Voltage Test Probe

Posted on Oct 3, 2012

Using inexpensive components, you can fit a simple probe circuit into a pencil-sized enclosure. When both LEDs are on, the probe indicates the presence of an ac voltage; either LED alone indicates the presence and polarity of a dc voltage. The diode-bridge arrangement allows one-way current source R1, R2, Q1, and Q2 to light either LED (or both) when the probe is activated by a test voltage.

Click here to download the full size of the above Circuit.

Diodes provide the necessary peak-inverse voltage rating; R3 and C1 provide a spike-suppression network to protect the current-source transistors.

Leave Comment

characters left:

Related Circuits

Instrumentation Amplifier

In a single difference amplifier configuration, the AM-427 exhibits excellent common-mode rejection and spot nois

Duty-Cycle Measurer

An Intersil 7101 31/2-digit A/D converter is used to display the duty cycle of a pulse train as a percentage. The frequ

Four-Trace Oscilloscope Adapter

This simple adapter uses an oscillator (567) to drive a counter (U2) and switch (U3) that selects the outpu

Digital Tachometer Circuitry

This system can be used with 4-, 6-, or 8-cylinder automobiles. The timebase formed by IC5 is an oscillator th

B-Field Measurer

This circuit develops an output voltage that is proportional to the magnetic induction, B, flowing through its probe"s coi

Simple Absolute Value Circuit

When an input voltage is positive, the output of ICA is negative and diode D does not conduct; hence the outp

Microfarad Counter

This circuit measures capacitance by the time it takes an unknown capacitor to reach 6.32 V (10 V 63% or 1RC time const

Electrolytic Capacitor Reforming Circuit

Sometimes electrolytic capacitors that are stored for some time will exhibit high leakage currents

New Circuits

Zero consumption Anti-theft High power Code Switch for E-Bike Batteries

This project fits more for custom made battery casing that can have more space to hide things inside. This switch with few alterations can be used for any other project as well but is designed based on e-bike needs! This project use a clever functionality of....
Ultrasonic distance finder circuit

The circuit described here uses ultrasonic oscillations and operates based on the propagation velocity of these oscillations in the air. Thus, we can easily determine the distance of two points if the time within which the wave travels this distance is....
Simple Metal detector circuit with CD4030

Sometimes the precious metals are hidden too deep and are not detected except with complicated devices. In many cases, however, small pieces of precious metal buried near the surface can be detected by relatively simple means. Everyone is very attractive to....
Fully automatic watering circuit for flower pots

Many times for various reasons we forget or can not water the plants that we have in our homes. And many humidity sensors units just notify us with a beeping sound or with a flashing light, that the pot needs watering. But what if we are away from home? This....
Simple deep searching metal detector circuit

The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the....
Sub-Harmonic Bass Converter for Electric Guitars

The sub-harmonic bass generator is a sound producing unit for guitars. The sound it produces looks very much like that of the bass guitar. The octave generator is sometimes quite unusual since it does not produce any of the usual ways of coloring the sound,....
Echo Depth Sounding Sonar for Boats

In the past the sounding (sea bottom measurement) was done with the "bullet", that is, with a heavy lead object that the seamen plucked into the sea hung from a calibrated rope. As soon as the "bullet" reached the bottom, the depth appeared directly from the....
100W HiFi Audio Amplifier Circuit

This Amplifier was designed to have the following specifications: Distortion less than 0.1% at full power of 100W even at 20KHz. Power has to be attributed to an extended bandwidth. The output transistors must be protected against short circuits. The power....
Plant-Pot Water Level Indicator Circuit

A series of LEDs serve to alert the gardener when plants need water. Using two conventional digital integrated LEDs and a series of LEDs, we make a very useful device for gardening. The device detects the amount of water in the pot and alerts the grower.....