On 24 January 2011 16:40, RussellMc wrote: > On 24 January 2011 16:48, Richard Prosser wrote: > >> Here's an alternative - similar idea to Russells >> Don't pay too much attention to device types or component values - >> thay will need tuning to your application but could be used as a very >> rough starting point . Possibly. >> =A0(Your device is the LED / Resistor at the top. It can equally well be >> moved to the earthy end of the battery etc.) >> RP > > That bends the brain just the wee-est amount until a few clever points > are noted. > Plus it is a flasher only with the LED flashing bright, then tailing off. > You may need a second Q to get it back to a DC or flash mode as mine > was. Maybe not. > > C! charges via path D3 R1 D1 C1 D2 - so starts bright and tails off. > Pulse on Q1 base turns Q1 on, clamps C1 top to gnd so C1 bottom goes to = =A0-Vin > > Q2e is drive to -Vin so Q2_b is at ground so Vbe is applied to Q2 to turn= it on. > LED now conducts Via Q2 to bootm on C1 so sees Vin + Vcap. > As C1 discharges brightness drops. > > More but that will do. > > Min is cleaner IMHO and does more like wyat is wanted. > But I lke the brain bending aspects of this one. > > Mine, (which is standard voltage doubler) can use bipolars instead of > FETS with due care. > > =A0 Russell > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > Yep, that pretty much describes the operation. It was originally intended as a relay drive circuit to allow 24V relays to operate from a 12-15V supply. Once pulled in the standing current was enough to hold the relay in. Not recommended for critical applications however! As only a quick pulse of energy was required in this case I thought it may be applicable. Q1 can be just a mechanical switch if required and the "off" current is effectively zero once C1 is charged. RP --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .