Simple RX Battery Monitor

update.gif (1811 bytes) 14 Apr 99 - Updated schematic & veroboard layout to improve consistency.

This is an extremely simple design for a battery monitor. As shown below it can be connected to a spare output of the RX. With the addition of a pair of output wires can be connected between the battery & RX. These output wires can be inserted in to the same holes as the input wires. It is recommended that the assembly be covered in heat shrink sleeving with holes made for the LEDs or potted in to a small Vero box.

In use the meaning of the 2 LEDs is as follows:

LED(s) showing Battery Condition Estimated Voltage
  Green only   Fully charged over 4.3V
  Red & Green   Beginning to discharge - safe to fly between 4.0V and < 4.3V
  Red only (or with dim green)   Discharged - Not recommended to fly below 4.0V

Please click on the Schematic and Vero board layout images below for the full size version. A parts list is presented at the bottom of the page. It should be noted that in order to minimise the size of the board, R4 and TR1(c) share a hole and D2(a) and D3(k) share a hole. These 2 holes may need to be enlarged to accommodate the 2 leads to be inserted. The board only needs to 1 track to be cut at 1 position, between R1 and D3/D2 (this can be done carefully by hand with a 1/8" drill bit).

Monitor SchematicMonitor Vero Board Layout

Parts List

Ref. No.

Description Comments
R1 560R W 5% Resistor  
R2 1K W 5% Resistor  
R3, R4 120R W 5% Resistor Increase to 270R for 6v NiCd
C1 47uF 6.3v Electrolytic Value not critical - voltage is. Use 10v for 6v NiCd.
D1 BZY88C4v3 4.3V W Zener Diode Change to BZY88C5v6 (5.6V) for 6v NiCd
D2 5mm Green LED Can be almost any LED
D3 1N4148 Signal Diode Can be any silicon diode
D4 5mm Red LED Can be almost any LED
TR1 BC239 NPN Transistor Can be almost any NPN small signal transistor (e.g. BC108)
  Servo lead to suit your system If using input & output, an extension lead can be cut in half.
  Piece of Vero board  

Vero board layout may need to be altered slightly to accommodate these changes

I have included suggestions for revised values of some of the components for operation with a 5 cell (6V) NiCd pack. This option has not been tested and users should ensure that they are happy with the point at which the LEDs changeover. It is likely that the Red LED will illuminate slightly earlier with this arrangement. If it proves too sensitive, a 5.1V Zener could be used instead.

Testing the completed unit

As it is extremely easy to do, I would recommend testing every unit to confirm the voltages. The method I used was:

1. Connect the monitor to a charged NiCd (should have Green LED).
2. Connect a voltmeter to the NiCd (or output connector of fitted).
3. Connect a S400 6v (or similar motor) to the NiCd (or output connector).
4. Apply load to the motor until the Red LED just starts to light, release load until it just goes out - note voltage 1.
5. Apply more load until the Green LED just extinguishes - note voltage 2.
The load can just be finger pressure on the shaft (use a cloth to stop your fingers getting too hot).

You now know that if the Red LED is showing you are below voltage 1, and if the Green LED is not showing you are below voltage 2.

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