Wichit Sirichote, wichit.sirichote@gmail.com
Build a two channels 4 1/2 digit DVM controlled by a nano328 board.
The kit interfaces two channels ICL7135 precision A/D converter with a nano328 board. The nano board reads the multiplexed BCD data from both chips and sends the readings through a 9600 serial port. The voltage input is +/-2.0000V with 100uV sensitivity. Kit also has a serial port for HC-06 bluetooth MODEM interfacing. The readings can send to the cell phone using Bluetooth connection easily.
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| Dual channel 7135 DVM kit |
Hardware
The nano board, U1 reads multiplexed BCD data from ICL7135, U2 , BCD1-BCD4 to PORTD4-PORTD7. STROBE signals from U2 and U4 are tied to external interrupt pins, INT0 and INT1. Transferring BCD data is done by reading the BCD digit from digit 5, 4, 3, 2 and 1 sequentially.
The polarity bit, POL_DVM1 is also captured by PORTC4. The ADC clock is generated by Timer1 output compare mode. The clock frequency, 125kHz and 121kHz is selectable by timer1 settings.
The reference voltage, +1.000V is generated by reference diode, LM385 and R3 10k precision trimmer.
The negative power supply -5V is generated by U3, ICL7660 voltage converter chip.
JP1 is 4-pin header for HC06 Bluetooth MODEM interface.
J1 is DC jack for external +9V DC supply.
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| Schematic, click to enlarge |
ADC clock generation
Timer1 is programmed for waveform generation mode using Timer1 counter and compare register. The frequency of the clock signal is then programmable. Below waveform is 124.8kHz (approx. 125kHz) measured at OC1A, pin 12 on nano board.
For 121kHz (approx. 120kHz), timer1 settings are modified (see the source code).
Software
The control firmware is developed using Arduino IDE. Main code provides serial commands scanning. The serial commands are single letter ASCII characters. External interrupt0 and 1 are for readings BCD digit from both ADC chips. BCD digit 5 is captured first, then digit 4,3,2,1 are followed. When reading all digits are completed, they are converted into integer number. Polarity bit, if activated is then converted into negative readings.
Connecting the kit to 9600 terminal
Kit can connect to PC using 9600 terminal. When connected, press ? or h for help menu.
Press Space Bar key, prints the readings once. The readings are displayed as integer number. The unit sensitivity is 100uV. Below displays 1288, or 0.1288V.
The interval between readings can be set with command i. Example below, enter 1 for 1 second interval.
Press command A, the readings will be printed every one second. Press Q to quit.
Connecting Bluetooth MODEM HC-06
JP1 is for connecting the Bluetooth MODEM HC-06. We can use the cell-phone with APP BlueTerminal to connect the kit easily.
Example below uses WEKO smart phone running blueterminal APP. The password for HC-06 MODEM is 1234. When connected, we can send command from the phone and receive the readings easily. The terminal can record the readings and save it as text file. We can import such text file to the spreadsheet software and making graph directly.
Example of using the kit for temperature recording.
The voltage input with +/-2.0000V range is very useful for interfacing many sensors. This example shows the use of LM35 precision temperature sensor with the kit for data logging experiment.
The sensor LM35 has 3 pin, +5V, Vout, and GND. We can wire it and put it into the protection tube. The input terminal pin3 provides +5V for sensor power supply.
The interval is 60s or 1 minute. The graph is air temperature on April 26, 2024 at my home town.
Data file, TEST3.CSV is saved in CSV format.
PARTS LIST
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DIY Kit is available on eBay. |
Download
Hardware schematic, Source code
More information, please contact Wichit Sirichote, wichit.sirichote@gmail.com