Friends interested digital voltmeter circuit very much. But cannot buy ICs since it is very ancient.
Now there is digital voltmeter panel meter that easy to use as
Figure 2 is wires connector or terminal by white is power supply of 9volts DC or AC and Input wires is a black as negative(-) and the next one a Red wire as positive (+). on 0-50V range full scale.
So fixed to most my working.
Therefore is need to use a power supply for it. As Figure 3 I selected old 12V DC adapter at 450mA as because I do not use it for any operation. (already for use)
But it uses 9volts we should reduce voltage down to it.
Then I open the DC adapter see obviously (Figure 4) they use LM7812 for 12 volts DC regulators
I will remove it then modify circuit as Figure 5 instead of them.
Figure 5 All parts to modify as 9-volts DC regulators. Which have one transistor MJE3055 or TIP41 or TIP31 is NPN transistor 2A 40V.
Next one is 10V Zener 1watts that is used as 10-volts steady voltage. And 470 ohms resistor to bias current to Zener and base of the transistor for high current output. Which this meter use current about lower than 100mA only.
Video: Testing, it is very accuracy. Compared with digital multimeter.
I measure a voltage of 9-volts battery results is 8.3V same them.
3 wires LED digital voltmeter panel
- RED, PW (+) is a positive voltage of Power Supply
- Black, PW (-) is a negative voltage of Power Supply
- Yellow, to measure a voltage
We need to use the external DC power supply to them, 3V to 30V. In this case, I use a 9V power supply and it should be a fixed regulator.
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