Step 1: Disassemble the Machine
Step 2: Trial Run the Circuit
I use a infrared emitter and receiver pair ITR20001 to sensor my hand. You may change to use other infrared emitter and receivers but you may need to change the resistors in series with the diodes.
I use a breadboard to try out the circuit and found that work perfectly. The 100ohm resistor may go very hot so you may change it to bigger value like 300ohm but that may shorten the sense distance.
Step 3: Solder Everything Together
Basically the two transistors are stick closely and the resistores are bend to stick on the transistors. The infrared emitter and receiver are solder together to become one unit that require only three wires.
Six lines come out from the complex. Three go to the infrared emitter and receiver. Three go to motor and power.
Step 4: Final Assembly and Water Proof
The complex is just fit and can put into original room.
Now put everything back and try with real soap.
Normally no issue at all as the circuit has been tested via breadboard.
That is all. Enojy it.