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into the holes . Be c areful; if y ou bend the pins , c onnec tions will not be made and the c hip may bec ome unus able.
3. Onc e y our LED bric k is ins erted, y ou need to find the c orrec t polariz ation. A "light emitting diode" is firs t and foremos t a
diode, whic h is a s emic onduc tor devic e that allows c urrent to flow almos t freely in one direc tion (while emitting light if it is an
LED), and almos t c ompletely bloc k s c urrent flow in the oppos ite direc tion. You c an eas ily determine the direc tion in whic h
y our LEDs are ac tivated by finding out whic h direc tion c aus es them to light up; unlik e the TTL c hips y ou will us e later, y ou
will not harm LEDs by apply ing voltage in the wrong (c urrent-bloc k ing) direc tion. However, y ou c an eas ily burn out an LED by
apply ing too muc h c urrent in the forward direc tion. Therefore, y ou s hould never c onnec t an LED direc tly between power and
ground; alway s plac e a few-hundred-ohm res is tor between ground and the negative lead.
4. Tak e a 330-ohm res is tor (one with two orange bands at one end) from the s mall res is tor pac k envelope in y our k it and ins ert
one lead of the res is tor into the neares t ground bus .
5. Ins ert the other hanging lead into the s ame terminal as one of the LED bars .
6. Connec t other end of y our c hos en LED to the neares t power bus with a wire.
7. Rec onnec t y our protoboard to the battery . If the LED lights up, y ou have found the c orrec t polariz ation! If not, s witc h the
power and ground s ides (moving the res is tor appropriately ), and try again. If this does n't work , c hec k y our wiring c arefully and
meas ure the voltages at both s ides of the LED and res is tor with y our voltmeter to mak e s ure that the one s ide has pos itive
voltage and the other is grounded. If it s till does n't work , repeat with a different LED in the bric k . If that s till does n't work , as k
y our lab ins truc tor or TA for help. W e rec ommend y ou leave one LED c onnec ted in this way to provide a c ons tant vis ual
indic ation as to whether y our board is c urrently powered; a s urpris ingly high frac tion of the time, the problem is that one
forgets or fails...
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- Fall '14