Figure 7.4. There are two input terminals on the left. The upper one goes to the inverting input of the op amp. The lower one goes to the positive end of the lower battery and also to the negative end of the upper battery. The negative power supply terminal of the op amp goes to the negative of the lower battery. The positive power supply terminal of the op amp goes to the positive side of the upper battery. The output of the op amp goes to the anode of a diode. The cathode goes to the anode of another diode. The cathode of this second diode goes to one side of a tungsten lamp. The other side of the lamp goes to the negative of the lower battery. Just to the left of the lamp is a photo resistor. The lower end of the photo resistor goes to the junction of the two batteries. The upper end goes to the noninverting input of the op amp. The upper end of the photo resistor also goes through a resistor to the positive of the upper battery. There is another photo resistor to the right of the lamp. There is no electrical connection between the components associated with this photo cell and the op amp circuit on the left. The bottom end of the second photo resistor goes to ground. Note there is no ground in the op amp circuit. The top end of the photo resistor goes through a pot connected as a rheostat, then a resistor to the positive of a third battery. The negative end of this battery is grounded. The top of the photo resistor goes to the upper output terminal. The lower output terminal is grounded. End verbal description.
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