Audible Milliohmmeter with 555

This project uses a four-point resistance measurement process also known as the Kelvin method. This procedure uses a current source to determine the value of an unidentified resistance. A constant current flows through the unknown resistance and the voltage drop across it is measured. Note that the current source and voltage measurement wires are soldered together at the test prod tips Figure 2. Because the voltage drop across the unidentified resistance is measured at the probe tips, the resistance of the test leads carrying the constant current is not included. The resistance under test can be found by dividing the voltage drop between the test prods by the current source.
Audible Milliohmmeter with 555 - schematic

The design is composed of the following: a) 50 milliamp Current Sink b) Amplifier with a gain of 470 c) Voltage Controlled Oscillator (VCO) d) Chopper e) Flip-Flop and Speaker Transistor Q1 forms a 50 milliamp current sink. To find the voltage drop for a one milliohm resistance, multiply the current source by the resistance: 0.05 * 0.001 = 5 uV. U1 is a differential amplifier with a gain of 470. For a 5 uV input, the output of U1 pin 6 is 5 uV * 470 = 23.5 mV per one milliohm of test resistance. U2 and U4 form a VCO. U2 is a current source whose output is controlled by the voltage at U1, pin 6. When the current source charges C2, its voltage ramps up in a linear fashion until it reaches U4’s threshold level (2/3 * 9V = 6V). Then the internal transistor of U4 discharges C2 until it reaches U4’s trigger voltage level (1/3 * 9V = 3V). The output of U4, pin 3 goes low during the discharge of C2. Then it goes high while C2 charges to 6V again. When the voltage input to the VCO increases, the source current also increases. This causes C2 to be charged in less time. The result is an oscillation frequency proportional to the voltage at the input of the current source. If the input voltage is zero, the output frequency (U4, pin 3) is zero. The frequency of the VCO increases at a rate of 1,250 Hz per volt. I found it easier to discern changes in pitch by chopping” the VCO on and off. U3 generates a 3 Hz signal at pin 3....

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