a simple robot made of legos accompanied by a laser as its light source. basically a night circuit with a twist:)
you can used some mirrors to make it more interesting as if you are on an action movie.. (the instillation of mirrors are up to you.. just bare in mind that the final beam must be striking the LDR 

Step 1: Materials Needed

-An LDR...
-any kind of switch( you can have those on the back of your toys)...
-about a 60000 ohm resistor...
-some wires...
-buzzers from your old toys..(a peizo buzzer will do but its a bit expensive, and it is irritating to ears(but you can use a 555 timer cgip to customize it)...
-an LED(any color will do)...
-a 9 volt battery(dont worry if you are using an old battery, 6 volts will do)...
-some electrical tapes for insulation purposes...
Lego pieces to build you robot(you can use bottles or any recycled materials based on your imagination on how you will build your robot...
-and an SCR.. its basically a transistor, the only difference is that.. one its triggered, it wont stop, even if the light came back, until it is switched off
-and of course , alaser and mirrors to build your laser mazed

Step 2: The Schematic

this my schematic for this project...following this will help figure out the connections that must be done:) and even for the trouble shooting purposes...for those who cannot follow the schematic here it is...

first, the LDR was connected to a 60000 ohm resistor connected to positive and the other end of the LDR connected to negative.

second.. the leg of the LDR connected to the resistor is connected to the base of the SCR..(the middle one)

third, the collector leg of the SCR( the right leg of the SCR with the circuit or the curve part facing you) is connected to the ground(negative).

fourth, the emitter of the SCR( the left leg with the circuit again facing you) is connected to the negatinve end of the LED and the buzzer..

lastly, the positive of the LED and the buzzer is connected to positive...(positive of the LED may have or may not have a resistor for limit depending on your battery..

hope this helps coz that all it goes... the only problem now is how are we going to install it into our robot..

Step 3: Check Your Circuit If Its Is Working

you may check you circuit on a breadboard if it is working( this is optional if you have your breadboard)

Step 4: Instillations

this steps is probably the most exhausting part
you solder now things in accordance to the schematic...
 and let the LDR be the eyes of your robot..
make sure that the LDR is capable of absorbing light or the laser must be able to strike it.. 
just make some hole to the lego piece to isert the wires..

Step 5: Finalizing the Head

place a screw between the head and the lego piece it was placed, tight enough to make it move, to make it turn its face left and right

Step 6: Make the Final Connections for Battery and Switch

a switch in here is use to manually turn turn it of because, as i said earlier, it wont stop until it was turned of, even if the light came back because of the SCR.
you can place the switch anywhere you want it. but in my case, i placed it in series with the negative of the battery to the collector of the SCR..
and the LED... is placed on its shoulders...
after all, make some insulation using electrical tape on open wires to avoid short circuit

Step 7: Pictorial and Checking

just point a laser on the LDR and turn it on, it must not make any noise until your laser is pointed on it.. and once the laser is off.. it must make noise until it is switched off..

and of coarse once you finished it.. take some pictures of it for remembrance hehe

for some errors, just message me here charleskim_kabiling@yahoo.com

for some comments.. and suggestions:)
You're so great! :)

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