OLPC India/Khairat Technical

Khairat school, OLPC India

Work on this document is in progress

XOs at Khairat school

Comments

1. All photos are by Carla unless otherwise stated.
2. The following are my views and may not necessarily reflect that of OLPC. For official clarifications on any of the technical aspects, please contact people at OLPC directly.

Overview of observed Bugs

The journal

Some XOs on the 613 images had their Journal gone. Image 616 fixed that bug. Once the teacher lost his files. As a result an automatic backup script was written by Amit in the site. --Carla 16:56, 30 October 2007 (EDT)

Alternative power

The power supply in town is not very reliable, so many days we didn't have power, or had it intermittently. The children tried to bring their XOs charged, but still some needed to charge them at school. --Carla 16:56, 30 October 2007 (EDT)

XOs are getting charged

Children working while charging

Cow Power Design Notes

Mechanical design prototype

22nd December, 2007

• Pranay, Sairam and Raghav - These are some of my friends in college with whom I've recently been having some detailed discussions regarding next steps for the design. Many thanks to all of them.
• Gear box - I scouted Chawri Bazaar and Meena Bazar (places in East Delhi) with Pranay to look around for gearboxes that we could find and also to look for suggestions from local people. We got to have a look at the gear box of an Enfield (mini Bullet model) motorbike - but after manually rotating the gears on it realized that the maximum reduction that we could get with this was only 3 (in gear 1). The person was selling the gearox at Rs 1100/- We also got to know about gear boxes used in cars , but came to know that these are immersed in oil so
• Planetary Gears - One of the options that Pranay has suggested in to use Planetary gear system. Some good links to understand the concept are here

http://auto.howstuffworks.com/gear-ratio4.htm http://www.howstuffworks.com/automatic-transmission1.htm

However it seems that this might not be what we want to go in for because we are looking for existing gear boxes and don't want to build a customized solution. Perhaps some gear boxes used in cars...

• Alternator vs Dynamo - In the local market people told me that contrary to the dynamo, the alternator would start giving current below its rated speed also. Although, the alternator would require some starting current, it seems that I would need to seriously consider this option.
• Tonga wheel - On my way back I saw a Tonga on the road. I observed that it has a large wheel. I am now thinking how I may use it to my advantage :)

See here to know what a Tonga is.

• Meeting Mr Vipin -
• Increasing rpm ratio - It seems that adding a 80cm radius pulley with flat belt drive may be the best solution. Mr Vipin said that one could get it from the junk market at the rate of metal (by weight). He estimated it to weigh 70-80 kgs.

Electrical design

Requirements

1. Regulate current. The XOs at Khairat school have LiFePO4 type batteries. Let us chose a 1A charging current so that with the given capacity of the dynamo, we can charge about 15 XOs simultaneously. See Battery Charging for more details about all kinds of batteries that are used in the XO
2. Regulate voltage. The XO power supply accepts voltage from 9V - 18V . The charging circuit switches on at 9.85V. Let us chose to fix the voltage that we give the XO at 12V

Design approach

• Using L200 as the central component - it is cheap, easily available and most of all robust
• Referring to this sample circuit and deriving parameters from this design guide
• To avoid effects of shorting or reversing of supply from dynamo to the L200 circuit a simple full wave bridge rectifier using 1N5408 diodes has been used. Also, a 100uF 35V decoupling capacitor is being used after the rectifier.

Deriving circuit parameters

1. For values of R2 and R1, I referred to table in right column of Page 4 of design guide. Selecting Vo=12V +-4% I get R1= 1K ohm +-1% and R2=3.3K +-1%
2. Since Isc = 0.45/Rsc (according to sample circuit design), I chose Rsc=0.47 ohm 1/2 watt
3. Current in R2, Ir2=(Vo-Vref)/R2 where Vref = 2.77V. Hence power dissipation in R2, Pr2 = 25.8 mW
4. Current in R1, Ir1=Vref/R1 = 2.77mA . Hence power dissipation in R1, Pr1 = 7.7 mW
5. Hence R1 and R2 can be chosen to be 0.25W type

Acknowledgments

Thanks to Richard Smith for help in the electrical design.

--Arjs 11:18, 16 November 2007 (EST)

Sharing Experience

In the classroom

Amit and I, together with the teacher and the children, did some testing on the Mesh by sharing activities. Children were working on the laptops and did different kinds of sharing to work with their classmates. The Sharing Activities wasn't that intuitive, we weren't really successful with it. Nevertheless, the children who shared activities were super-happy. --Carla 16:56, 30 October 2007 (EDT)

Comments

I was very involved with the technical side with Amit, until Manu and Arjun arrived. I still was in the loop but more involved with the educational side. So, I'll focus on the aspects that had an impact on the educational side Khairat Chronicle. We worked with the developers daily to tackle these issues. --Carla 16:56, 30 October 2007 (EDT)