On Sat, 12 Jul 2003 06:26:36 +1000, you wrote:

>PICers - I'm thinking of breadboarding a pulsed IR spotlight. The idea =
is to
>pulse LEDs on for ~10 msec then switch off for ~100 msec. =
Straightforward to
>programme.
>
>It'll pull a hefty current pulse from the battery - 12V gel - and i got =
to
>wondering what could possibly go wrong as the battery discharges and =
terminal
>volts go down?
>
>ie Could the PIC could freeze up and latch the LEDs in an ON state as =
Vcc
>declines? What should I be on the look out for & ways to guard against? =
If I
>get round to building a full array, it'll be 9x7 IR LEDs @ maybe $1 each=
 so u
>absolutely, positively don't want a latch on. :( Looking for ideas ....
>Thankz - Debbie
A simple way to guarantee against sticking on would be to AC couple the =
drive signal with a
capacitor and resistor/diode clamp. Choose the time constant such that it=
 limits the 'on' time to a
safe value if the output sticks high.=20
If you don't do this, then at the very least you  want a pulldown =
resistor to the Mosfet gate drive
so it can't float high if the PIC goes into reset and tristates the pin.

$1 seems a bit expensive for the LEDs. especially in that quantity..!

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads