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HowTo: Build a 16 Ch PicDimmed Renard with SSRs onboard!
( Dave Johnson)
Step 25 : Zero Crossing detector (H11AA1)
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| | R1 = 27K – quarter Watt
U1 = H11AA1
R2,R3 = 15K – halfWatt
There are several approaches to detecting when the AC voltage is zero. It could be done by bringing 110VAC onto the board (and using the H11AA1 circuit as shown above, and used in the 16ch PicDimmed Renard with SSR design), by using a dedicated pin on the cat5 (That is, having a H11AA1 or another device located near the computer, and sending the ZC signal down the Cat5), or by using a CAT5 pin for the power input and zero-crossing at the same time.
Renard doesn't have interrupt-on-change enabled, so there is no interrupt from the zero-crossing logic. Also, the input to the transistor circuit that has occasionally appeared in connection with Renard is a full-wave rectified AC signal, so the output of the transistor is very similar to the output of the H11AA1.
(http://forums.parallax.com/forums/default.aspx?f=21&m=61194)
This device provides notification of when the AC voltage is zero. It consists of two cross-coupled LEDs on the AC side and a phototransistor on the logic level side. Thus the one or the other LEDs is on during any non-zero portion of the AC sine wave. Both LEDs are off at the zero voltage point. By adding a pull-up resistor to the photo transistor output on the logic level side of the device, the zero crossing detector will pull the output low during the non-zero portion of the sine wave and the pull-up resistor will pull the line up to 5 V when the photo transistor turns off. The zero crossing detector will produce a positive going pulse every 8.333 milliseconds (1/120 = .0083333). This device also electrically isolates the 120 VAC side of the circuit from the logic level side of the circuit.
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