When testing circuits and fault finding there is often a need to inject a defined DC voltage level at some point on the circuit. It may, for example, be necessary to apply a logic level on the input of a digital gate or a reference voltage level on the input of an analog circuit. With this handy piece of kit you can select one f five fixed voltage levels of 0 V, 2.5 V, 3.3 V, 4.096 V or 5 V., In addition, the output can supply a variable voltage in the range of 2.9 V to 7.3 V adjustable via a pot.
A stable 2.5 V reference voltage is sourced from a REF192 low voltage reference chip. This level is then multiplied by the gain of an opamp with switchable DC gain to produce the output voltage levels. A ten-stage Johnson counter type 4017 selects the amp’s gain setting. A pushbutton provides the clock signal to the counter. An RC filter followed by Schmitt trigger IC2.A suppresses unwanted signal transitions generated by pushbutton contact bounce. A Johnson counter only ever has one output high at any one time so each press of the button advances this high to the next output. The outputs switch FETs which in turn connect the voltage divider networks to ground to produce the different gain settings. The outputs also have LEDs, giving a visual indication of the output voltage level setting.
Counter output 6 is connected back to the reset input so that after the sixth output the counter is reset to the beginning again. To produce a 0 V output level the voltage reference chip is switched into sleep mode and the 1 KΩ resistor R4 ensures a 0 V output. Power supply for the circuit is provided by a standard or rechargeable 9 V battery. To make the design easy to use it can be mounted in a plastic casing to fit comfortably in the hand. The output signal can then be connected to a test probe extending from the case. An earth connection is also necessary and can take the form of a flying lead terminated in a croc clip for connection to the test circuit earth.