This project was created to replace the AA batteries of the super scope with something better. The super scope expends batteries quickly so a rechargeable system would was preferable. NIMH rechargeable AA batteries are a lower voltage (1.2V) than standard alkaline batteries (1.5V) and they just can’t power the super scope. So a li-po system was the best option. Keeping modifications to the SUPER SCOPE minor was also a goal.
A DC-to-DC converter takes in power from one source like a li-po battery and outputs a selected voltage. The DC-to-DC converter maintains that voltage as long as the battery can provide enough power. One chip that does most of the work in a DC-to-DC converter is the MC34063. Episode 110 of the EEVBlog has a nice discussion of this part.
The Super Scope needs 30 mA of current at a voltage of 9 V. in sizing the components an 80% efficient DC-to-DC converter was assumed. The equipment required to measure actual efficiency was over budget. 8 hours of use per charge was deemed acceptable for battery sizing. Current Li-Po batteries in the size range acceptable for this project were 3.7V. So, (9*.03*8)/(.8*3.7) = .730Ah = 730mAh battery would do the job. A reasonably priced 900mAh Li-Po battery was found on Sparkfun and was purchased. A li-po battery needs a charger that can handle li-po batteries. The charger sends current into the battery until it hits the target voltage. Once the target voltage has been hit the chip stops sending current. Though-hole solutions for a Li-Po charger were elusive. There are several SMD chips for li-po battery charging, and after review the MCP73833 was found suitable for this project. This particular chip sets the charge rate by a resistor off chip. The battery datasheet said that it could be charged at a standard rate of .2C or a max rate of 1C. Since this battery is 900mAh the standard charge rate is 180mA. From the datasheet for the MCP73833 to get the 180mA charge rate there is a simple calculation. 180 = 1000/Rprog this gives the result 5.556KΩ. The closest resistor in the cabinet was 5KΩ which gives a 200mA charge rate.
After deciding on the ICs, all necessary parts were ordered from digikey. Once the parts came in they were prototyped on a breadboard. The adapter for the li-po charger was purchased from proto-advantage.com. It came with solder paste and a solder paste stencil. The solder paste was reflowed in a small electric skillet.
The breadboarded circuit worked rather nicely. The battery charged and the DC-to-DC put out a nice clean 9 V and the Super Scope worked.
Once the circuit was working eagle was used to design the PCB. The schematic was laid out and then the PCB was routed.
With the design files completed a physical board was ordered from BatchPCB. After the boards arrived, visual inspection revealed the battery connector polarity was reversed. When the board was populated the battery connector was flipped on its back and the problem was fixed. The power plug for the charger was also reversed; some trace cutting and two wires fixed the problem there. With the two fixes the board works great. When the board was first connected to the super scope it did not work. After troubleshooting it was discovered that the controller port number 2 on the SNES was broken. A console swap fixed the problem and now the project is finished and working.
This design is not perfect. One of the possible improvements that could be made would be to redesign the PCB to replace the Battery cover. Another improvement would be to use some sort of spring contacts to connect to the battery terminals instead of soldering wires to the battery terminal PCB. Another problem is that there is no low voltage auto off. If the board was turned on for an extended period of time the battery would be completely drained and could shorten the life span of the battery.
This is the first project I have ever put on the internet. Any suggestions on how to make this a better post would be appreciated.