a known way the time delay of the pulse produced can be used to measure light level fig6 g is a circuit diagram illustrating a circuit used to determine the state of a switch s 1 in one application the switch s 1 could be attached to a door so an alarm is sounded when the door is opened when the switch s 1 is open capacitor c 1 is charged through r 1 producing a pulse of a known delay when the switch s 1 is closed resistor r 6 is connected in parallel with r 1 and a shorter delay is produced fig6 h is a circuit diagram illustrating replacing resistor r 1 with a rheostat r 9 the value of the rheostat r 9 can be varied by a physical connection to a movable object for example a rheostat with a rotating shaft could be attached to a float to determine the level of a liquid fig7 a is a circuit diagram illustrating a third wire 717 in addition to the pair of wires 115 added to the series of sensors 105 1 105 n to return pulses to the controller 220 the resistor r 9 of fig2 b in series with the sensor drive is eliminated so the voltage applied to subsequent sensors 105 is increased allowing more sensors 105 to be connected in the series of sensors 105 1 105 n before the sensor input voltage drops to an unacceptable.