Matrix user forums

kersing wrote:Have you considered using a resistor divider? Connect your input to 400K resistor (R1). Connect other side of R1 to analog input of PIC and to 100K resistor (R2). Connect second side of R2 to ground. Connect ground of input with ground of circuit.

The voltage at the analog input should be Vin * 100K / (100K + 400K) = Vin * 1/5 = Vin / 5. So for input 25V the output voltage would be 5V.

Make sure to use high quality metal film resistors with 1% tolerance for these kind of circuits. Total cost should be < 1 euro.

stevenvh@electronics.stackexchange.com wrote: Best is to use an optocoupler to detect zero-crossings. Put the mains voltage via high resistance resistors (R1, R2) to the input of the optocoupler. Vishay's SFH6206 has two LEDs in anti-parallel, so it works over the full cycle of the mains voltage.



If the input voltage is high enough the output transistor is switched on, and the collector is at a low level. Around the zero crossing, however, the input voltage is too low to activate the output transistor and its collector will be pulled high. So you get a positive pulse at every zero crossing. The pulse width depends on the LEDs' current. Never mind if it's more than 10% duty cycle (1ms at 50Hz). It will be symmetrical about the actual zero-crossing, so the exact point is in the middle of the pulse.

To detect power outages you (re)start a timer on every zero-crossing, with a timeout at 2.5 half cycles. Best practice is to let the pulse generate an interrupt. As long as the power is present the timer will be restarted every half cycle and never time out. Upon a power outage however, it will timeout after a bit longer than a cycle, and you can take the appropriate action. (The timeout value is longer than 2 half cycles, so that a spike on 1 zero-crossing causing a missed pulse won't give you a false warning.)
If you create a software timer it won't cost you anything, but you can also use a retriggerable monostable multivibrator (MMV), for instance with an LM555.