This Idea for Design provides a visual indication of an input voltage’s relationship to upper and lower threshold values and can be used as a quick pass/marginal/fail tester. It uses a modified Wien-Bridge oscillator structure to create an op-amp circuit with three output modes (Fig. 1).
With the input voltage VIn above a first threshold voltage, the op amp is in a high-voltage output mode. With input below a second voltage threshold, the op amp is in a low-voltage output mode. For an input voltage between the two thresholds, the op-amp output oscillates between its high and low output values.
Oscillation also means the op-amp circuit will be alternately a current source and a current sink. Connecting a single package bi-color green/red LED to the op-amp output thus provides a visual indication of the input voltage. The bi-color LED has a green LED with an opposite polarity from the red LED. So when the op amp is sourcing current (high output mode), the green LED will be on, and when the op amp is sinking current (low output mode), the red LED will be on.
For input voltages in the window between the thresholds, the oscillating output will alternately light each LED. The oscillation is fast enough, however, that the bi-color LED will appear to the eye not as two blinking lights but as a single, steady light of a yellowish color. Because the oscillator’s duty cycle varies with input voltage, the ratio of red to green light being emitted—hence the exact shade of yellow seen—will depend upon the input voltage.
With an input voltage at the high end of the op-amp oscillation window, the duty cycle will be close to 100% and the light from the bi-color LED will appear a yellow-green color. At the low end, the duty cycle will be close to 0% and the light emitted will appear red-orange. In the middle of the oscillation window the light will appear pure yellow.
The two stable conditions for the circuit occur when VIn is high enough that the op amp drives to and stays at its maximum output value or is low enough that the op amp drives to and stays at its minimum output value. The thresholds for these stable conditions occur when the voltage at VSet is such that there is zero current through the op amp’s inverting input. At a threshold point, then:
where VOut is either the op amp’s minimum or maximum output voltage.
Resistors R1 and R2 form a voltage divider that, in conjunction with R3, sets the voltages where the oscillation window is active. A general rule is that the higher the value of R3, the narrower the input voltage range for oscillation. Resistors R1 and R2 may be replaced by a potentiometer to custom tune the circuit for a desired operating range.
The circuit as shown uses a Linear Technology LT1637 op amp with maximum output (at VCC = 6 V) of 5.36 V and minimum output 0.64 V. Substituting these values into the equation yields an oscillation window starting at 4.34 V on the high end and 3.15 V on the low end. These minimum and maximum output values will, of course, depend on VCC and may differ if other op amps are used.
The circuit’s oscillation frequency depends on the RC network of R4, R5, and C1. The center frequency of the oscillation window is given by:
which yields a center frequency of 32.15 Hz using the values shown.
Figure 2a shows the circuit’s response to the input voltage as VIn ramps down from 4.5 V to 3.0 V and triggers all three modes of operation. The op-amp oscillations begin where VIn equals the maximum value of VSet and stop where VIn and VSet are again equal at the minimum value of VSet.
Current through the green LED as a function of VIn appears in Figure 2b. The green LED is on when VIn is above 4.34 V and off when VIn is below 3.15 V, and it flashes on and off between the two voltages, as calculated. Measurements show that the oscillation’s duty cycle is 79% at the beginning and 18% at the end of the oscillation window. The current through the red LED has the opposite polarity of the green LED and so is on when the green LED is off.
Resistors R6, R7, and R8 serve as current limiters and are based on the specifications of the Stanley Electric BRPG1204W bi-color LED used here. The op amp was selected for its ability to operate with input voltage and supply ranging up to 44 V, which makes the circuit useful over a large voltage range. The current limiting resistors will have to be scaled appropriately, though, so they don’t overdrive the bi-color LED.