Visor Cradle USB/Serial Conversion

The above pic & the following is an email I received for Sean after his successful conversion of his USB Cradle to Serial. Great Job Sean!


Here is a diagram of the circuit I added. closing SW1 (grounding KBD/pin2) will allow you to hotsync over serial. If you're just going to use the serial lines to communicate to a modem, GPS, etc, you don't have to ground that pin.

Radio Shack Part #'s: 7805 (5V regulator) 276-1770 1N4148 (diodes) 276-1122 47uF (capacitor) 272-1027 20-pin IC socket 276-1991

I assembled the whole thing on two of the radio shack IC experimenter boards (sorry, don't have the P/N handy)... The perfboard I got snaps in half, and each half accepts a 20-pin DIP. I trimmed them to fit in the space in the cradle, then stacked them - 1 board has the IC, the other the voltage regulator and other goodies.

The chip I used is the Maxim MAX203, in the 20-pin DIP package. If you use the SO package, be careful, the pinout is slightly different. I ordered a couple of chips over the web, and they were nice enough to send them as samples, free of charge.

Hmm, a few other notes... I doubt the regulator circuit is all that efficient, but it seems to work okay, even on my laptop, where the signaling lines are +/- 5..7V. I'm no EE, so if you see any way to improve this, I'd be interested. :)

Might be a few days on the pics, I'll have to borrow the digital camera from work. Sean

I will be building the above circuit as soon as my 302 chip arrives. I'll probably include a transistor in the ckt. so that it can autosense a serial connection. This will eliminate the necessity for the switch. Or, maybe I'll just get lazy & use a jumper. ;) With a new daughter around, not much time for hobbies.

Using the 7805 regulator is definitely the way to keep the 203 happy. We're only talking milliamps here, but the regulator is still pretty efficient. Once again, great job Sean!


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