Light Sensitive and Differential Temperature Switch

Light Sensitive and Differential Temperature Switch

Circuit Project Description

The circuits were designed to illustrate the functions of two switches with the use of a single op-amp to show the operations of a precision light sensitive switch and a differential switch.

Terminology LM741 – a general purpose single operational amplifier with features such as offset null, compensated internal frequency, voltage range with high input, good stability of temperature, and protected from short circuitBC214L – a complementary silicon planar epitaxial transistor used in AF small signal drivers and amplifiers as well as for low noise preamplifier applications due to its feature of good linearity of DC current gain Circuit Explanation

The first figure denotes the switch which is precisely sensitive to light. It becomes activated when the amount of light goes above or below the preset values. This is determined by the light dependent resistor (LDR) that can be made of cadmium sulfide (CdS) cells. Physically, it is yellow in color and also functions as a semiconductor. At the trip level, its resistance ranges from 500R to 20K. To fully maintain the operation at normal light level, LDR is being adjusted by 22K ohm potentiometer RV1. The resistance of LDR varies with the amount of light that passes on it.

The second figure of switch circuit is based on the differential temperature. It makes use of an ordinary silicon diode to function as temperature sensing element which responds proportionally to differential of a fraction of a degree. Silicon diodes are the cheapest and the most common compared to other types of diodes while possessing a forward voltage drop of about 0.65 Volts. The offset of several degrees to the two diodes D2 & D3 is being handled effectively by the 1K ohm potentiometer RV2. The required differential temperature is applied to the two diodes for the adjustment of the circuit following the regulation of RV2 to let the relay to just turn ON.

The measurement of the two diodes is based on relative temperature rather than absolute temperature. Relative temperature uses the scales of Celsius and Fahrenheit and a unit in terms of degree. The scales can be positive or negative and a measurement of zero degrees is considered as a temperature. Common numbers are used to express the temperature of humans in normal environment. The absolute temperature on the other hand uses the scales of Kelvin and Rankine which is referenced to absolute zero that denotes no temperature. In this type of measurement, very large numbers are used for the normal environmental temperature of humans. Also, the scales are not using degrees as their unit and negative temperatures are not used.

Part List

R1=LDR *see text
RV1=22Kohm  pot.
RV2=1Kohm  pot.
RL1-2=RELAY 12V >120 ohm


The use of light sensitive switches with light dependent resistors may be widely seen in street lights because of its very low cost feature. In street lights, they are used for auto dimming and twilight or darkness detection for tuning ON and OFF the lights. The circuit can also be used in outdoor clocks, alarms, clock radios, and photographic exposure or camera light meter type applications like the photoswitch in electronic toys, auto flash for camera, photoelectric control, and industrial control.

For differential temperature switches, they are used in solar heating systems where a pump is activated to pump hot water from the solar collector to the storage tank of hot water. This will happen on the event that the water in the solar collector was heated on a preset temperature differential with reference to the water in the storage tank.

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