Dear Children, I thought I would show you a circuit that is not only for fun, but can also be useful at home for your parents and siblings. Surely it has happened before that someone forgot to close the refrigerator door, or didn’t shut it properly. This device gives a warning in such cases.
Most modern refrigerators already have this feature. If the door is left open for too long, the appliance warns you with a beeping sound or a flashing light. The simpler models only give an audible signal after about 1–2 minutes, while the “smart” refrigerators can even send a notification to your mobile phone. Many older appliances still lack this function. By placing this device on one of the shelves near the lamp, you can make up for the missing feature.
The device can also be used in other ways: for example, it can indicate if someone has left the light on in the bathroom, toilet or pantry, though in that case it is advisable to set a longer timer. The use of this simple light-monitoring device is limited only by your imagination.
The creation of the circuit was inspired by the BK-0624 “Refrigerator Door Monitor KIT, DC 9V” offered by HESTORE. However, I was not satisfied with the solution presented there. It is a simple timer built with a single NE555 IC, whose power supply is switched on and off by the photoresistor through a series pass transistor. It runs on a 9 V 6LR61 alkaline battery, which is rather expensive and does not tolerate moisture well. If the transistor does not shut off properly, or if the circuit board becomes damp, the battery slowly discharges on its own, and the alarm falls silent.
A better solution is to use a lower supply voltage (3V) and a TLC556 timer IC. This IC contains two CMOS timers equivalent to two NE555s.
You can find the circuit on Tinkercad here:
👉 https://www.tinkercad.com/things/4sa98ZEBjhD-refrigerator-door-monitor
What do you need?
| Name | Quantity | Component |
|---|---|---|
| Mini Breadboard | 1 | BB-170 prototyping board |
| U1 | 1 | TLC556 dual CMOS timer |
| D1 | 1 | 1N4148 or similar diode |
| R1 | 1 | 10 MΩ resistor |
| R2 | 1 | Photo resistor (any type) |
| R3 | 1 | 2,2 MΩ resistor |
| R4 | 1 | 1 MΩ resistors |
| C1 | 1 | 10 μF, 5 V polarized capacitor |
| C1 | 1 | 100nF Polyester Capacitor |
| PIEZO1 | 1 | Piezo sounder (incorporating 3KHz oscillator) |
| BAT1 | 1 | 3V battery (2 x 1.5V AAA or smaller type button cells) |
How it works?
The circuit uses a photoresistor to detect when the refrigerator door is opened (by sensing the fridge’s internal light). If the door is left open too long (the lamp stays on for an extended period), it alerts the user with an audible signal.
With the door closed, the interior of the fridge is in dark, the photo resistor R2 presents a high resistance (>200K) thus clamping Timer 1 by holding C1 fully charged across R1 and D1.
When a beam of light enters the photo resistor lowers its resistance (<2K) stopping C1 charging current. Therefore Timer 1, wired as an astable multivibrator, starts oscillating at a very low frequency and about after half a minute its output pin (#5) goes high, enabling Timer 2. This chip is also wired as an astable multivibrator, driving the Piezo sounder (incorporating 3KHz oscillator) intermittently at about 5 times per second. The alarm is activated for a third of a minute, then stopped for the same time period and the cycle repeats until the fridge door closes.
You can read about the NE555 timer IC in this article. The TLC556 contains two such timers in a single package and is a CMOS version with very low power consumption, requiring only 2 V to operate. You can find the datasheet of the IC here.
Notes:
- Delay time can be varied changing C1 and/or R3 values.
- Beeper repetition rate can be varied changing C2 and/or R4 values.
- Stand-by current drawing: 150µA.
- Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.
- Do not put this device in the freezer.
If the circuit works, you can build it on a solderable prototyping board and mount it in a small case.
Note: This circuit requires a piezo sounder with a built-in oscillator. Such a type produces a continuous beeping sound when DC voltage is applied. If the piezo does not have an internal oscillator, it will only make a single click when DC is applied, not a beep. The built-in piezo element in Tinkercad also lacks an oscillator, which is why during simulation you hear only clicking instead of intermittent beeping. In reality, the circuit produces an intermittent beeping sound.
If you also have a piezo sounder without a built-in oscillator, then replace C2 with a 10 nF capacitor, and R4 with a 22…33 kΩ resistor. In this case, the second timer will act as the oscillator generating the sound – but you will get a continuous beep instead of an intermittent one.