From: mwp@ms9.mis.semi.harris.com (Mark W. Poole)
Newsgroups: rec.models.rc
Subject: Re: Airborne Battery Monitor Schematic
Date: 5 Jan 93 15:47:26 GMT
Reply-To: mwp@ms9.mis.semi.harris.com
Organization: Harris Semiconductor
X-Newsreader: TIN [version 1.1 PL6]


I received several requests for additional information regarding
the schematic I posted for an improved version of the "go-light".

Here goes.

Helpful Circuit information:
---------------------------

The leds have two leads, the SHORT one gets connected to the IC and
the LONG one gets connected to V+.

The C1 capacitor is of tantalum type, rather than ceramic, and has 
polarity indicated by a "+" on one end.  This end gets connected to V+.

I placed the IC in the center of my blank PC board, which was nothing
more than a bunch of holes with copper around them.  I then placed all 
the components as close as possible on the same side of the board
to reduce weight and size.  I cut away the excess PC board, leaving
a circuit about 3/4 inch by 1 1/4 inch, and I could've made it
smaller if I would have been smarter about where I placed some of the
parts.  

I used the legs of the components for all solder connections, insulating
the ones that had to cross over others with insulation stripped from
small-guage wire. 

After completing the circuit and verifying its functionality, I spread
epoxy all over the connections to ensure that nothing could get bumped
and shorted.  Its crude but functional.

I purchased all components except for the pc board from a local
electronics distributor.  Their prices were significantly lower than
Radio Shack for most components, and Radio Shack didn't carry the IC. 


Where to connect it:
-------------------

You can connect the circuit to ANY point in your receiver hardware where
there is battery voltage and ground available.  I connected mine into
one of the unused servo slots in the reciever, but you could connect it
to your battery pack or switch harness.  I had an extra servo
lying around, so I cut off the connector and wired it to the battery
monitor circuit.  You can determine which pins are + and - by looking 
at your receiver where the switch harness or battery plugs in.  For
example, if the switch harness has connections to pins 1 and 2 in the
receiver slot, place a voltmeter across pins 1 and 2 in an unused
SERVO slot to see which pins are + and -.  Wire the connector to your
battery monitor circuit and simply plug it into this extra servo slot.


Setting the Threshold Voltage:
------------------------------

I wanted the red led to illuminate when the battery voltage (under load)
reached 4.75 volts.  I connected the battery monitor to my receiver
pack and connected my voltmeter to any V+ and gound I could find.
Fully charged the battery pack will be around 5.6 volts, and only the
topmost green led will be illuminated.  I started exercising the servos,
watched some TV, and did this until the voltmeter indicated 4.75 volts
with the servos idle.  At this point I adjusted the potentiometer to 
illuminate the red led.  Voila.


Current Drain:
-------------

The monitor is on all the time, with one led completely illuminated
or a couple of leds partially illuminated(dot mode).  The circuit 
CAN be wired in "bar mode", where MANY leds would be illuminated, by
connecting pin 9 to V+.  This would cost approximately 15-20ma PER LED, 
and is not advised.  In "dot mode" (recommended) the drain is 
approximately 15-20ma, which isn't significant in my opinion, especially 
given the information you gain from it.  I no longer "wonder" if my 
batteries are OK. 


Where to mount it:
------------------

I mounted mine in the rearmost window of my Goldberg Anniversay Cub, 
with the Green leds at the top and the red led at the bottom.  I can 
see the condition of my batteries at any time by performing a "fly-by". 


Experiences:
------------

After a couple of weeks of usage, it seems that my transmitter batteries 
get low long before the receiver batteries do, but this could be specific 
to your configuration, battery age, and flying style.  Also,  I broke
one of the yellow leds by trying to bend it to align with the others.
The plastic top of the led seems to be fairly brittle.  

This was the first time I had ever tried to build a circuit, and it was 
really rewarding to have it work perfectly the first time.


Disclaimer:
-----------

I assume no responsibility for crashed planes due to incorrectly built
or configured "battery monitors" (unless its my plane, of course). 


Good Luck!  It was a fun weekend project and I'm anxious to build 3 more
to fit my other planes.


Mark.