Electronics laboratory

1) Design and construct a circuit similar to the one below that will utilize a photo eye to control an HVAC system. Use a photo eye, a voltage controlled current source (a Transistor), dc motor, a DPDT switch, a relay, and two power supplies. The resistance of the photo eye changes with light. Its resistance is inversely proportional to light. The TIP47 transistor turns on at approximately .7Vdc. Assume that the control lead of the transistor (called the “base”) requires no current to operate – just the .7 volts. Reversing of the motor to change from “heat” mode to “cool” can be accomplished by wiring a double pole double throw switch as illustrated below. Your primary challenge is to set up the system so that the transistor is at the edge of turning on at the current temperature of the lab. The easiest way to do this is to calculate what resistance R1 must be in order to apply the turn-on voltage to the gate of the transistor while the photo eye is halfway between full light and full dark (oh yeah…… the voltage divider rule can be used here). Use your fingers to cover the photo eye enough to trigger the transistor and relay and turn the motor on.
Electronics Laboratory

2) Explain the purpose of every component based on your task of having to control a high current high voltage motor with that teeny tiny photo eye…

Purpose of:

Photo eye –

Transistor –

Relay –

DPDT Switch –


1. Purpose of photo eye is to control the triggering of on-off motor at a certain external illumination. By having its resistance inversely proportional to the external light , when external light becomes brighter its resistance increases and hence the transistor is blocked and the motor is turned off. (if the motor were directly wired to the output of the transistor the frequency of the motor were diminished).

2. Purpose of transistor is to amplify the small variations of current through the resistor divider formed by the photo eye and R1. By having a resistance load on its collector it also amplifies its the base voltage to a voltage high enough to drive the relay.

3. Purpose of the relay is to trigger on-off the motor based on the current (or equivalent voltage) provided on its primary coil by the transistor (which amplifies the external illumination variations).

4. purpose of DPDT switch is to switch the polarity of the voltage applied to the motor and hence reversing the motor from “heat” mode to “cool” mode.