Troubleshooting guide for the light beam circuit
Left to right on the schematic
1
LED – Does it light?
a. If not, check power supply connections
b. Check to make sure LED is oriented properly
c. Test LED in a known good circuit
d. Replace LED after making sure the current limiting resistor is there
2
CdS Photocell – not typically a problem
a. Check to make sure that the photocell voltage varies between about 2V
(lit) to 4V (dark) ___________ and _____________ (LM324 pin 3)
b. Check the circuit connections
c. Test the photocell in a known good circuit
3
Variable resistor and reference voltage
a. Check that the reference voltage is between the voltages recorded in step
2 (LM324 pin 2) _____________
b. Check that the variable resistor is oriented so that it is not shorted
c. Check the connections
4
LM324 Op-Amp
a. Check that there are no bent or broken pins on the LM324
b. Check that the connections are correct
c. Check the photocell and reference voltages (pins 3 and 2 respectively)
d. Check that the output switches between 0 and about 4.5V (LM324 pin 1)
e. Replace LM324 op-amp
5
NE555 Timer
a. Check that there are no bent or broken pins on the NE555
b. Check that the connections are correct
c. Check the input from the LM324 (NE555 pin 2)
d. Check that the NE555 output switches from 0 to about 4.5V (NE555 pin
3)
6
PN2222 Transistor
a. Check all connections
b. Test the transistor on the voltmeter transistor tester
7
General considerations
a. Ensure that the circuit is being powered by 5V and that the power and
ground are not reversed (this will fry both chips)
b. Ensure that all components are properly inserted in the breadboard
c. Ensure that no pins are bent or broken on the integrated circuits
8
For the debounced switch, use steps 5 through 7
Lab 4: LED Troubleshooting