A simple logic probe in concert with a signal source can be used to check digital as well as linear circuits. With the flick of a switch, the probe can inject a test signal or check the logic state of a digital signal. Comprising just one CMOS IC (CD4011B) and a handful of components, the whole unit fits inside of a very small enclosure. It can be used as an add-on unit for test equipment that lacks such features (see the figure).
When the switch is put in the "2" position and the probe is connected to an IC in a digital circuit, giving a logic "1" (high), LED2 is off. When given logic "0" (low), LED2 is on. The corollary takes place for the other gate. Therefore, LED1 is on in an inverse fashion to LED2.
Gates C and D with the associated circuit constitute an oscillator whose frequency can be set by VR1. When the switch is put in the "1" position, the circuit functions as a signal source. This source is most useful for checking audio circuits. Faults can be easily located by injecting this output to each stage of an audio amplifier.