Creating a Versatile Voltage Injection Tool
In the process of circuit testing and troubleshooting, there’s often a requirement to introduce a specific DC voltage at a certain point in the circuit. This necessity might arise, for instance, when applying a logic level to a digital gate’s input or a reference voltage to an analog circuit’s input. This convenient device allows you to choose from five fixed voltage levels: 0 V, 2.5 V, 3.3 V, 4.096 V, or 5 V. Additionally, the output provides a variable voltage ranging from 2.9 V to 7.3 V, adjustable through a potentiometer.
Generating Precise Output Levels with Voltage Reference Chip
A stable 2.5 V reference voltage is obtained from the REF192 low voltage reference chip, serving as the foundation for generating the desired output voltage levels. The reference voltage is multiplied by the gain of an operational amplifier, which has a switchable DC gain, resulting in the output voltage variations. The selection of the amplifier’s gain setting is facilitated by a ten-stage Johnson counter of type 4017. A pushbutton generates the necessary clock signal for the counter.
To eliminate unwanted signal transitions caused by the pushbutton’s contact bounce, an RC filter followed by Schmitt trigger IC2.A is employed. The unique characteristic of a Johnson counter ensures that only one output is high at any given time. Each press of the button advances the high output to the next stage, activating the corresponding FETs. These FETs connect the voltage divider networks to ground, producing distinct gain settings. Additionally, LEDs are integrated into the outputs, providing a visual representation of the output voltage level settings.
Intelligent Reset and Zero Voltage Output
The output of the sixth stage of the counter is looped back to the reset input. This arrangement ensures that after the sixth output, the counter resets to the initial state, maintaining a continuous and cyclical operation. To achieve a 0 V output level, the voltage reference chip is transitioned into sleep mode, and a 1 KΩ resistor (R4) guarantees a 0 V output. Power for the circuit is supplied by a standard or rechargeable 9 V battery. For user convenience, the design can be encased in plastic, allowing comfortable handheld use. A test probe can extend from the casing to connect with the output signal, while an earth connection, often a flying lead terminated in a crocodile clip, is necessary for linking to the test circuit’s earth.