This circuit detects spikes in voltage around 18v. It is used to detect when a battery becomes disconnected on a running motorcycle. When that happens, the regulator is unable to maintain a steady voltage so the voltage fluctuates wildly. Even though the average voltage may remain near 14v, the instantaneous voltage can be extremely high leading to a large RMS voltage. This over-voltage can burn out electronics and lighting in the vehicle. This circuit detects the peak voltages and when there is significant peak voltage, a red LED lights up as a warning.

C1 is used to make the circuit less sensitive to transient spikes which don't contain much energy. This prevents false warnings. Only a spike with significant energy will set off the detector. If the voltage is pure DC, the threshold voltage is near 18v. If the voltage is in the form of a spike, the threshold is higher. The less energy the spike contains, the higher the threshold will be.

Since spikes can be very brief, the LED normally would only flash very briefly. In order to make the LED stay on for a longer period, C2 is added. When a spike is detected above the threshold, LED1 is lit and C2 charges very rapidly through R3. When the spike is gone, Q1 turns off and the base of Q2 gets current from C2, thus keeping the LED lit while C2 discharges. The layout of Q2, LED1, R4, and C2 is such that Q2 is in the active region when the LED is lit. This means the base current of Q2 is very small. This was done so Q2 would discharge slowly while keeping the LED lit.

Changing R1 will alter the threshold voltage. 470 ohms yields about 17.5v DC, while 1.5K yields about 18.7v DC.

Here is the what the circuit board of the prototype looked like. It was a tight fit into a serial cable connector!