I'm trying to design a monitor circuit for my folks steam boiler furnace. For whatever reason, when the weather changes, the boiler demands water and then the internal valve gets stuck on... the boiler eventually overfills, filling the steam registers and it's a BIG MESS

so I'm going to throw a microcontroller and a liquid flow sensor at the problem.

The uC will monitor the thermostat circuit - when the thermo requests heat, the uC will switch on an external valve and monitor the water flow. When the boiler is operating normally, it doesn't need water but every few days, but the internal safety system needs to see water pressure, otherwise the boiler won't fire. The thermostat circuit is 24vac.

So what I need to do is detect when the thermostat circuit is closed, and pass that information to a uC, which will open a valve, monitoring the water flow, and shut down the valve if it flows for more than a few seconds.

How do I read 24vac with a 5vdc microcontroller?

I don't think there is much current involved in the thermostat circuit, as far as I can tell, a relay in the thermostat closes the circuit when it calls for heat, and that in turn completes a circuit for the gas valve ... a steam pressure sensor and a water pressure sensor are in series with that circuit and are normally closed except when their values are out of bounds (steam pressure too high, water pressure too low).. as far as I can tell, there are no "electronics" in the furnace, its from the mid 1900's.

Originally Posted by justDIY

So what I need to do is detect when the thermostat circuit is closed, and pass that information to a uC, which will open a valve, monitoring the water flow, and shut down the valve if it flows for more than a few seconds.

How do I read 24vac with a 5vdc microcontroller?

Just rectify it, add a smoothing capacitor, then a series resistor to feed the PIC I/O pin - make sure the pin you use has protection diodes, most do!. Calculate the resistor to give a suitable current using ohms law, you only need a mA or so - or even less (so, 36 v = 36kohm for 1mA). You will also need a resistor across the smoothing capacitor, to discharge it when switched off, the small current through the limiting resistor won't be enough.

its from the mid 1900's.

You mean like 1905?

BTW...can u use a voltage divider and the ADC from a cheap uC like 12F683 working at 20MHz?

Originally Posted by Tarsil

BTW...can u use a voltage divider and the ADC from a cheap uC like 12F683 working at 20MHz?

Yes, the speed makes no difference - you also don't need a voltage divider, just a single resistor in conjunction with the protection diodes - although a divider won't do any harm.

But why would you want to use an ADC?, all you're looking for is ON or OFF.

Originally Posted by Nigel Goodwin

Yes, the speed makes no difference - you also don't need a voltage divider, just a single resistor in conjunction with the protection diodes - although a divider won't do any harm.

But why would you want to use an ADC?, all you're looking for is ON or OFF.

Wel if the speed doesn't mater u can use a uC at 8MHz (not with external clock like in my orig idea) and 1or2 resistors. Simpler circuit then rectifier, smoothing cap etc.

thanks for the know-how mramos and Nigel ... I'll toss something on a breadboard and test it out tomorrow.

would a half wave rect suffice, combined with the smoothing cap? I'm not lookin for ultrapure DC here, and like you all said, there's minimal current involved here, even a small cap should hold up during the crossing?

the uC i'm thinking about using will either be a 16F 628a or a 630, depending on how many I've got of either left in storage... I know they have internal protection diodes on the digital inputs, would an external pair help or hurt, as extra insurance?

Hi
I think it is better to use an AC opto isoltator to detect when the thermostat circuit is closed. You only need a series resistor to limit the current in the photo diode and a load resistor for the photo transistor . The following web address is a datasheet of TLP620 which i have used to read AC signal and convert them to 5V
http://www.alldatasheet.com/datashee...BA/TLP620.html

Originally Posted by justDIY

would a half wave rect suffice, combined with the smoothing cap? I'm not lookin for ultrapure DC here, and like you all said, there's minimal current involved here, even a small cap should hold up during the crossing?

Should be fine!

the uC i'm thinking about using will either be a 16F 628a or a 630, depending on how many I've got of either left in storage... I know they have internal protection diodes on the digital inputs, would an external pair help or hurt, as extra insurance?

Won't help, but it won't hurt either.

Use a 12F683..is much cheaper.

Originally Posted by mramos1

Tarsil. Use the f683 at 31Khz with the internal OSC. Use a serial resistor. It will save parts and and draw less current. No ADC required.

No adc required?...I'l use the comp or just a usual digital imput? In that case i can use a puny PIC like 12F629.

Originally Posted by kamdy

I think it is better to use an AC opto isoltator to detect when the thermostat circuit is closed. You only need a series resistor to limit the current in the photo diode and a load resistor for the photo transistor .

interesting ... it looks like a DC opto, but with an extra LED along for the ride. I imagine a DC opto would work as well, just add a reverse diode externally, to protect the led from reverse voltage... except the led is going to be pulsing with the zero crossing event, so I'd need to rectify and add a smoothing cap and etc.

I'll try both the straight in and dc opto methods ... I haven't seen any double-diode isolators for sale in my available surplus shops, so I'll use a single with an external diode.

thanks all!