A project log for Dodo: 6502 Game System

Handheld Game System featuring the 6502

peter-noyesPeter Noyes 12/16/2016 at 05:420 Comments

Dodo has nearly reached its final form by now having integrated batteries! The Hackaday Superconference badge gave me the inspiration I needed to finally figure out a way to mount the batteries. By mounting the battery holders sideways on either side of the board, they are unobtrusive yet still accessible. To fit, I needed to downside to AAA batteries, but I think the tradeoff is worth it.

I also now have a working boost regulator circuit on board. The previous revision (Rev 2) had the MAX756 but it didn't work. For Rev 3 I switched to the LT1303 and very carefully laid out the PCB after researching the best practices for laying out such a circuit. I also experimented with different capacitors. I am happy to report that it is running at 85% efficiency, which is as good as expected for this part.

Unfortunately, Dodo does not run for long on typical alkaline batteries. However, with Ni-MH rechargeable batteries, or with lithium batteries the life is much better. My rechargeable batteries yield a life of around 3 hours and the lithium batteries from Energizer last nearly 5 hours.

Dodo uses around 100mA at 5V. Below is the formula for the current draw from the batteries based upon the 85% efficiency. At 3v, the current draw is around 200mA. Once the batteries are nearly depleted at 2V, the current draw is nearly 300mA.

200-300mA is quite high for an alkaline battery and significantly degrades the capacity. For instance, a 750mAh alkaline battery might instead only provide 500mAh when the current is large. The voltage also falls quickly with the alkaline batteries. On the other hand, a lithium battery holds a voltage closer to 1.5V for much longer. Also, the lithium battery is able to provide a high current without losing capacity. The rechargeable batteries seem to behave similar to the lithium batteries. I therefore recommend that rechargeable batteries be used with Dodo.

The battery indicator is also working spot on. I have it tweaked to go off when the voltage reaches 2V. The LT1303 has an internal voltage comparator and the LBO pin goes low when that threshold is reached. The trip voltage is 1.24v which is too low for a pair of AAA batteries. I use a voltage divider to scale that threshold. I am using values of 255k and 412k resistors for the divider. For my rechargeable batteries, the battery indicator goes off when there is right around 10 minutes of life remaining.

Unfortunately, version 1.3 is still not right :( There is a problem with the wiring of the low battery indicator on the top board. I was able to fix it with patch wires but a new PCB will be needed. I guess the 4th try will be a charm.