On Fri, 9 Jan 2009 00:25:16 +0100, "Dr. Ernst Baumgartner"
wrote:

Hi,

my problem ist the following:

With a foot contact I switch a tiny power mosfet (BS 170) on, which in turn
(softly) turns on a small 9V DC motor. After a moment, my foot often gets
tired, and I would like some circuit to hold the gate of the mosfet positive
for me.

After a minute or two, I would like to be able to turn off my mosfet again
by a short touch to the pedal.

I was thinking of a combination of 555 timer(s) opening a time window where
I could take the foot off the pedal, and a sample an hold ic, but do not see
how to combine them, I am just an amateur...

Is there a genius out there who can help me to accomplish this?

Any input will be greatly appreciated, thank you in advance!

---
View in Courier:

+V--+---------+-----+------+-------+------+------------+
| | |K | U1 |8 | U2A|
[10K] [100K][1N4148][1M] +---+---+ | +---+---+
| | | Rt| 2|_ Vcc |3 +-------O|S Vcc | D
+-[0.1µF]-+-----+------|--O|T OUT|--|--------| Q|--G BS170
| | 6| | | | HC74 | S
| +---|TH 7555| +-1M-+--O|R _|
O | | 7|_ _|4 | | +-|D GND Q|-+
|--S1 +--O|D R|O-|----+ | +---+---+ |
O | | | GND | | | +-----|-----+
| | +---+---+ | | |
| |+ |1 | [0.1µF] |
| [1µF] | [0.1µF] | |
| Ct| | | | |
GND-+----------------------+-------+------+----+-------+
| U2B
| +-------+
+--O|S |
+---| HC74 |
+--O|R |
+---|D |
+-------+


U1 is a CMOS 555 timer which puts out about a 1.5 second pulse (with Rt
and Ct as shown) when S1 is made.

U1 isn't retriggerable, so as long as the switch is released before U1
times out, the contact bounce on release won't cause another pulse to be
generated.

U2A is a "D" type flip-flop wired to toggle on successive high-going
edges at the clock input, and is reset on power-up by the 1 megohm
resistor and the 0.1µF capacitor, as is the timer, guaranteeing a low
input to the MOSFET gate.

In use, once power is applied to the circuit, S1 is tapped, which will
cause the motor to start turning and then tapped again some time later
in order to stop the motor. Tapping it again will cause the motor to
start again and tapping it after that...

JF