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Reply to topic  [ 5 posts ] 
Interested in Building Fridge Door Alarm Circuit using 555? 
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Joined: Thu Oct 26, 2017 7:14 am
Posts: 1
Post Interested in Building Fridge Door Alarm Circuit using 555?
Here I want to share a project about building a fridge door alarm circuit. The name of the circuit itself infers the application. This circuit triggers the alarm if the door of Fridge is left open for the long time. When the door of the refrigerator is left open, the temperature inside the cabin will increase. This rise in temperature will be sensed by thermostat and try to cool down the cabin. It will always try to maintain constant temperature of the system. The compressor will be working continuously to remove the heat from cabin, this increases the power consumption from the receptacle. Also, continuous usage under this condition would reduce the life of compressor and probably do malfunction.

Hence, this Fridge Door Alarm Circuit is a good solution which will indicate the user about the door in prolonged open condition. We can also set different pre-set time after which the audible indication has to be given. This is done here by using the versatile 555 timer IC under astable multivibrator mode and LDR. As soon as we open the Door of refrigerator, LDR senses it and start the countdown using 555 Timer, and after a preset time the buzzers starts beeping as alarm signal.


Components Required:

    ·555 timer IC – 2Nos(datasheet for reference:
    ·5mm LDR – 1No.
    ·Buzzer – 1No.
    ·Diode (1N4007 or 1N4001) – 1No.
    ·Capacitor, 47uF(Electrolytic) – 1No.
    ·Capacitor, 0.1uF(Ceramic) – 1No.
    ·Resistors (10kὨ - 1; 470kὨ -1; 150kὨ -2; 100Ὠ -1)
    ·Connecting wires

Fridge Door Alarm Circuit Diagram and Explanation:


The two 555 timers are connected in Astable multivibrator mode. The key components in the circuit are LDR (Light Dependent Resistor) and 555 Timer IC.

LDR (Light Dependent Resistor):

LDR works under the principle of Photo Conductivity. The conductance of the material inside the element increases when light falls over it. In terms of resistance, the value of resistance decreases when light falls over it and resistance will be large in dark surrounding. The resistance is directly proportional the light over the material, check the table below:


There are several types of LDR like 3mm LDR, 4mm LDR, 5mm LDR, 7mm LDR and etc. The part used here is 5mm LDR. Using the above data we have considered the resistance divider as 10k with 5mm LDR.

555 Timer in Astable operation:

Astable multivibrator has no stable states. The output swings between high and low based on the timing resistor and capacitor.
The formulae to calculate the time delay is as below,

Time (Sec) = 1.1 x (R2 + R3)  x C1

Here in this Fridge Door Open Alarm Circuit, we have used two 555 ICs, one for calculate the ‘Fridge door Open time duration’ after which the Buzzer should be triggered, and second 555 IC is for controlling the Buzzer beeping pattern.
Below we have calculated Time delay for Buzzer to be triggered and selected the resistor values accordingly. Here the Time delay means the duration for which the Refrigerator Door is left open. This is done by first 555 IC in the circuit.
Time (Sec)=1.1 × (620kὨ ± 5%) × 47uF
Time =30.4 secs
Hence, R2=150kὨ, R3=470kὨ in series and C1=47uF

Below we have calculated the Time Delay for second 555 IC, which is controlling Buzzer Beeping Time Period. In this case time delay is calculated as,
Time (Sec) =1.1 × (470kὨ ± 5%) × 0.1uF
Time = 0.5 secs
Hence, R5=470kὨ and C2=0.1uF (The buzzer turns ON and OFF at this time frame)

Working of Refrigerator Door Open Alarm Circuit:

The whole circuit is powered by a 9V battery. When the fridge door is closed, it is dark and the resistance of the LDR is nearly 1MὨ as given in datasheet. The output voltage of the potential divider appears across the capacitor and it remains in charged condition (Voltage higher than 2/3Vcc) making the output LOW. When we open the Fridge, the light falls over LDR which lowers down the resistance of LDR and causing the capacitor to discharge which in this RC combination it is 30 secs. After this (Voltage lower than 2/3Vcc), the output starts to oscillate at certain frequency and output is HIGH. Again, the capacitor charges and reaches a threshold continued by discharge of the capacitor. This continues till the LDR resistance goes high which will happen in the absence of light (door is closed).

This makes the second 555 timer to oscillate and the output becomes HIGH and LOW causing the buzzer connected to output to beep in a pattern which is combinational cause of the first timer oscillations and the second timer internal oscillation. During HIGH condition of first timer output, the second timer master reset will happen. Thus, the capacitor C2 charges (Voltage higher than 2/3Vcc) and output goes LOW. In a short span the capacitor starts to discharge (Voltage lower than 2/3Vcc) causes the output HIGH. Hence, the buzzer connected to output becomes pulsed beep sound.

