Thankz evgerybody! got some good ideas from that thread. I swapped the relays
for a lower current (50mA) coil and used a BC549. works fine - I have the pic
port going to a diode -> 1K resistor -> 390 ohm resistor --> GND.
The BC549's base goes to the junction of the 1k/390 ohm. The 390 ohm makes sure
the base is at GND (hence OFF) when the pic's o/p is LOW & the diode in the
base cct makes sure the pic is isolated from the switching cct (worst case
scenario)

Hmmm ... I had a notion u had to bring the base up to the same level as the
collector - something about collector/base junction bias? which is why i was
sorta preoccupied with fets. But 5V drive works ok.

> No, the diode is in the wrong place.  The diode must be accross the relay
> coil, not the transistor.

i'll say. The coil is +ve at rail, -ve at collector when current flows but on
switch off back emf makes it +ve at the collector so u need the diode's anode
conected there to short the back emf across the coil. I'm using a separate 12V
battery for the motors and a 9V for the pic so transients don't get into the
pic's supply rail. haven't connected any sensors yet - the 'bot just runs round
in a figure 8 for the moment.

thankz again - Debbie :)
>
>

http://personals.yahoo.com.au - Yahoo! Personals
New people, new possibilities. FREE for a limited time.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics