Various Circuits

This very simple circuit acts as a high sensitivity capacitive sensor. Lamps and/or Buzzers are operated at half the mains supply voltage when a part of the human body comes in contact with the sensor or approaches it at a close distance. The circuit can be used as an alternative to the Door Alarm or Capacitive Sensor circuits, already available on this website...

You can use a multimeter with capacitance-measurement capability to measure the length of wire or cable to an open circuit. The capacitance of a pair of wires (or a wire to a shield) is directly proportional to the length of the wire. If you know the capacitance per foot of wire, then you can calculate how far it is to the open circuit...

The circuit in Figure 1 allows you to effect long-term, unattended collection of animal-behavior data using any standard PC with an RS-232C port. The circuit detects changes in the proximity of the animal subjects to a capacitive sensor electrode. The multivibrator comprising IC1D, IC1E, and IC1F drives the electrode with a 5V p-p, 200-kHz square wave...

The simple circuit in Figure 1 uses a low-current-drain MAX4073H amplifier to sense the current flowing through a 4- to 20-mA loop. The circuit senses the current through a 1 resistor with a fixed gain of 100 and uses no battery or dc power supply. The low current drain of the amplifier (0.5 mA) enables the circuit to tap its power from the 4- to 20-mA loop to power the amplifier chip. Note that the current flowing in the amplifier`s power-supply Pin 3 (nominally 0.5 mA but may vary slightly) is not part of the sensing loop...

This circuit was designed on request, to remotely monitor when a couple of electric heaters have been left on. Its sensor must be placed in contact with the feeder to be able to monitor when the power cable is drawing current, thus causing the circuit to switch-on a LED...

High-speed current sensing presents a designer with some significant challenges. Most techniques for sensing current involve measuring the differential voltage the current produces as it flows through a sense element, such as a resistor or a Hall-effect device. The differential voltage across the sense element is generally small and is often riding on a common-mode voltage that is considerably larger than the differential voltage itself...

The simple ADC in Figure 1 is perfect for getting analog signals into a purely digital microcontroller. Using just five surface-mount parts, you can assemble it for less than 50 cents (1000), which is approximately half the cost of a single-chip-ADC approach in the same volume. Moreover, this design takes only one pin from the microcontroller to operate. Although you can purchase many microcontrollers with built-in ADCs, in some circumstances, this solution is impractical. For example, you might have an all-digital microcontroller already designed in...

The residual-current circuit breaker in Figure 1 continuously monitors the supply lines for any leakage current and immediately disconnects the supply if necessary. Load-supply wires, both live and neutral, pass through the magnetic core of the CR4311-5 transducer (http://www.crmagnetics.com), which monitors the supply current. Under normal circumstances, because the current flowing in both conductors is equal and opposite, no flux is generated in the transducer core...

The simple circuit design in Figure 1 lets you measure all components of a current flowing in a dc servo motor. The rectified output of the circuit uses ground as a reference, so you can measure the output by using a single-ended A/D converter. The current-sense resistor, R1, has a value of 0.1. The Zetex (www.zetex.com) ZXCT1010 IC converts the differential signal across R1 to a single-ended signal. Two of these ICs form a signal rectifier...

The accurate, high-side, current-sense circuit in Figure 1 does not use a dedicated, isolated supply voltage, as some schemes do. Only the selected transistors limit the common-mode range. The circuit measures the voltage across a small current-sense resistor, RS. The operation of the circuit revolves around the high-side current mirror comprising Q1 and Q2. All the circuit components have one overall function: to make the collector currents equal in Q1 and Q2...

The accurate, high-side, current-sense circuit in Figure 1 does not use a dedicated, isolated supply voltage, as some schemes do. Only the selected transistors limit the common-mode range. The circuit measures the voltage across a small current-sense resistor, RS. The operation of the circuit revolves around the high-side current mirror comprising Q1 and Q2. All the circuit components have one overall function: to make the collector currents equal in Q1 and Q2...

High-side current-sense amplifiers, such as IC1 in Figure 1, monitor current from a battery or dc power line in systems for which the ground-return continuity is crucial. IC1`s 36V maximum operating voltage, however, excludes some applications, including traction-motor batteries and central-exchange power supplies, both of which require operation at voltages exceeding 72V...

CMOS-interface ICs serve as gateways to the wired connections in electronic systems. Mishandling those external connections can damage the interface ICs by causing shorts to ground or by applying voltages that result in latch-up. The circuit in Figure 1 protects CMOS interface circuitry by sensing excessive currents. IC1 monitors the supply current (IS) to the interface circuitry and quickly removes current and voltage from the interface if IS exceeds a programmed threshold...

The simple circuit in Figure 1a can sense both low and high current levels without low sensitivities or loss of accuracy either at the low or the high end of the scale. The circuit is useful for discerning either low or high currents in noisy environments...

Several integrated high-side current-sense amplifiers, such as the MAX4172 (Maxim Integrated Products, www.maxim-ic.com), make it easy to measure the current from a positive power supply. With a couple of resistors, these devices provide a ground-referenced voltage output that is proportional to the delivered current (Figure 1a)...