Here is the video of this project:
Thanks for reading and welcome any suggestion or questions.

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Thu Oct 26, 2017 8:02 am

Joined: Wed Aug 21, 2013 7:38 am
Posts: 49
Post Re: Interested in Building Fridge Door Alarm Circuit using 5
Well, way back in 1972 I bought a 555 and it was an amazing chip. I was a young man in college then, you wouldn't know it to look at me now.

Fortunately, time has progressed and today we have Hobby Components selling
viewtopic.php?f=73&t=1900 W1209 Temperature Control Switches
STM8Sxxx breakout boards that can run eForth.
DS18B20 digital sensors.

I am not criticising your technology but 45 years later there are far better ways to waste your time. :idea:
A few years ago I was on the tube and overheard a guy telling his companion about it, while the other guy and I were rolling our eyes. I had just programmed the latest NXP LPC810 8-pin, 32 bit ARM Cortex-M0+ as a 555, ref: ... mable-555/
Sadly the latest CPUs don't have 200 mA output, but then again a 555 can't be programmed by I2C.

Working with the 555 is the best learning experience possible but it is old iron. I once quit a job in engineering as they had their analog buffers, followers, inverters, comparators on plug-in cards, knew the exact cost, employed circuit board stuffers, building backplane racks, and didn't need some hot-shot to tell them how to save a lot of money by using a 555 in 1974.

These days, a few lines of code in a Hobbycomponents STM8Sxxx or the ancient Arduino will do the job without all the analog circuitry and trial and error calculations. I commend your efforts but encourage you to move on.

Thu Oct 26, 2017 6:20 pm

Joined: Wed Aug 21, 2013 7:38 am
Posts: 49
Post Re: Interested in Building Fridge Door Alarm Circuit using 5
This circuit appears to be plagerized.
"This circuit was awarded with publication in ELECTRONICS WORLD "Circuit Ideas", April 2005 issue, page 52."
I can't see what this post has to do Hobby Components unless you bought some components here.

Fri Oct 27, 2017 6:20 pm

Joined: Sat Oct 28, 2017 10:15 am
Posts: 4
Post Re: Interested in Building Fridge Door Alarm Circuit using 5
Good catch :lol: :lol: :lol:
The same circuit for a fridge alarm is all over the web.

"Here I want to share a project " should indicate the source.
To be fair, it's not a direct lift, he's swapped the R.C values around a bit, so may have been lifted from somewhere else (there are only a few configurations for using a 555 in it's "standard" modes.) 555 circuits tend to be the same, it's the application that changes.

Hint: due to leakage of the caps, you'll get closer to calculated values (frequency or delay) using high resistances and low capacitances (low leakage types, and tants work well too for bigger values)

You might want to try a cmos 7555 instead, dual 555 and lower power! Standard 555s are hungry.

Or much better, use a Arduino nano etc. and give it much more functionality (probably for the same price!!!)

Since you have an LDR and a sounder, add a temperature sensor too - still going to be dirt cheap!

For instance, off the top of my head
    *Play tunes! you might squeeze in a voice playback!
    *depending upon lux levels you could indicate a blown bulb
    *Indicate the temperature range (too high/too low) with a different beep pattern.
    *You could also note if the fridge had become too warm (power cut) when you were not there
    *Note when the door was opened over the last 24hrs - who's stolen your --item here--- ?
    *Record peek activity times at the fridge
    *From a profile of dropping vs rising temperatures estimate power usage (motor running)
    *Profile how long the door remains open during a visit

You could probably think of a few or own and publish a novel project :D

Sun Nov 05, 2017 12:29 pm

Joined: Sat Oct 28, 2017 10:15 am
Posts: 4
Post Re: Interested in Building Fridge Door Alarm Circuit using 5
barewires wrote:
Well, way back in 1972 I bought a 555 and it was an amazing chip. I was a young man in college then, you wouldn't know it to look at me now.

LOL back then I was making synthesisers at school using mainly 741s & 555s, job lots of OC51's etc, keyboards made with bits of wire coat hanger and clothes pegs! And the paint scratched off of OC71's to make photo transistors (thereminesque effects). Even played with this weird digital stuff, 4001's etc. :D
Couldn't wait to get my hands on a 4040 and then a z80A OMG! I used to dream in assembly! No high level languages for me! Not even an assembler for a long time (all hand assembled and entered into a hex kbd) My bible's name was zilog!
Later at uni, because I was so plugged into assembler, I drew a table of op-codes for a 'risc' processor we were given (something didn't ring true), and extrapolated a lot of 'missing' instructions (I suspect that they were 'failed' Z80s). Finished my project in super fast time with more flexibility available like like indexed addressing! And then had to explain why I was using impossible instructions in my code, and yet it weirdly worked :shock: :roll:
Kids today, with there high level languages and levels of abstraction! Don't know they're born! :lol:

Sun Nov 05, 2017 1:29 pm
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