I posted this message before, but it seems to have failed. Here goes
I've been able to start soldering, and to study more, and I've come up
with a couple of problems.
When the air pressure rises, the input voltage drops as it becomes
harder to pump. However, as the reference voltage is set by the same
battery, won't they drop together? This could be fixed by using two
There is a much bigger and more fundamental problem. When the motor
is turned off, the battery voltage will jump back up to 9v, causing it
to switch back on. (this would loop until the air pressure lowers
again). This can't be simply solved by having two supplies. Unless
there is a clever solution to this floating around out there, it looks
like a pneumatic pressure microswitch is the way to go.
Ok, so the goal is to control pressure, not just protect
the motor. Use 2 pressure switches - one for low
to turn the motor on, and one for high, to turn it off.
Below is a complete diagram, but since the pressures
are unspecified, you will need to determine if the
switches in the parts list at the bottom will work for
| | |
| PNP [MOTOR] [D3]
+9 ---+---+----+------ ----+------+ | |a
| | | e\ /c | | +-----+
| | o --- | [RY1] |
| | / HP | [D1] | |
| [1K] o | |a [33R] |
| | | | | | o---
| | +--------+ +------+ o--^
| | | | | | RY1-2
| | | | | |
| | | /c o--- |
| +----|--------+-----| NPN o--^ |
| | | \e | RY1-1 |
[47uF] [1K] o | | |
| | / LP | | |
| | o | | |
| | | | | |
How it works: When turned on, both HP and LP switches are
open. The PNP transistor is biased on by the 1K resistor
to ground. The NPN transistor is turned on by the 1K
resistor to +9, which energizes the relay, closing the
RY1-1 contacts. When pressure builds up higher than the
lower limit, the LP switch closes, removing the + bias
from the NPN and turning it off. However, the RY1-1
normally open contact maintains a path to ground for the
relay coil, so the relay stays energized. Pressure
continues to build, until the HP switch transfers,
removing the negative bias from the PNP, which turns
off. That causes the relay to de-energize, and the motor
stops. Pressure decreases, and the HP switch opens,
removing the + from the PNP base, so it can turn on.
However, there is no path to ground, as the relay is
de-energized and the NPN is held off by the closed LP
switch, until the pressure drops below the lower limit.
That turns on the NPN, and the cycle repeats. The 47 uF
cap and D2 keep the voltage to the transistors stable
when the motor switches on and off.
Parts from Allelectronics:
Cat # PSW-12 (pressure switch, $1.00 each) and
Cat # RLY-642 6 volt DPDT relay. The 33 ohm resistor
will drop the 9 volts down so the relay will work fine.
They also sell the transistors (any PNP and NPN
will work), cap, resistors and 1N4002 diodes.
You may need to make TEE fittings to connect the
tubing to the switches, with one leg of the TEE
going to a piece of tubing where you can bleed
off some of the pressure to get the switch to
operate at the right level.