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

Rate this post
 • ▲ useful ▼ not useful
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?

Rate this post
 • ▲ useful ▼ not useful
Hi,

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

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

Rate this post
 • ▲ useful ▼ not useful
PS.
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...

• $formula (LaTeX syntax)$