Just a short article about the battery charger I just assembled.
Charging Ni-Cad Batteries
Ni-Cad batteries can be fairly easily charged using a constant current source. For best results with this type of charger you should set the current no higher than C/10, where C is the Amp-Hour rating of the battery. Here is a link to a site which discusses several methods of charging Ni-Cads.
This simple charger can be built with very few parts.
- One 3-pin header (H1)
- Voltage Regulator (U1)
- Four 82 ohm resistors
- One 2-pin cable connector (H2)
Here is the schematic:
I used four 82 ohm resistors simply because that is what I had on-hand. The required resistor value can easily be calculated using Ohms Law: R = E/I. Our battery packs are either 1600 maH or 2200 maH so I chose to use 160 ma for the charger. Therefore, the resistance needs to be 3.3 V/160 ma = 20.6 ohms. Four 82 ohm resistors in parallel yield 20.5 ohms – close enough for government work! There will also be the current required by the regulator (about 5 ma) but that is relatively insignificant relative to the total current.
Pictures of the unit
The first is a close-up and you can see that it is built on a piece of breadboard material I cut from a larger piece. The second shows the connector assembly that mates to the connectors of the batteries we purchased.
The second shows the connector assembly that mates to the connectors of the batteries we purchased.
Puck and I discussed what to use for sensors for the Rover. We came to the conclusion that there are basically three possibilities for “long distance” sensors: sonic, infra-red and camera. I did an Internet search to see what is available for the sonic and infra-red.
The first place I went was to Digi-Key and entered “optical sensor” into the Part Search box. It found 26 items but most were listed as “Non-stock”. I did not want to use something that is not stocked. The few that were listed as available have very short ranges.
The next attempt was a little more adventurous – I used ASK (my preferred search engine) to search for robot accessories. A number of interesting site were listed. I was unable to find anything useful until I looked at Ehow.com. The site has quite a bit of useful information.
The most useful link I found was: RobotStore. This store has a really good selection of parts and accessories. Selecting the Sensors link from the left panel led me to the sensors I wanted to take a closer look at.
SRF-05 Sonic Sensor
They have quite a few sensors. The first one of interest was an IR sensor, but its minimum distance is listed as 40 inches which is obviously too far out to be of use.
Shortly below that one are a number of sonic sensors which have various ranges most of which would work just fine in our application. We finally decided on the SENSOR-SRF05 for $28. There were two deciding criteria: its range is 1cm to 4m and we also like the way it looks. The output of the sensor is a pulse whose length is directly proportional to the distance of the object it detects.
Our current thinking is that we will mount the sensor on a rotating mast. The mast will be rotated using an RC-type servo controlled via a low duty cycle PWM signal from the Rabbit processor.
We will also probably mount a small camera on the mast. Rabbit has a Camera Application Kit that looks like just what we need for both the camera and the software to use it.