High Impedance Voltmeter

(Phototransistor Circuit Test Voltmeter)

  This circuit is designed to provide an inexpensive way to to create a High Impedance Voltmeter while making use of an inexpensive analog or digital multimeter.

  The circuit is specifically designed for testing phototransistors when they are used in the circuits shown at this site. It has a very high input impedance that will not "Load Down" the sensor that is being checked.


  When measuring voltages in high resistance circuits the resistance of the voltmeter itself has an effect on the circuit. For example if the voltage across a 1 megohm resistor is measured with a voltmeter that has an internal resistance of 1 megohm 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 megohm resistance is placed in series with a 1 megohm resistance there will in effect be two - 1 megohm 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.

  Many inexpensive digital multimeters will have an internal input resistance in the 1 megohm range. Analog voltmeters that are not battery powered will have much lower internal resistances. The more expensive meters will have higher input impedances and therefore will have less loading effect on the circuit under test.

  The circuit on this page will compensate for the low input impedances fo the meters and thereby provide more accurate voltage readings in circuits that have high resistances.


  Phototransistors, when used to detect a trains 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.

High impedance Test Voltmeter Schematic


High Impedance Voltmeter Notes

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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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