Dc circuit help

Add a pull down resistor. This will notify the timer board that the sensor is closed.

I have an liquid optical sensor that has a 5vdc output when open and a .4v  when closed (LGZD sensor v1.1)
I have a l298n H-drive motor board
And a dw44 ne555 4pin timer
12 volt power supply supply power for l298n board and 5v from the L298 board to supply the optical sensor board and the dw555 board power
The output of the optical board is 5vdc when open and .4v when closed the input on the dw44 needs a ground (not sure if needs a ground but if I ground it it works) signal to start the timer, the output of dw44 is 5vdc goes to in2 of the L298 board to start the motor. And I need the timer to start when optical is closed 

Connect a 4.7k resistor from the optical board output to the optical board ground.

Then connect the optical board ground to the main circuit ground and power up the optical board. Check whether the output is getting 5V and 0 now.

Direct current (DC) is the one directional or unidirectional flow electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current. A term formerly used for this type of current was galvanic current.
The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.
Direct current may be converted from an alternating current supply by use of a rectifier, which contains electronic elements or electromechanical elements that allow current to flow only in one direction. Direct current may be converted into alternating current via an inverter.

Direct current has many uses, from the charging of batteries to large power supplies for electronic systems, motors, and more. Very large quantities of electrical energy provided via direct-current are used in smelting of aluminum and other electrochemical processes. It is also used for some railways, especially in urban areas. High-voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.