The whole process of designing and making the voltmeter + ammeter LCD panel

　　This is a multimeter designed to measure the output voltage and load current of the negative voltage rail by sensing the shunt resistor which is in series with the power supply. Only one supply voltage is required which can be acquired from the main power supply. An additional feature of the multimeter is that it can control (turn on and off) the electric fan used to cool the main heat sink. The power threshold at which the fan switches on is adjustable using a single button setting.

　　Technical specifications

　　A single UC ATMEL ATmega8 is used to handle all multimeter functions.

　　Voltage range 0 - 30V.

　　The voltage measurement resolution is 10mV.

　　The current measurement resolution is 10 mA (depending on the current sense resistor value).

　　Single, non-isolated supply voltage.

　　One side of the PCB.

　　Compact structure, can use multimeter panel meter.

　　The HD44780 controller is based on compatibility with standard LCD displays.

　　Schematic diagram

　　Bellow you can see the schematic diagram of the multimeter. There are some components in the parts list marked as "Do not assemble". In previous software versions, those components were needed. The current software version does not need them, so you just don't add them. Maybe in future versions of the multimeter will have a simple PCB with simple electronics diagram too.

　　A printed circuit board

　　I think that anyone who makes PCBs will know which one should be used to produce the correct PCB. After soldering all the parts on the PCB:

　　Crossover connections on both PCBs (see “Layout - Bottom”).

　　Short L1 pad.

　　Place the U2 element (+12 V voltage regulator) on the heatsink.

　　Pay attention to the polarity of the D1 and D2 molecules. Extensive silkscreen layout with part number placement, one side is negative.

　　Layout - Bottom

　　Layout - Top

Programming

　　Since the C TQFP package, after soldering all the components on the PCB, we can program. After that, programming is very easy to perform. The programming signals are passed through the LCD connector. To make the programming cable, you can use the hard drive cable of an old computer. The picture of my programming cable is shown:

　　Remember that the circuit must be supplied with +5 V during programming. Depending on your programmer, the supply voltage may be provided by the programmer itself or from a separate power supply.

　　Programming Cable Connection List

　　After connecting C to the prog, you should check if C is "visible" to the prog. When everything is good, you can upload the code to C. It is assumed that C is new and its internal RC clock frequency is 1MHz. If not, set the appropriate fuse bits to achieve the above conditions. In addition the brown-out detector should be turned on, enabling the fuse bit BODEN. The recommended brown-out reset threshold voltage is 4V. The next thing to do is to solder across LCD pads 1 and 5. This is necessary to provide ground for the LCD RW signal. After all, connect the LCD module to the PCB with a multimeter. It is recommended to use a detachable connector for further expansions such as software upgrades.

　　Installation in power supply and regulation

　　The multimeter power supply is according to the following figure:

　　Connectors and Control Elements

　　S1 Setting Button

　　When this button is pressed the shunt resistor value appears. If the resistor value is known, repeat the button push until the correct value is reached. If the resistor value is unknown (e.g. homemade resistor), short the power supply meter output, set the power supply current limit to adjust the current, then push the button, causing the resistance value to be set, the button must not be equal to the current indicated by the ammeter and multimeter. Press for about 5 seconds. The next parameter sets the fan switch power threshold. It is not the real power output triode (transistor) loose, because the multimeter has information on the voltage drop of the transistor and the drive current. To avoid unstable switching the turn-off threshold is automatically set to 20%, less than the switch at one.

　　R9 is the potentiometer for adjusting the fine voltage circuit

　　In order to reduce ADC conversion errors such as un-nonlinearity, gain factor etc. the measuring range is divided into two sub-ranges 0 - 10V and 10 - 30V (depending on output current and switching threshold element tolerance can be between 7 - 13V)

　　In order to regulate the fine sub range connect the voltmeter, the power supply output voltage is established at about 9V and turn R9 until the signs of the voltmeter and the multimeter are equal.

　　R10 is the voltage adjustment potentiometer for the rough circuit.

　　There is an oversampling multimeter software application, so that the measurement resolution is the same, the fine and coarse circuits are 10mV. Due to the reasons described above, there are two measurement circuits

　　To regulate the coarse range, connect a voltmeter to the power supply output, set the voltage to about 19V, and open R10 until the voltmeter and multimeter indications are equal. (If you have a 4.5 digit voltmeter, you can adjust the voltage to 30V)

　　R3 is the contrast potentiometer for the LCD display. Turn the potentiometer first if there is no LCD display.

　　J1 fan connector.

　　If +12 V DC is your power supply, connect it to this pin. In this case, you should not assemble the +12 V regulator U2 on the PCB

　　This solution is convenient for multimeters because it eliminates the heating of U2 and allows connecting the fan and LCD with higher

　　Current consumption. If you don't have a +12 V DC power supply in your case, leave this pin floating.

　　J3+35 V

　　Rectifier bridge voltage. For the element of the U2 band, refer to the datasheet you are using to find out the maximum voltage at which it can work properly. On the other hand, the minimum voltage on this pin must not drop the bellows ca 9V or 6.5V low dropout type U2 and U3 voltage regulators

　　This pin should be connected even if +12 V DC is connected to J2 pin. The voltage from this pin provides information on the fan switching.
 

J4 Connector for measuring signals

 

　　The multimeter is suitable for measuring voltage and current at the power supply, where the current sense shunt resistor is connected in series with the load and at the negative rail. Pin 1: Voltage measurement U - connect to the "+" power supply output, preferably directly to the output terminal; Pin 2: Current measurement I - connect to the "-" power supply output, preferably directly to the output terminal, Pin 3: Ground - connect to the shunt resistor connected to the opposite terminal of the "-" power supply output.

　　LCD connector.

　　The multimeter 2X8 (most of the LCD monitors available on the market) controls the LCD type 1x16 logic for normal operation, because the output current of the linear voltage regulator used by the multimeter is limited. The fan and the LCD backlight are therefore the main current consumption elements: - Use LCD with LED backlight display (typical current consumption is less than 15mA); - Use a low speed, low current fan. The additional advantage of this solution will be silent.