|The monstable is formed by IC1. R1 and C1 trigger the monostable when the power is connected. D4 helps to stabalise the IC supply from the motor when running. VR1 alters the time duration from:
T = 1.1 x C x R
T = 1.1 x 0.00047 x ( 47000 + 0 ) = 24s (with VR1 at minimum)
T = 1.1 x 0.00047 x ( 47000 + 470000 ) = 268s (with VR1 at maximum)
Q1 drives RL1 while D1 reduces EMF from the relay coil. As long as the ignition is off and RL2 is off. The NC contact of RL2 will supply the power for RL1 and the sensor only. These draw no current when off and so no current is drawn at all until the sensor is activated - saving a flat battery if left for many weeks.
When the sensor is activated, RL1 is turned on by Q1. The NO contacts supply the power to the rest of the circuit, which immediately triggers the monostable sending pin 3 high and keeping Q1 turned on - thus a latching effect. When the time duration is over, Q1 switches off, turning off RL1 and the rest of the circuit.
Supply to the mortor is through D3. This is needed so that is the pump is activated manually, the circuit doesn't activate also and start to provide a timed ouotput. If this happened, when you switch the pump off, it would still be running if you activated it for less time than the timer did.
It is rated at 3A which is fine for smaller bilge pumps like mine which only draws about 1.3A under load. If this is used for a bigger pump, then either add another diode or 2 in parallel with D3, or use a DPDT relay for RL1 where the second set of contacts activate the pump only and the first set activate the circuit as before.