The brushless dc fans found in many types of equipment can be crucial to the performance and longevity of that equipment, and a quick indication of fan failure can be essential in preventing major damage. Among the many approaches for identifying and indicating stalled fans, the circuit of Figure 1 is very simple and reliable.
The fan's tachometer output connects to the watchdog input of a uP supervisor (U1). The LED remains off during normal operation, but if the tachometer does not change state within a watchdog timeout period, U1 lights the LED by asserting its reset output. (As a result, the LED pulses on and off as the supervisor goes through its watchdog/reset cycle.) The LED in this example has a 200ms on time and flashes with a period of 1.6 seconds, which is suitable for most purposes.
Figure 1: This uP supervisor monitors a fan's tachometer output.