I have a water tank with a built in simple 'float switch'. It acts like any other switch. Last year I tried connecting it to a buzzer so that I would know when the tank would be empty. This burnt out the switch almost instantly, so I came up with something much too complicated for my likings:

Old system

Some things to note:

  • My battery system is 24v
  • The timer relay is 12v (hence the transformer), when powered it connects the output pins for 5 seconds (the length of the alarm).
  • I only added the entire 'switching part' to reduce the current, because when I would put the switch between the supply and transformer directly it would 'burn out' after 1 use.

From the start on I have found that the buzzer is not loud enough. So I am planning to 'upgrade' to a 12v buzzer with 85db. And I thought this would then be a good time to simplify things. See below:

New system Now the main question is:

  • How do I avoid burning out the switch? I thought of adding a resistor, but at 12v and lets say 100ma (the pull in for the relay is 50ma and the nominal use of the buzzer 25ma), the resistor would get too hot, wouldn't it? The switch is rated 500ma max. fyi.
  • Should I stick to the in my opinion over complicated old idea? I would like to make it a lot simpler and not have to use the extra 1.5v battery.


  • \$\begingroup\$ You are right, this is vastly overcomplicated. Start with what really matters: what are the ratings of the buzzer? And is the application safe for mains power at all? Also not that transformers only work with AC, while the rest of your circuit seems to assume DC. \$\endgroup\$ – Chris Stratton Sep 30 '19 at 14:17
  • \$\begingroup\$ Could you use a lower power piezo buzzer? Those used on smoke alarms easily carry through heavy doors. \$\endgroup\$ – Chris Stratton Sep 30 '19 at 14:20
  • \$\begingroup\$ @ChrisStratton Thanks, sorry the transformer should have been named dc-dc converter then. It runs of a 2x12v series battery, not connected to mains. The old system used a 5v piezzo buzzer, which I could hardly hear (it is in the basement). As for the ratings of the new buzzer: Nominal voltage 12vdc and nomimal current 25ma at 12vdc. \$\endgroup\$ – Thomas van Gerven Sep 30 '19 at 14:28
  • \$\begingroup\$ 25 mA should not be burning out a switch, probably not even with extreme inductive kick. Something odd is going on, and you need to understand what before you can design around it. \$\endgroup\$ – Chris Stratton Sep 30 '19 at 14:37
  • \$\begingroup\$ @ChrisStratton Thats exactly what I don't understand. Why is it killing the switch when it should be using only a small amount of current. Must be something with the relay then I would think.. \$\endgroup\$ – Thomas van Gerven Sep 30 '19 at 14:53
  • you need to stop making bad assumptions and define what you must have/use, the - float switch and any power source, not just the 24V "bell" {low VA} transformer
  • Define any important feature , like connect to a power relay to disable burning out the tank water heater? Beep ? instead of steady and have a self test?

Is it SPST-NO or NC? What is the current rating? Is it gold-plated (<2A) or non-corrosive?

  • small Buzzers come in 2 common types.
    Magnetic 10~100mA 3~24Vdc
    Piezo 1~10mA 3~24Vdc with built in oscillators
    Others range from 1.5Vdc to 220Vac

These are estimates. DigiKey et al will show what is in stock (hundreds of types)

So you need very little power P=V*I and varying resistance with fairly low current I=V/R

You can often waste power with a series voltage drop resistor if it is too loud, or put tape over the acoustic hole to reduce 100 dB to 85 dB when looking at rated current and loudness.

  • \$\begingroup\$ Thanks for your answer. The specs of the switch (its NC) are: Switch power(Max): 10W Switching voltage(Max): DC 100V Switching current(Max): 0.5A Max Breakdown Voltage: DC 220V Max Carry Current: 1.0A Max Contract resistance: 100mΩ. I just don't understand why the switch breaks down when I connect it between the battery and the relay, as you stated the buzzers use very little current. And so does the relay. \$\endgroup\$ – Thomas van Gerven Sep 30 '19 at 14:50
  • \$\begingroup\$ you also need to show your work and define the switch position when low water on float and answer my questions \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 30 '19 at 14:51
  • \$\begingroup\$ I already took it apart to experiment. Ill do my best to answer: - The switch closes when the water gets lower than the switch. It then lets current flow. - There is no water heater - The only function should be to sound an alarm for about 5 seconds when the water is low. That's what I used the timer relay for (it has a 555 timer and potentiometer on it to set the amount of time for it too close the loop) - Current rating 0.5A as stated before - It's sealed plastic so I can't check the inside. The floating switch itself is suspended in water if that was not clear. \$\endgroup\$ – Thomas van Gerven Sep 30 '19 at 15:17
  • \$\begingroup\$ Now show your wiring diagram of inputs and outputs. This should be no more difficult than a flashlight right? must be something missing like grounds? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 30 '19 at 15:19

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