Monitor status of slow-switching (1 second) signal
Hard to get any simpler than below. The pulses drive a charge pump to turn on an N-channel MOSFET which drives an LED. When pulses stop, the gate voltage drops, shutting off the LED. It doesn't matter if the input signal stays high or low. As long as pulsing stops, the LED turns off after a few seconds.

Only downside is that your circuit has a heavy capacitive load on the rising edge. Shouldn't be a problem for most dumb logic. If it's a problem, don't add another transistor like most would do. Just add a 5k resistor in series to limit the current to 1 mA. Hope that helps, -Vince