stickyfox
Key Veteran
Location: Troy, NY, US
| Here's a little more technical rundown of how the thing works (I should say, how I intended it to work), and also an explanation of how to pick the right values for R2 and R3. I realize not everyone wants to become an honorary Ph.D. in electronics for the sake of making a buzzer so I will just hit on the practical points. Skip down to the ----- if you don't care how it works and just want the numbers.  And since it's 1am here, watch out for mistakes.
Q1 turns the buzzer on by conducting when current is available. When the battery is fully charged, Q2 keeps Q1 turned off by sucking all the available current up. In other words, the buzzer stays off as long as Q2 is saturated.
Q2 can be saturated whenever the voltage at its base exceeds a certain value. This brings us to the heart of the circuit, the voltage divider formed by R2 and R3. Basically, the voltage at the base of Q2 is a fraction of the battery voltage. That fraction is determined by the ratio of R2 and R3 by:
Vout = Vbatt * (R3 / (R2 + R3))
Q2 will suck the voltage down and change Vout (and also introduce some temperature dependency), but because they're fairly large resistances, and because R1 is there to keep current down, total current is pretty low, and there is little or no influence from Q2 on the switchover voltage.
For the transistor I used, a 2N2222, saturation occurs when the base voltage is around 1V. So if we want Q2 to saturate at a particular Vbuzzer, we would use:
-------------
Vsat = Vbuzzer * (R3 / (R2 + R3)) ... where Vsat is 1V
or
Vbuzzer = Vsat * (R2 + R3) / R3
Here is the tricky part, though: Vsat depends on the type of transistor you use, and even varies between manufacturers, so I suggest you breadboard this circuit before building it. If the circuit activates too early (at too high a voltage), make R2 bigger. If it activates too late (too low), make R2 smaller. Don't worry, Vsat won't change after you've assembled the circuit; it just varies from one transistor to the next.
If you want to make this circuit adjustable, substitute a 5k pot for R2, or use a single 5k pot for R2 and R3, with the wiper connected to R1. Be sure to check it periodically because the vibration on the heli may change the pot's resistance.
This circuit is not the most accurate, nor the simplest, way to do this, but what it has going for it is that anyone who tinkers with electronics has the parts lying around. If you don't, you can get them at RS for dirt cheap. I've analyzed the circuit and the error margin is safe enough if you're flying in typical Earth climate zones. If you are flying on the surface of Venus it may not protect your battery.
-fox |
But I am a techie...
