A device used to monitor water oil temperatures and battery charge voltage for automotive. The heart of the circuit is PIC16F628 microcontroller car temperature measurement is provided by 2 DS18B20 temperature sensors. Nokia 5510 LCD module is used for the display. Software PicBasic Pro There is no analog-digital converter in PIC16F628. They solved the voltage measurement problem with Sigma Delta calculation algorithm. It is a very good source in terms of software and hardware.
It performs a series of iterations (2nd bit of ADC resolution) comparing the input voltage with the reference voltage (programmable) generated internally by PIC16F628 (RA0 pin). If this value is greater, it produces a low pulse delivered by R2 to pin RA3, which slightly reduces the voltage across RA0, while the opposite produces a high pulse with the opposite effect. At the end of the cycle (1024 iterations or 10 bits), the ratio between the high/low pulses depends on the ratio between the input voltage and the reference voltage.
Car Display Device Circuit Diagram
Starting from the bottom left, the power supply section consists of a 3.3 volt AMS1117 linear regulator surrounded by capacitors for rectification and filtering of input and output voltages.
The upper section forms the front end of the ADC circuit: a multi-turn trimpot for calibration, a correction capacitor, and the RCR group previously seen connected to the RA0 and RA3 lines of the PIC16F628. Diode D1 is a protection against polarity reversals or negative spikes that may occur in the vehicle’s electrical circuit.
At the top, the optocoupler forms the auxiliary input that connects the display backlight control pin to ground when connected to the light circuit. In this way, the backlight for night use is activated when the headlights are turned on. Jumper JP8 is used to fix this situation.
On the right side, in addition to the connections to the display, there is the JP4 jumper, which activates the “inverted” mode of the display. Finally, in the lower part, there is a transistor push-pull amplifier, which is used to strengthen the buzzer signal in cases where the volume is insufficient.
After numerous revisions following the implementation of various functions and clumsy and unsuccessful attempts to get the ADC routine working, the final version of the firmware includes:
- Reading the supply voltage
- Reading the water and oil temperatures
- Management of audible and visual alarms
Currently the alarm thresholds for the minimum battery voltage are 12.2 volts, 95 degrees for water and 110 degrees for oil, which can be easily changed from the picbasic software and the activation thresholds of the bars on the battery icon. When an alarm is activated, the relevant value is underlined and highlighted and an intermittent note is heard from the buzzer; after a few seconds the note becomes silent. When the circuit is opened, the acoustic note is also suppressed for a few seconds, allowing the engine to be started without being disturbed by the “beep” sound that indicates a significant drop in voltage.
From a graphical point of view, the thermometer on the right, which shows that the water temperature is maintained; automotive style icons for oil, water and battery in the middle, finally the detected values on the left and finally the animated small phone style icon showing the battery charge level.
When used in a car, the card and screen must be positioned in a way that does not distract the driver. The situation is different for temperature probes that are placed in the engine compartment and must be in thermal contact with engine fluids. There are waterproof coated versions of the DS1820, the installation process is shaped according to the structure of the vehicle and your experience.
source: digilander.libero.it/praz2004/5510/5510.htm
