Phototransistor for High Impedance Voltmeter

Posted on Feb 7, 2014

This circuit is designed to provide an inexpensive way to create a High Impedance Voltmeter while making use of an inexpensive analog or digital multi meter. The circuit is specifically designed for testing phototransistors. This the figure for the circuit; When measuring voltages in high resistance circuits the resistance of the voltmeter itself

Phototransistor for High Impedance Voltmeter
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has an effect on the circuit. For example if the voltage across a 1 Mega ohm resistor is measured with a voltmeter that has an internal resistance of 1 Mega ohm then the total resistance in that part of the circuit is effectively halved (two 1 M resistors in parallel = 500K ohms). In another example; If a voltmeter with a 1 Mega ohm resistance is placed in series with a 1 Mega ohm resistance there will in effect be two - 1 Mega ohm resistances in series, the resistor in the circuit and the resistance of the voltmeter. Under these conditions the maximum voltage that the voltmeter could show would be 1/2 of the supply voltage. This is the principle work of the circuit. Phototransistors, when used to detect a train position essentially have two states of conductance. When the phototransistor is dark it has LOW conductance and the voltage across it will be HIGH if the phototransistor is has either visible or infrared light falling on it then its conductance will be HIGH and the voltage across it will be LOW. If the high impedance voltmeter circuit is used to measure the voltage across the phototransistor when it is dark it will not load down the circuit and should indicate almost 100 percent of the supply voltage.

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