Measurement Circuits


Celsius Thermometer

The circuit of the Celsius thermometer in the diagram is based on the well-known Type LM334 from National Semiconductor. This IC is a sensor that provides a current which is directly proportional to the temperature in kelvin (K). Unfortunately, this is a quantity that is not suitable for use in most practical applications. In the […]


Simple Voltmeter

This circuit provides a simple means to determine the voltage of a low-impedance voltage source. It works as follows. P1, which is a 1-W potentiometer, forms a voltage divider in combination with R1. The voltage at their junction is buffered by T1, and then passed to reference diode D1 via R3. D1 limits the voltage […]


High Side Current Measurements

It’s always a bit difficult to measure the current in the positive lead of a power supply, such as a battery charger. Fortunately, special ICs have been developed for this purpose in the last few years, such as the Burr-Brown INA138 and INA168. These ICs have special internal circuitry that allows their inputs to be […]


Precision Inductance and Capacitance Meter

This circuit for measurement of inductance and capacitance can be used to test whether the values of inductors and capacitors quoted by the manufacturer are correct. The principle used in the circuit is based on the transient voltages produced across inductors and capacitors connected as series R-L and R-C networks, respectively, across a constant voltage […]


Absolute-Value Meter With Polarity Detector

This circuit breaks an input voltage signal down into its components: (1) the absolute value and (2) the polarity or ‘sign’ (+ or –). It will handle direct input voltages as well as alternating voltages up to several kHz. With a supply voltage of ±9 V, the input level should remain below ±6V. The circuit […]


Measuring Inductors

Often you find yourself in the position of needing to wind your own coil for a project, or maybe you come across an unmarked coil in the junk-box. How can you best find out its inductance? An oscilloscope is all you need. Construct a resonant circuit using the coil and a capacitor and connect it […]


TV Relative Signal Strength Meter

This circuit was designed to assist the installation of TV antennas. The signal is monitored using a small portable TV set and this circuit monitors the output of the TV’s FM detector IC via a shielded lead. To initially calibrate the meter, adjust trimpot VR2 to zero the meter. Trimpot VR1 is a sensitivity control […]


Linear RF Power Meter

The National Semiconductor LMV225 is a linear RF power meter IC in an SMD package. It can be used over the frequency range of 450 MHz to 2000 MHz and requires only four external components. The input coupling capacitor isolates the DC voltage of the IC from the input signal. The 10-k? resistor enables or […]


Car Tachometer

A 555 is configured as a monostable or one shot in this project. The period of the 555 is determined by the 47k and the capacitor from pin 6 to ground (100n). Time “T” = 1.1 RC or 1.1 X 50,000 X 0.1 X10 -6 = 0.0055 or 5.5 mS (milli-seconds).The 555 receives trigger pulses […]


Bicycle Speedometer With Hub Dynamo

The idea for this circuit came when the author had problems with the wireless speedometer on his bicycle. Such a device consists of two parts: the cycle computer itself and a transmitter that is mounted on the front fork. A small magnet is attached to the spokes so that the transmitter sends out a pulse […]