From: Brian Gr (brian_gr_at_hotmail.com)
Date: 2001-12-30 01:57:23
Hi,
I built a BrainMaster when it was an open design. I went through similar issues with power and isolation. Here's what I did with the bm, maybe they could be of some use in this design:
I put only one connector on the unit, a 25 pin D connector. Depending on which of two cables were plugged into this unit, one could either operate theunit or charge the internal 6 nicads in the unit, but one couldn't run theunit with the charger, at least with these connectors.
The nicads were connected to the 25 pin D connector in 3 battery pairs. Theeeg inputs were connected to the 25 pin D connector. The input and ground and isolated serial was connected to this connetor as well.
One D connector/circuit was used to charge the batteries. It connected a wall wart to a couple of resistors to charge the internal batteries. You couldn't operate the unit when this connector was connected as the rs232 and eeg inputs were not connected on this connector/circuit.
A second connector/circuit was used to operate the unit. By connecting pinson this connector, the three pairs of batteries were connected in series and fed to a voltage in and ground pin on the 25 pin connector. This powered the unit. This connector had eeg inputs and ref ground and isolated rs232connected as well.
The Vin and ground were available on the connector, so I suppose someone could or would think of connecting an unisolated supply to this input and notusing the internal batteries. Well, I'm no dummy, I never would.
To isolate rs232, I used 2 pairs of ir leds and integral ir recievers from Digikey. They were Fairchild parts. Current Digikey part #s are QSE158-ND ($1.42, qty 1)(http://rocky.digikey.com/scripts/ProductInfo.dll?Site=CA&V=46&M=QSE158) for the logic detector and QEE113-ND ($.74, qty 1) (http://rocky.digikey.com/scripts/ProductInfo.dll?Site=CA&V=46&M=QEE113) for the ir diode. The reciever has built in circuitry to generate logic out from IR in. The leds and current limiting resistor were connected to the tx on both sides and the logic detector was connected to the rx on both sides.Pointing the appropriate led / detector pair at each other allowed isolation. The BM board had one pair and the isolation board had another pair. Youmight think of these as optoisolators in two packages. You could control the distance between the pairs to increase isolation. There was no problem running 9600 baud across these parts.
If I was to build this again, I would put a seperate connector for the isolated RS232 to increase the distance between the isolated and non isolated portions of the circuitry. On my brainmaster, the isolated and non isolatedcircuits go to the same connector, with a couple of blank pins between.
Brian.
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST