Voltage Sensing Hcpl3700 Code May Be

The AC input module doesn;t work for 24 VAC, but a few component changes and it should.

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I reviewed a number of posts on the HPCL-3700 Arduino and found some circuits that looked promising.

Attached is what I assume is correct, but wanted to see if anyone sees any issues with resistor values or other details.

One thing I am not sure of is if I need to add a capacitor to the DC DC- on the optocoupler or not That was referenced in a post here.

I would build it and make sure it works and does what you want.

Ron.

I have a simple Arduino Sketch watching for my digital pin (connected from pin 6 on the HCPL-3700) to switch from HIGH to LOW, but nothing changes when the 24v comes into pins 1 4.

I have the 10 uF cap on 2 3, 20k resistors on the lines to 1 4, and the 10k resistor between Vcc (pin 8) and Vo (pin 6).

I have my Vcc (pin 8) connected to the 5V pin on my Arduino and the GND (pin 5) to GND on Arduino, as shown in the circuit design.

I assume I should do some testing to figure out if things are configured as expected.

I wonder, what should I be expecting to come from pin 6 (Vo) when 24v comes across Pin 1 4.

Can I check with a voltmeter for something perhaps Maybe my resistance values or the cap are an issue I have seen some designs calling for a cap between pin 8 (Vcc) and pin 5 (GND) Any other thoughts Thanks for any assistance.

Voltage Sensing Hcpl3700 Code May Be

If thew LED comes on but you cannot read it, than the code may be in question.

I first tested this sketch using a pull-up pull-down circuit example I found online (see attached) to confirm that the logic was working before connecting to the heating system - and it worked as expected.

In the end, I followed the configuration for Example 2 (AC Operation) along with the following Input Filtering for AC Operation to add the 10uF cap for ripple suppression.

After doing the calcs on resistance on pages 5-7, Rx V - Vth Ith along with the specification for calculating RMS at 65 when using a capacitor for ripple (specified on page 7), I got 24vRMS 0.65 15.6v for V, Vth is specified as 5.1 and Ith is 2.5: Rx 15.6 - 5.1 2.5 4.2kOhms for resistance total on pins 1 4 (AC) - so split, 2.2 on each leg.

This didnt work, so I checked the voltage I was getting at pin 1 4 when I connected my 24v and it was just over 6v, which seemed a touch out of spec of 5v for the Vth (input upper voltage threshold level).

So I increased resistance slightly to 6.6kOhms total which brought my voltage at pin 1 4 down to 5v (perfect).

After checking for logic on the other side (pin 6), I get still got nothing.

I tested pin 6 as well with my voltmeter, assuming I would see some voltage when I expected logic out, but it stayed at 0.

I tried the LED with resistance from pin 6 on the HCPL-3700 and got nothing of course, since no voltage was coming out) Any thoughts Thanks.

In any case, wouldnt I see some kind of voltage come from pin 6 of the HCPL-3700 if the detector side switched I saw nothing in any case which leads me to believe I am not triggering the detector side for some reason.

That or set the digital input of the Arduino for an internal pull up.

Now if you want to use an internal pullup you would change the code to read.

The AC module is cheap and so is the backplane, Its a pretty nice system.