This circuit allows the measurement of voltages in the scale of 0 to 6 V or another with changes of the components and a good input sensitivity. The indicator is analog (microammeter) and can be obtained from old sound equipment.
The unique setting is zero which will be performed by a trimpot. The power is made by a 9 V battery, but the consumption is very low. In Figure 1 we have the complete diagram of the voltmeter.
Scale calibration can be done with the help of a variable source of known output voltages.
To use, clear the scale and then measure the voltage of the circuit. Be careful not to connect it to very high voltages that would burn the transistor and remember the circuit only measures continuous voltages.
Material list
Q1 - BC548 - NPN general purpose transistor
D1 - 1N4002 or equivalent - silicon diode
M1- 0-200 uA - microammeter
S1 - Single switch
B1- 9 V - battery
R1 - 47k ohms x 1/8 W - resistor - yellow, violet, orange
R2 - 10 k ohms x 1/8 W - resistor - brown, black, orange
P1 - 10 k - potentiometer
Miscellaneous: terminal strip, battery holder, wires, welding, etc.
