Note: this article was published in a Brazilian magazine and so transleted in English by the author to be part of one of his books published in USA.
The project is based on a light dependent resistor (LDR) or cadmium sulfide (CdS) photocell and works as follows.
As soon as the door of a freezer is opened, light falls onto the LDR, activating the circuit (i.e., IC1-a output goes from low to high).
The circuit is then actuated, and a warning tone is produced by a piezoelectric transducer until the door is closed again (the circuit is not timed).
IC1-b acts as a low-frequency oscillator, and IC1-c as an audio oscillator.
The oscillators’ signals are combined by IC1-d to drive the piezoelectric transducer with an intermittent tone.
The circuit can be powered from AA cells or a 9 V battery, and current drain is on the order of 0.5 mA when off and about 5 mA when the tone is on.
A schematic diagram of the Freezer Alarm is given in Fig. 1.
Proper positioning of the polarized components (electrolytic capacitor and power supply) should be observed.
You can use any type of LDR in this project.
Types with diameters ranging from 1/4 to 1 inch are suitable, as sensitivity can be adjusted by R1.
All the components can be housed into a plastic box, but it needs to incorporate a hole to allow the light to fall onto the LDR and generated tone to escape.
The sound pulses can be altered by varying R2 or C1.
C1 can range from 0.22 µF to 0.47 µF.
Audio tone is determined by R3 and C2. These component values can also be varied.
IC1 - 4093 CMOS integrated circuit
X1 - Piezoelectric transducer or crystal earpieceLDR - CdS photocell (any type)
S1 - SPST toggle or slide switch
B1 - 6 V or 9 V, four AA cells or battery
R1 - 1,000,000 Ω - trimmer pot.
R2 - 2,200,000 Ω, 1/4 W, 5% resistor
R3 - 47,000 Ω, 1/4 W, 5% resistor
C1 - 0.22 µF to 0.47 µF ceramic or metal film capacitor
C2 -- 0.022 µF ceramic or metal film capacitor
C3 - 100 µF, 12 WVDC electrolytic capacitor
