im trying to make a diy lost rc model alarm/strobe. i need it to activate when main battery power is disconnected. i want to use a small lith-ion battery to power the alarm circuit when the main lipo battery gets disconnected. ive found lots of diy alarm circuits but they all seem to be using a arduino or a IC. I was hoping i can accomplish this with maybe just a mosfet and a capacitor?.?.? not sure if its possible though... i was thinking i could use the main lipo to hold the mosfet "open" and when its disconnected the mosfet will close allowing alarm circuit to activate. and by using a capacitor that is charged by the lipo as well in order to somehow only trigger the alarm if the lipo was just supplying power and the capacitor was charged. or do i not have the right idea?

if this idea is not feasible, what is the simple analog way to archive this? and if needed maybe including a 555 timer to sence when lipo was just connected and suddenly removed.?.?

I know I could just buy a premade one but wheres the fun in that? also they arent "very loud"

 

Feasible. I made a circuit using a 555 alternating between flashing a super bright white LED and sounding a loud beeper. Powered from a A23 12 volt battery. A 12 volt lipo for power would last longer.
You mentioned triggered when the main lipo is disconnected. What does that mean exactly? When the main battery is dead or from a broken wire or something?
SG

 

If you need to latch the output then use a NPN bjt with a CD4013 in RS mode, if you don't need it then just use a CMOS 555 as LMC555 with monostable circuit, if you need more current then you can add a bjt or MOSFET, if you don't care about the draw current of 555 then you can choose NE555, its current drive ability is enough.

 

im trying to make a diy lost rc model alarm/strobe. i need it to activate when main battery power is disconnected.

Those seem to be two different functions. Why would the battery auto-disconnect when the model is lost?

For a lost model (as in out of range), you can use a spare servo channel (or Y-connector to one in use) and configure a 555 as a missing pulse detector. The 555 is then your trigger.

For low or no battery, there are several diode-based circuits. Old fashioned battery backup circuits allowed one battery to be used until depleted, then switched to the backup battery. That switch over can be your trigger.

 

A comparator is more precisely for the voltage detection.

 

wow thx for all the responses..

i fly freestyle/racing quadcopters. when we crash they have a tendency to loss the lipo. just comes flying off... so the only battery remaining may be just the tiny lith-ion im using for the alarm.

if the main lipo stays connected i can trigger the on-board alarm connected to the flight controller

we dont use ppm receivers or servo controls. just one wire protocols. (sbus,ibus,etc...)

and if possible please point me to a schematic or in the right direction. thanks

 

Don't you have observers who can spot the crashes?

 

The Vout will be output a low voltage level when the main power is OK, when the main power is failure then the Vout will be output a High voltage level and close to +V, you can in series with a resistor and LED or whatever you like to drive, but be sure the R1 could provides enough Ib drive current or use a P-MOSFET.

 

ScottWang you have the wrong transistor shown, should be a 2N3906.
SG

 

ScottWang you have the wrong transistor shown, should be a 2N3906.
SG

Thanks.

 

if the main lipo stays connected i can trigger the on-board alarm connected to the flight controller

So why not just use the smaller lipo as backup power for the flight controller. What are the battery voltages as far as main and backup?
SG

 

thank you so much ScottWang... i like that circuit and will build it asap. Should the ground (-) of this circuit or battery be tied into the main lipo ground? im assuming yes since it will be sensing the main lipo voltage...

the reason i cant use the tiny lith-ion battery to power flight controller is because it will only be 1s (3.7v) maybe 200-300mah. and the flight controller also powers the receiver,camera, video transmitter, etc... and the drone is stuck, or if it takes me a couple of hours of searching (god forbid) the battery will be dead.

remember this is only in case of emergency. since we only fly freestyle or racing the drone is normally a max of 50ft maybe 100ft for a brief moment and within less then 1/2 mile normally not a issue but... you never know where it will end up... top of tall tree etc...

 

thank you so much ScottWang... i like that circuit and will build it asap. Should the ground (-) of this circuit or battery be tied into the main lipo ground? im assuming yes since it will be sensing the main lipo voltage...

Yes, they should be have a common Ground.