Unless it reaches really high levels, what we refer to as ‘voltage’ is totally invisible, so test and measurement equipment is a must in every electronics lab. However, in many cases you’ll just want to know if a voltage is present at or not a certain point. The tester described as this month’s Mini Project is an unusual one because it can work with just one test lead — all you have to do is use the single probe to touch the point you want to check.So where’s the other test lead connected to, after all, a voltage always exists between two points? The answer is: the voltage is measured against ‘ground’ connected to the metal enclosure of the test instrument. In this way, it is defined by the person holding the test probe in his/her hand. With each voltage test, a safe current of just a few micro-amperes flows through the person’s body. The tester will amplify this current to a level where it can be indicated by an LED.