Forum: µC & Digital Electronics Pump for a water-bearing stove

von Rolf (Guest)

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I have written a program for the Raspberry Pi to control a pump for a 
water-bearing stove. The program works fine - but I still need a 
computer independent safety device. If the Raspberry fails - then it 
must be ensured that the pump runs in continuous operation to avoid 
overheating of the boiler. My first approach was - to evaluate a clock 
signal to be output from the Raspberry. As long as the clock signal 
comes from the Raspberry - as long as a retriggerable monostable toggle 
stage is reset again and again. If the clock signal is missing - the 
pump goes into continuous operation. But if the clock signal remains 
HIGH - then this idea does not work. One must also evaluate the falling 
edge of the clock signal and connect it with logic. At the moment I am 
on the hose - can someone help me?

von minifloat (Guest)

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I suggest to address the problems from the root, in following 

1 put an overpressure blowoff valve at the output of the boiler. This 
acts against the fault effect and prevents severe injuries, as the 
boiler tank cannot explode anymore.

2 put an overtemperature switch in series to the heater element. 
Consider also using a non-resettable type. This acts against the root 
cause and also prevents fire.

3 now it is time to think about the additional safety device you're 
planning. Simple solution is make the toggling signal AC by adding a 
capacitor in series and a resistor after the cap to GND. Then feed this 
into the input of a retriggerable monoflop. A simple form of this is the 
attached circuit. (from 
https://cpldcpu.wordpress.com/2021/02/27/the-transistorpixel/ )
Higher capacitor values have to be used, and a resistor in series to 
Q7:collector of a few hundred Ω is advisable.

br, mf

von minifloat (Guest)

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A pressure switch of NC type in series with the heater element could 
also serve as another safety mechanism. But pressure switches could  be 
rather bulky and maybe also expensive...

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