The circuit shown in the figure is recommended for applications where a visual indication is desired when the input voltage is outside a certain range of values. For example, in a power supply for TTL circuits, it can be adjusted to indicate when the voltage is below 4.5 V or above 5.5 V. Two operational amplifiers with field effect transistors of the CA3140 type are used, but equivalent types can be tried, including bipolar types such as the 741 or the dual operational MC1458. You just need to pay attention to the operating voltages of the chosen amplifiers. The two trigger values are adjusted in the trimpots, which, for greater precision, can be of the multiturn type. The diodes are general-purpose, and the indicator LED can be of any color. An amplifier stage can be used to trigger a relay or even a warning system. The circuit can be used with 5 to 12 V sources.
