Using this voltage probe, three voltage levels of 12-V standard car batteries can be displayed-normal, high, and low (see the figure). It can also indicate levels for dc power supplies with output voltage above 6 V.
When the battery is operating normally between 11 V ("Low") and 15 V ("High"), the LED remains off. If the voltage climbs above 15 V, the LED begins blinking at about 1 Hz. The frequency depends on the capacitance of C. When the voltage falls to 11 V or other desired Low level, the LED glows continuously.
The probe contains an LED (Jumbo Bright Red or something similar), a low-cost 7400-series gate, two Zener diodes, one oxide capacitor, and several resistors.
If the input voltage is less than the desired low threshold, both the A and B outputs will be High, pulling the C and D outputs low, and causing the LED to glow. When the input level is above the Low threshold but below the High, B will have a low output; pins 4 and 5 will be above the switching threshold, and thus C and D will have a high output, and the LED won't glow. In addition, when input pins 2 and 5 are above the High threshold, the circuit will operate as a two-NAND-based multivibrator, causing the LED to blink.
R3 must be larger than R1 for the circuit to run normally. Due to the IC's tolerances, the values of R2 and R4 may need to be adjusted to set the thresholds. Once adjusted, the voltage thresholds' stability will remain within 5% between 60 and 100° F, which suits many applications.