Laser Level Detector

Rotating laser levels, which are very handy  for setting objects in a room or garden at the  same height, are available at prices of a few  dozen pounds. At relatively large distances  and for outdoor use, the light from the rotating laser beam is often not easy to see, and  the laser beam detector described here can  be useful in such situations. The detector  works well at distances up to 50 metres (150  feet) and consists entirely of standard components. The detector is housed in a plastic case  that can be fixed to an object (such as a post  or a beam). It has three LEDs and a beeper that  indicate whether the object should be raised  or lowered. 

LEDs with a transparent package and integrated lens (round surface) are used as sensors. The top and bottom detection zones  each have five LEDs and two opamps (IC1a &  IC1b or IC1c & IC1d), which drive the ‘Move  Up’ and ‘Move Down’ indicator LEDs. The middle sensor LED drives the ‘OK’ indicator LED  via two opamps (IC2a & IC2b).  The rising edges of the opamp output signals  trigger three separate monostable multivibrators (type CD4047). If desired, the circuit  shown inside the dashed outline (one gate of a  CD4044 quad RS latch) can be used in place of  each of the monostable multivibrators. In this  case the output signal has the opposite polar-ity, so the BS170 N-channel MOSFET must be  replaced by a P-channel type. 

The monostable time of the middle retriggerable MMV should be longer than the rotation period of the laser (e.g. with a 2 rpm laser  it should be longer than 500 ms) so that the  beeper will emit a continuous tone. Most  rotating laser levels have variable speed, so  this can also be achieved by adjusting the peed if necessary. The monostable times of  the upper and lower MMVs are dimensioned  to generate clearly distinguishable short and  long beeps, respectively. The three MOSFETs  (T1, T2 and T3) are configured as a wired-OR  gate to drive the shared beeper. The fourth  MOSFET (T4) drives the ‘OK’ LED. 

The circuit can be housed in an enclosure  together with three penlight cells.