Electronic – AC Watchdog Circuit

capacitorwatchdog

I've designed a small AC-based watchdog circuit for my Raspberry Pi (schematic below). A buddy of mine looked at it and said that it was complicated, and there might be problems with asymmetric charge/discharge of the two capacitor's will be dependent on the rate & duty cycle of the watchdog tickle signal. I'm planning on tickling it with a software watchdog timer at around a 1Hz rate (the software watchdog will be running at sufficient priority to make that happen). VREF_MID is a resistor divider providing 2.5V.

Any problems with the circuit that jump out at anyone?

Thanks in advance!

Circuit diagram http://www.krten.com/ac.jpg

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Detection circuit using AC charge pump.

Would this circuit work for your application? C1 passes alternating signal through, D1 and D2 charge C2 which holds on the relay as long as the AC tickle runs.

I haven't worked out component values.

How it works

  • As the left side of C1 rises and falls so does the right side. If the right side goes negative D1 conducts and holds it slightly below zero.
  • When the tickle goes high the right side of C1 goes high. D1 is reverse biased and D2 is forward biased. Some charge is transferred from C1 to C2 raising its voltage.
  • After a few cycles C2 voltage will be high enough to turn on Q1 and RLY1.
  • If the tickle stops alternating - either fails high or low - no charge will be pumped and C2 voltage will decay through Q1. The relay will drop out.
Related Topic