For my first post, a very simple circuit that I threw together in an evening. The reason for this circuit was that this aircraft happened to be on my building bench:
It's a Binary 900 Discus Launch Glider, which is available from different online sources. Anyways, the kit belongs to a friend who asked if I would build it for him. The build is pretty simple and there are plenty of links where it is detailed. What I'll describe here is an extra feature I added to the glider. The feature is a low voltage warning circuit to tell you if you need to charge the glider before you toss it into the air again.
On most electric powered RC aircraft, you have a motor, which sucks a majority of the on board electrical power. RC speed controllers have a cut-off circuit which serves to warn you when the battery is getting towards empty. There are two reasons this is useful. First because it isn't much fun being airborne when your battery drains completely (ask me how I know), and second the lithium polymer batteries used in RC aircraft don't like being discharged below about 3 volts per cell.
Anyways, the motor controller typically either pulses or cuts off the motor completely when it's time to land. On a glider, however, not having a motor means that it doesn't have an easy way to tell if the battery still has enough juice for another toss. The radio and servos will continue to work well below a voltage that would result in damage to the battery.
What I needed was something light and simple that you could glance at before tossing the glider back into the air for another flight.
Some researching and I found an op-amp circuit that was just simple and light enough that I could add to a 150g glider without hurting the performance too much:
http://www.reuk.co.uk/LM741-OpAmp-Voltage-Indicator.htm
Here's the schematic I sketched up:
Op amps are a surprisingly simple circuit element, but one that can be incredibly useful. Here I'm using the op amp in probably it's simplest form - a comparator. The basic operation of an op-amp is that when one input (the one with a "+") is at a higher voltage than the other (the one with a "-") the output is high. Otherwise, the output is low. Add a light to this output and you can have an indicator to tell you if your voltage is above or below a certain range.
In the circuit above, I am using a zener diode , D1, to set the voltage at the non-inverting input (pin 3) to a constant voltage of roughly 1.8V. The inverting input of the op amp (Pin 2) is fed by the wiper of a variable resistor. This variable resistor forms a voltage divider and allows me to set the trip point of the circuit by adjusting the variable resistor. When the voltage at pin 3 is greater than the 1.8V from the Zener Diode, the output, LED1, will be OFF. When the voltage at pin 3 is lower than the 1.8V at pin 2, the output will be ON.
I ordered some LM741 Op-Amps from Digikey so I could do some bread boarding and validate my design. It wasn't long before I had something working on the breadboard.
The circuit is near the bottom of the picture above. The LM741 is the 8 pin DIP package.
To test the circuit, I used my giant rheostat to load the battery down. This is nothing more than a large 3 ohm variable resistor that is rated up to 12.9Amps continuous current. I connected it across the battery in the circuit and adjusted it to get whatever battery voltage I wanted.
Once I had the circuit working on the breadboard, it was time to squeeze it onto the smalled piece of circuit board I could find. Here is the final result soldered onto some prototyping circuit board.
Here's the circuit squeezed into the fuselage of the Binary 900. It is the small black backage between the silver metal wing mounts in the left area of the picture below. You can just make out the wire going to the LED glued into the side of the fuselage.
Finally, here's the final product in action. Can't wait to play with it when there are some thermals in the area.
It's a Binary 900 Discus Launch Glider, which is available from different online sources. Anyways, the kit belongs to a friend who asked if I would build it for him. The build is pretty simple and there are plenty of links where it is detailed. What I'll describe here is an extra feature I added to the glider. The feature is a low voltage warning circuit to tell you if you need to charge the glider before you toss it into the air again.
On most electric powered RC aircraft, you have a motor, which sucks a majority of the on board electrical power. RC speed controllers have a cut-off circuit which serves to warn you when the battery is getting towards empty. There are two reasons this is useful. First because it isn't much fun being airborne when your battery drains completely (ask me how I know), and second the lithium polymer batteries used in RC aircraft don't like being discharged below about 3 volts per cell.
Anyways, the motor controller typically either pulses or cuts off the motor completely when it's time to land. On a glider, however, not having a motor means that it doesn't have an easy way to tell if the battery still has enough juice for another toss. The radio and servos will continue to work well below a voltage that would result in damage to the battery.
What I needed was something light and simple that you could glance at before tossing the glider back into the air for another flight.
Some researching and I found an op-amp circuit that was just simple and light enough that I could add to a 150g glider without hurting the performance too much:
http://www.reuk.co.uk/LM741-OpAmp-Voltage-Indicator.htm
Here's the schematic I sketched up:
Op amps are a surprisingly simple circuit element, but one that can be incredibly useful. Here I'm using the op amp in probably it's simplest form - a comparator. The basic operation of an op-amp is that when one input (the one with a "+") is at a higher voltage than the other (the one with a "-") the output is high. Otherwise, the output is low. Add a light to this output and you can have an indicator to tell you if your voltage is above or below a certain range.
In the circuit above, I am using a zener diode , D1, to set the voltage at the non-inverting input (pin 3) to a constant voltage of roughly 1.8V. The inverting input of the op amp (Pin 2) is fed by the wiper of a variable resistor. This variable resistor forms a voltage divider and allows me to set the trip point of the circuit by adjusting the variable resistor. When the voltage at pin 3 is greater than the 1.8V from the Zener Diode, the output, LED1, will be OFF. When the voltage at pin 3 is lower than the 1.8V at pin 2, the output will be ON.
I ordered some LM741 Op-Amps from Digikey so I could do some bread boarding and validate my design. It wasn't long before I had something working on the breadboard.
The circuit is near the bottom of the picture above. The LM741 is the 8 pin DIP package.
To test the circuit, I used my giant rheostat to load the battery down. This is nothing more than a large 3 ohm variable resistor that is rated up to 12.9Amps continuous current. I connected it across the battery in the circuit and adjusted it to get whatever battery voltage I wanted.
The aircraft is powered by a 2-cell Lithium Polymer battery. This battery is fully charged at 8.4V, nominally operates at 7.4V, and shouldn't be drained below 6V. I chose 7.0V as my desired trip point to prevent from discharging the battery too much during normal use.
With the battery just below 7V, I adjusted the variable resistor in the circuit until the LED just turned on. At this point there's nothing more to do. The low battery warning light will stay off until the battery voltage drops below the 7V point. In the image below you can see the input is 6.77V, and the LED is on.
Once I had the circuit working on the breadboard, it was time to squeeze it onto the smalled piece of circuit board I could find. Here is the final result soldered onto some prototyping circuit board.
A quick final test and adjustment of the pot was all that was needed before sealing up the circuit board.
Here's the circuit with a battery level of 6.9V, the LED is off.
Here it is with the battery voltage slightly lower (although the Power Analyzer is still showing 6.90V due to round-off error). The LED is now on.
With this circuit on board, all you need to do is glance at the LED and wiggle the controls on the glider. If the LED stays off, you're good for another flight. If the LED flickers ON when you wiggle the controls, you might want to think about calling it a day and charging the battery inside the glider.
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