circuit to tell Microcontroller when Voltage is Low
batteriescircuit-designvoltage
Related Solutions
It just looks like magic doesn't it?
If you haven't taken a course in semiconductor device physics, I suppose it does look like magic.
PN junctions are not resistors. Whereas the voltage across a resistor is proportional to the current through
\$v_R =R \cdot i_R \$
the current through a diode is approximately:
\$i_D = I_S(e^{\frac{qv_D}{kT}}-1) \$
In words, the diode current goes up exponentially with the diode voltage.
The derivation of the diode equation isn't trivial but it isn't mysterious either; it's based on well understood device physics.
For zener and avalanche diodes, the physical explanation is a bit easier to grasp. From Wiki:
Under a high reverse-bias voltage, the p-n junction's depletion region expands, leading to a high strength electric field across the junction. A sufficiently strong electric field enables tunneling of electrons from the valence to the conduction band of a semiconductor leading to a large number of free charge carriers. This sudden generation of carriers rapidly increases the reverse current and gives rise to the high slope resistance of the Zener diode.
The Zener effect is distinct from avalanche breakdown which involves minority carrier electrons in the transition region which are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bound electrons. Either the Zener or the avalanche effect may occur independently, or both may occur simultaneously. In general, diode junctions which break down below 5 V are caused by the Zener effect, while junctions which experience breakdown above 5 V are caused by the avalanche effect.
This circuit won't work - the transistor will be saturated, and the GPIO pin held low, until the battery voltage drops to ~ 0.7 volts.
Also, your thought of using an LM7805 to regulate your 7.2 volt battery down to 5 volts will only work with fully-charged batteries, as the 7805 need a minimum of 2 volts across it to maintain regulation.
Best Answer
If you don't care much about accuracy, you can use a circuit like the below:
simulate this circuit – Schematic created using CircuitLab
This circuit uses the Vbe of the transistor as a crude reference and thus has a threshold of around 4V. It's crude, has a mushy transfer function and will vary from unit-to-unit and with temperature.
If you replace R1 with a resistor + zener and pick the value of R2 to pass a properly selected current through the zener you'll get a more consistent threshold of about 0.7V + zener voltage, but zeners less than about 5V have very mushy knees, so it won't be as much of an improvement as you might expect. You could consider replacing the zener + resistor with a green LED + resistor, but it would require some experimentation. It's also possible to use a TL431 with a transistor and some resistors.
If you need something accurate that doesn't drift much with temperature, have a look at supply voltage supervisor chips which contain a reference and a comparator (and sometimes a timer).