Car mirror heater timer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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The timer is designed for installation in a car equipped with exterior rear-view mirrors with heated mirror elements. Its distinguishing feature is that the driver has the ability to change the heater operation mode quickly, without interfering with the timer microcontroller program.

The following timer modes are provided:

"BASIC" - a simple inclusion of the heating elements of the mirrors by pressing a button. After a set time interval, they will be turned off automatically. Before this interval has elapsed, the heater can be turned off by pressing the same button. The next press on it will turn on the heater again, but the countdown of its operation time will start again;

"AUTO" - automatic switching on of the heating elements of the mirrors when power is supplied to the timer with automatic switching off after a specified time;

"SETTING" - in this mode, set the duration of the heating elements before automatic shutdown.

Fig. 1

The timer circuit is shown in fig. 1 The DD1 microcontroller works according to the program written in its FLASH memory, receiving clock pulses with a frequency of 4 MHz from the built-in RC generator. The heating elements of the mirrors are switched on using relay K1, controlled by transistor VT1 according to the signals generated by the microcontroller at pin 5 (GP2).

Fig. 2

The timer is assembled on a small-sized single-sided printed circuit board, shown in fig. 2. Many elements on it (including the microcontroller) are for surface mounting and are installed from the side of the printed conductors. On the same side is the HL1 LED, and the SB1 button is located above the microcontroller. The wire going to the heaters is soldered to a pad next to the relay output (TR20-12VDC-SC-AD) and then passed through a hole in the board.

The codes from the t_mirror_12.HEX file are written into the PIC675F12-E/SN microcontroller (or its analogue with a lower maximum allowable temperature PIC675F675-I/SN). Without any modifications to the board, PIC16F629 microcontrollers with the same indexes can also be used, but for their programming, the t_mirror_629.HEX file should be used.

The microcontroller is programmed by installing it on a board on which the SB1 button and the HL1 LED are not yet mounted. The wires leading to the programmer are soldered to the pads A-C, as well as to the printed conductor of the common wire. If the microcontroller is supposed to be powered from the programmer during programming, then the integral stabilizer DA1 is also temporarily not installed on the board, and the +5 V voltage from the programmer is supplied to the contact pad designed to output 1 of the stabilizer. Otherwise, the board with the installed stabilizer should be supplied with a 12 V supply voltage for the time of programming. After loading the program, all the wires temporarily soldered to the board are removed and the missing parts are mounted.

I placed the board in a square plug for the dashboard of a VAZ-2110 car, making two holes in it. One (in the center) - for the SB1 button pusher, the second (offset) - for the HL1 LED. After checking the operability of the timer, I fixed the board in a plug, for which almost all the free space in it was filled with sealant. A 5 A fusible link not shown in the diagram protects the timer power circuit from overload.

At the beginning of its work, the microcontroller program configures the corresponding pins of its GPIO port as inputs and outputs, disables those peripheral devices built into the microcontroller that are not required for its operation. Then the status of the SB1 button is checked.

If it is pressed, the "SETUP" mode starts, as indicated by the HL1 LED, which remains on until the button is released. Within 3 seconds after the LED turns off, press this button again and hold it down until the required heater operation time is set. It increases in steps of one minute every 2 s. Upon reaching 20 min, the specified duration is reduced by a jump to 1 min, then a new cycle of its increase begins. Each step is marked by LED flashes. Short flashes (about 0,3 s) mean units of minutes, there can be from one to four. Long flashes (about 0,8 s) - five-minute intervals. For example, four short flashes - 4 minutes, long and three short ones - 8 minutes. three long and two short - 17 min.

When the desired duration is reached, the SB1 button is released. Approximately 5 seconds after that, the LED will indicate with one long flash that the set value has been written to the EEPROM of the microcontroller. Then the program continues to work in the "BASIC" mode.

If the program did not detect the pressed button immediately after power-up, it checks whether the "AUTO" mode was enabled in the previous cycles of operation. If not, the program switches to work in the "BASIC" mode. When the "AUTO" mode is enabled, a pause of 20 seconds is counted, during which the LED flashes. After it, the mirror heaters are turned on for the time specified and recorded in the EEPROM. While they are working, the HL1 LED shines continuously. By pressing the SB1 button, you can turn off the heaters and the LED before the specified time has elapsed. The described procedure is performed once each time the timer is turned on, after which the operation continues in the "BASIC" mode.

To enable or disable the "AUTO" mode, you must press the SB1 button after the power is turned on and hold it until the HL1 LED emits three long or short flashes. Long ones indicate that the mode is on, short ones indicate that it is off. The "AUTO" mode activation sign is stored in the EEPROM of the microcontroller. This mode is convenient to use in the off-season, when moisture settles on the mirrors in the morning or even frost appears, or drops remain on the mirrors after rain.

Microcontroller programs can be downloaded from ftp://ftp.radio.ru/pub/2013/02/t_mirror.zip.

Author: S.Kashutin

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