Electrical appliance start and stop reminder

The reminder circuit diagram is shown below:

The 220V AC is stepped down by the transformer, and the 7812 voltage regulator provides 12V power for the subsequent stage. If the refrigerator is not plugged into the power socket of the reminder or the refrigerator is plugged in but the compressor is not running, there is no induced voltage on the secondary of the transformer, the base voltage of Q1 (2SC1815) is 0, Q1 is cut off, and the collector of Q1 is high level to turn on Q4. The 12V power supply voltage charges the electrolytic capacitor C5 (2200/16V) through the resistor R6 and the emitter of Q4. Since the positive input terminal 5 pin of the operational amplifier block (LM358) is divided into half of the power supply by R10 and R11, the negative input terminal 6 pin is charged by the capacitor, so the voltage of the positive input terminal 5 pin of the operational amplifier is greater than the voltage of the negative input terminal 6 pin, and the operational amplifier output terminal 7 pin outputs a high level, and the indicator D3 is on. The high level is added to the base of Q5 (2SCl815) through the resistor R12 to turn on Q5, and the 9V relay starts to work. The buzzer is connected to the 220V AC power supply through the contacts of the 9V relay and emits a musical chirping sound, indicating that the refrigerator and freezer are in a certain state. After C5 is fully charged, the voltage at the negative input terminal, pin 6, of the op amp is greater than the voltage at the positive input terminal, pin 5, as a result of the voltage division by resistors R6 and R7. As a result, the output terminal, pin 7, of the op amp becomes low level, indicator D3 goes out, Q5 is cut off, the relay contacts are disengaged, and the buzzer loses power and stops beeping, thus completing a reminder task.

At the same time, the high level of Q1 is added to the base of Q2 (2SC1815) through resistor R5, Q2 is turned on, the collector is at a low level, and is added to the base of Q3 (2SC1815), Q3 is cut off, and has no effect on the subsequent stage.

When a refrigerator is plugged into the reminder power socket and the compressor starts running, the 15A AC ammeter begins to indicate the working current of the compressor at this time. The working current of the compressor flows through the primary coil of the mutual inductor, and the secondary generates an induced voltage. This induced voltage is added to the base of Q1 after rectification and filtering to turn on Q1, and its collector becomes a low level and is added to the base of Q4. Q4 exits conduction and enters the cut-off state, and the large electrolytic capacitor C5 begins to discharge through the 100K resistor R9 to prepare for the next recharging. At the same time, the low level of Q1's collector is added to Q2's base through resistor R5, Q2 exits conduction and enters cutoff state, its collector becomes high level, and is added to Q3's base to turn on Q3, and the 12V power supply voltage charges electrolytic capacitor C4 (2200 μ/6V) through R6 and Q3's emitter, so the voltage of the negative input terminal 6-pin of the op amp LM358 is lower than the voltage of the positive input terminal 5-pin, and the output terminal 7-pin of the op amp block outputs a high level again, D3 lights up, Q5 turns on, the 9V relay is energized, and the buzzer is powered on to make music, indicating that the refrigerator freezer is in another working state. After C4 is charged. The op amp 7-pin outputs a low level, Q5 is turned off, the relay is disengaged, and the buzzer loses power and stops beeping, completing a prompting work.

When the temperature in the refrigerator reaches the temperature controller requirement and the compressor stops running, the induced voltage on the transformer disappears, Q1 is cut off again, Q4 is turned on again, the electrolytic capacitor C5 is recharged, and the voltage of the 6th pin of the op amp is less than the voltage of the 5th pin, so the 7th pin outputs a high level, Q5 is turned on and the buzzer sounds again. At the same time, Q1 is cut off to make Q2 turn on, Q3 is also cut off, and the electrolytic capacitor C4 discharges through the 100K resistor R8 to prepare for the next recharging. After C5 is fully charged, the 7th pin of the op amp outputs a low level, Q5 is cut off, the relay is disengaged, the buzzer loses power and stops sounding, and the prompting work is completed again. This cycle repeats itself, and the start and stop prompting function of the refrigerator is automatically realized. It frees us up and allows us to do other work during the test period without having to deliberately pay attention to whether the refrigerator compressor is running or not. Switch K can choose to turn on or off the power supply of the reminder, thereby deciding whether to use it or not.

Production Note: The selection of mutual inductor is critical. The primary coil enameled wire should be as thick as possible. When it is connected in series in a small refrigerator with a low power compressor (92W), the secondary induced voltage should reach about 1V after rectification so that Q1 can be reliably turned on. I chose a line-drive transformer on the motherboard of a scrapped Yellow River 29-inch color TV (used in reverse, the original secondary winding is used as the primary winding, and the original primary winding is used as the secondary). The primary coil resistance of the line-drive transformer is about 45 ohms. At the same time, I also tried the line-drive transformer for the M11 movement with a primary coil resistance of 115 ohms, but the induced voltage was too low, only about 0.5V, and it could not make Q1 turn on at all.

The buzzer uses an AC 220V music buzzer, and the volume and music are selected according to personal preference. The switch K uses a household light switch. All components and ammeters except the switch can be put into a suitable box. I use a scrapped electric meter with a transparent cover. Remove all the components inside and install it, exposing the indicator light. The surface of the AC ammeter faces upwards, so that you can easily understand the working current and usage status at any time. The power plug board can use a commonly used multi-hole power plug board. The power cord, plug board and reminder are fixed together on a small piece of wood, which can be conveniently moved and used at any time.