Wiring Material

Simple Electronic Multimeter  High cost deters many hobbyists from buying a conventional multimeter. Since it is difficult to obtain cheap 50 uAor 100uA meters which are essential for a highly sensitive multimeter, an electronic alternative device is suggested to serve the purpose. The circuit shown here gives economic and safe ranges covered to three values : (i) 0-15V, (ii) 0- l5mA, and (iii) 0-150mA.
The ranges can be extended with suitable modifications. The 741 operational amplifier acts as a null detector. its output is equal to the voltage at point A minus the voltage at , point B, multiplied by the device’s very high voltage gain. lf VA is slightly greater than VB, the output is limited by the supply voltage to about 7 volts. lf  VA is slightly less than VB, the output is about 2 volts. At the point at which the output changes from low to high or vice-versa, VA is equal to VB to within a very small margin of error. With the switch set to the position shown in circuit diagram (15V range) the potential difference between points A and C is

R3+R4+R5/R1+R2+R3+R4+R5 * input voltage

or 1/30 * input voltage


 The forward voltage drop of the diode D3 is about 0.6V and largely independent of battery condition. About 0.5V appears across the variable resistance VRl and a known fraction of this indicated by a scale on the potentiometer appears between points A and C, i.e. it compares the known reference voltage with a known fraction of the input voltage. On the two current ranges, the reference potential difference is compared with the voltages developed across R4 plus R5 on the l5mA range and R5 only in 150mA range. T D2, a light emitting diode, with its current limiting series resistor R9 indicates whether the output of the operational amplifier is high or low. The diode D1 and the condenser Cl provide the facility of measuring alternating voltages and  currents. lf the voltage at A momentarily exceeds the voltage at B, then Cl will charge up via Dl maintaining D2»alight until the peak of the next cycle. Without Cl there is no sharp point at which D2 extinguishes for AC measurement. ln use it must be remembered that the indicated readings are all peak values and will thus need to be divided by square-root of 2 to give RMS value for a sinusoidal input. The meter is calibrated directly VRl is scaled 0-15 on the _ 15V range by comparison with a standard meter. This calibration will hold quite closely for current ranges—the agreement depending on the tolerances of Rl to R5. Ten per cent tolerance resistors have been found to be quite successful for these but, if desired, 5 or even 2 per cent resistors provide a worthwhile increase in the accuracy of the current ranges. ln use, the meter is switched to the appropriate range and V connected as for a conventional multimeter. The potentiometer VR] is rotated until D2 is at the point of changing from off to on and then the reading is directly indicated. on the potentiometer scale. The diodes may be any silicon diode. For use of the multimeter in 0- l 50V range, the values of the resistors Rl to R5 may be changed proportionately so that the value of the ratio

R3+R4+R5/R1+R2+R3+R4+R5

is l/300. Then with these values, the multimeter will operate on 0- l 5OV range in the first position (as shown in the existing diagram). The accuracy no doubt will be somewhat ham- pered. VRl has to be calibrated accordingly.

Electronic voltmeter, ammeter circuit using a single IC 741