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• 09.22.2011
• Eight LEDs make a 100-division voltmeter
• Please Note: 1) This circuit works with a single supply of 5 to 5.5V. 2) Input voltage is with respect to the reference voltage of 2.5V. 3) The -2.5V should be 0V. Further if there are any queries perhaps refering the circuit layout may help.
• 05.26.2011
• Logic gates form high-impedance voltmeter
• The input current is 0.01nA and not 0.1nA. If one has a PS with output voltage display one has no need for a DVM, also for estimating voltages the moving coil meter may be unnecessary, LED alone is enough. However using an OP-Amp one requires a DMM at the output and also a dedicated dual supply to power the OP-Amp. One should consider a typical value than considering an extremum value. There could be temperature dependence but for estimating voltages it shouldn't be a serious problem. The values quoted below 100nA, 1uA etc are extremum values. I do not know well about the price difference but according to my experience there is a large difference between a CMOS input OPAmp and a more commonly used FET input OPAmp. This article describes a setup for estimating voltages across large resistances and not about accurately measuring them, using commonly available CMOS gates. The meter has been included to have a better understanding of the transition point. Upper limit of 3V for supply voltage is when CD4011 is used as a logic gate, but here it is being used for a different purpose.
• 02.17.2011
• Circuit lets you test capacitors
• Firstly it is not simple to measure currents less than 1uA. The leakage current (0.3uA) would be just 1/2 a division on a 0-100uA current meter. Assuming 50 divisions on the meter. secondly to measure leakage current one will have to first charge the capacitor to the max voltage and then measure the leakage current. This again requires switching arrangements. So if we set out to measure current we may again require an OpAmp. So we have meter + OpAmp + switching. Transistors with near by CE-drop voltages could be choosen for this design. It seems to some extent this could also be solved by interchanging the transistors, i.e give the test capacitor Cx a lead. If 22M seems to be a more stringent constraint it could be relaxed to a lower value. This idea was to have a go/no-go kind of test setup with a simple LED display and completely do away with current measurement. Lastly improvements in the circuit are possible with use of better components, this is just a design idea.
• 11.18.2010
• Use LEDs as photodiodes
• It seems an easy way to find out whether an LED can be used as a photo diode is to check whether under room illumination conditions the voltage produced at the terminals of the LED is about 0.3-1.0mV when measured with an ordinary 1M impedance DVM on 0-200mV scale. when covered with hand or so this voltage should vanish. On the other hand shining extra light should increase it.
• 03.17.2011
• Transistor tester identifies terminals
• In view of this degradation problem the supply voltage has been kept to be variable. So that a minimum voltage(current) for break down could be reached. However I have not measured, how much of degradation occurs. Atleast the beta of the transistor appeared to be the same before and after testing a transistor. Since the testing is done for a short time only. However yes, an estimate of degradation is required.
• 12.15.2010
• Logic probe uses six transistors
• The figure shown in the appendix for reference, figA, is a misprint. Here a map of appendix to() poster(print) values has been given. (I3)R1R1, (I1)R2R2, (Ib3)Rb3R15, (Ib1)Rb1R14(10k), (Ib2)Rb2R3, (Ic')Rc'R4, (IE)RER7, (Ic)RcR5, R3R14(6.8k), R4R12, I2 is the current leaving from the junction of R1 and R15.
• 11.18.2010
• Use LEDs as photodiodes
• The output logic level of the NAND gate in the light/no-light conditions is opposite than mentioned in the text.
• 11.04.2010
• Detect live ac-mains lines
• As pointed out by Steffano the coupling is by charge distribution on the hot wire and not by magnetic field. The coil is used for the sake of simplicity of fabrication. It serves the purpose of a metal cylinder. The coil can be given sufficient insulation coating(synthetic enamel+plastic sheath) for safety.