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 Home » Tests and Measurement » Voltmeter + Ammeter LCD panel
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Voltmeter + Ammeter LCD panel    
This multimeter was designed to measure output voltage and current in a PSU, where the current sense shunt resistor is connected in series with load at the negative voltage rail. It needs only one supply voltage that can be acquired from main PSU. An additional function of the multimeter is that it can control (switch on and off) an electric fan used to cool the main heatsink. The power threshold at which the fan switches on can be adjusted using One Touch Button Setup.


  • single uC ATMEL ATmega8 used to handle all the multimeter functions.

  • voltage range 0-30V.

  • voltage measure resolution 10mV.

  • current measure resolution 10mA (depended on current sense resistor value).

  • single, non isolated voltage supply.

  • one side PCB.

  • compact construction allowed to use the multimeter as panel meter.

compatibility with standard LCDs based on HD44780 controller.

Bellow you can see the multimeter schematic. There are some components in the parts list marked as "Do Not Assemble". That components was needed in a previous software edition. Current software version doesn't need them, so you just don't add them. Maybe in a future version of the multimeter there will be a simpler PCB with simpler electronic diagram too.




Layout - bottom side


Layout - top side

Parts List


Element Value/Type Case Remarks
R1         100k                  1206  
R2         100k                  1206  
R3         10k          Potentiometer
R4         30k                   1206  
R5         10k                  1206  
R6         10k                  1206  
R7          7k5                  1206  
R8          7k5                  1206  
R9          500R Potentiometer
R10          500R Potentiometer
R11          5k1                  1206  
C1 - C9  100n          1206  
C10          22u/6V          SMD A   
C11          10n                  1206      Optional element - protect Q1 against voltage peek after switch off fan. Most of the                                                        computer type fans which I tested didn't produce  voltage peeks dangerous for Q1
C12          10u/50V  
L1          47u                  1210        Do Not Assemble - cross PCB pads
D1         DIODE         SMD A Optional element - protect Q1 against voltage peek after switch off fan. Most of the                                                           computer type fans which I tested didn't produce  voltage peeks dangerous for Q1
D2         DIODE          SMD A           e.g. SK310A
U1         7805                 TO-252         Voltage regulator +5V, e.g. LM7805
U2         7812          TO220        Voltage regulator +12V, e.g. LM7812
U3      ATMEGA8    TQFP32  
LCD   GOLDPIN    1x16  
J1      GOLDPIN    1x2            FAN_CON - fan connector
J2      GOLDPIN     1x1             +12V_CON - optional +12V supply connector
J3      GOLDPIN    1x1            +35V_CON  - main supply connector
J4      GOLDPIN    1x3            ground and measured signals
S1      SWITCH  
Q1        MOSFET N       SOT-23        e.g. BSS-138 (fan current lees than 200mA)

ELEMENTACTION
S1Setup button
When pushing this button the shunt resistor value appears. If the resistor value is known, repeat button pushing until correct value reached. If resistor value is unknown (e.g. self made resistor), short PSU output by ammeter, set some current by PSU current limit regulator and then, push button,  lead to equal current indication on ammeter and multimeter.

After  resistor value setup, button must not be pressed for about 5 seconds. The next parameter to set up is fan switch-on power threshold. It is not the real power loosed on output transistor (transistors), because multimeter has information on voltage drop on transistor and driving current. To avoid instability switch-off threshold is automatically set to 20% less than switch-on one.
R9Fine voltage circuit regulation potentiometer.
To reduce ADC conversion errors like un-linearity, gain factor etc. measuring range is divided into two sub-ranges 0-10V and 10-30V (switch threshold can be between 7-13V depend on sourcing current and elements tolerance).
To regulate fine sub-range connect voltmeter to PSU output, set up voltage at about 9V and turn R9 until voltmeter and multimeter indications are equal.
R10Coarse voltage circuit regulation potentiometer..
There is over-sampling applied in multimeter software, so measuring resolution is the same in fine and coarse circuit and is 10mV. Because of the reason described above multimeter has two measuring circuits. 
To regulate coarse sub-range connect voltmeter to PSU output, set up voltage at about 19V and turn R10 until voltmeter and multimeter indication are equal. (If  you posses 4.5 digit voltmeter, you could regulate at voltage 30V)
R3LCD contrast potentiometer. Turn that potentiometer first, if nothing is visible on LCD.
J1Fan connector.
Pin no. 1: Fan "+"
Pin no. 2: Fan "-"
J2+12V
If +12V DC is available in your PSU, connect it to that pin. In that case you shouldn't assemble +12V voltage regulator U2 on PCB. 
That solution is convenient for multimeter, because eliminates U2 heating and permit to connect fan and LCD with higher current consumption.
If you haven't got +12V DC in your PSU, left that pin unconnected.
J3+35V
Rectifier bridge voltage. See U2 element you used data sheet to know about maximum voltage it can work properly. On the other hand the minimum voltage on that pin mustn't drop bellow c.a. 9V, or 6.5V if low drop type U2 and U3 voltage regulators were used.
That pin should be connected even if +12V DC is connected to J2 pin. Voltage from that pin deliver information for fan switching.
J4Measuring signal connector.
Multimeter is suitable for voltage and current measurement in PSU, where 
current sense shunt resistor is connected in series with load and is in negative rail

Pin no.1: voltage measurement U - connect to "+" PSU output, best directly to output terminal;

Pin no.2: current measurement I - connect to "-" PSU output, best directly to output terminal;

Pin no.3: ground - connect to shunt resistor terminal opposite to that connected to "-" PSU output.
LCDLCD connector.
Multimeter works properly with LCD's 1x16 logical controlled as 2x8 (most of LCD's available on the market).

Because of linear voltage regulators used in multimeter, sourcing current is limited. Main current consumption elements are fan and LCD backlight, so:
- use LCD with LED backlight (typically current consumption is less than 15mA);
- use low speed, low current fan. Additional advantage of that solution will be silence.

Download .hex file
PCB layouts



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