In my house I have a mechanical water meter with a 2 wire reed switch that sends a pulse every 10 liters to (I suppose) an electronic meter to witch I don't have access. I'd like to connect a circuit to the same switch to read the pulses to track my water usage, without interfering with the electronic meter, but I don't know what circuit is already wired to the switch. The only specs I've got on the sensor are " 24 Vca/Vcc, 0,1 A " and a simple diagram of a switch.

reed switch

I suppose, being a switch, that whatever circuit is currently connected, it must push a current through it when the switch closes to sense the switch closing. Maybe I could measure when current is flowing to sense the pulses? How could I do that without interfering with the electronic meter? Hall sensor? would that work? There's probably a simpler solution I'm not seeing.

  • \$\begingroup\$ Yes any comparator will do with shielded pair \$\endgroup\$ Jan 21, 2021 at 19:59
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 no idea what that means ahah. I know little about electronics. Could you be more extensive? \$\endgroup\$
    – nicari
    Jan 21, 2021 at 20:16
  • \$\begingroup\$ It's an IC that in a small circuit compares an input with a threshold in between to make a logic level out \$\endgroup\$ Jan 21, 2021 at 20:32
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    \$\begingroup\$ If this is the meter that controls your utility bills, there are probably quite strict clauses in the contract against modifying it. You might get away with contactless sensing, maybe even taping a Hall sensor to the wire, but anything involving cutting the wire or joining another to it is likely to be right out. \$\endgroup\$ Jan 21, 2021 at 20:54
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 much more clear. Thanks! \$\endgroup\$
    – nicari
    Jan 21, 2021 at 21:56

2 Answers 2


Maybe add a hall sensor to the outside of the meter to sense the magnet that pulses the reed switch.

Or perhaps add a current sensor to the reed switch circuit, like a clip-on hall sensor or current transformer.


A simple way that doesn't involve any electronics is to use a relay repeater.


simulate this circuit – Schematic created using CircuitLab

Figure 1. A relay provides a simple way of repeating the operation of a contact multiple times while providing electrical isolation between the circuits.

  • SW1 feeds the coil of a small 12 V relay. The relay will energise and de-energise with SW1.
  • Use one contact of the 2-pole relay to drive the meter.
  • The second contact can be used for your other circuit.
  • D1 is a snubber diode to prevent arcing and destruction of the contacts of SW1.
  • You'll need a small 12 V power supply.

You're looking for a 12 V, 2-pole relay, ideally with a low coil current (high resistance coil). Given that they're saying 0.1 A max, we'll derate by a factor of two and say 50 mA. From Ohm's law you're looking for a coil resistance of more than \$ R = \frac V I = \frac {12}{0.05} = 240 \ \Omega \$.

You might find something suitable in a car parts shop.

Why 12V if the sensor specify "24 Vca/Vcc"?

24 V is the maximum rated voltage that the contact can switch reliably. The rating is determined by the gap between the contacts when open and the switching speed. A larger gap is required to provide insulation at higher voltages. We can use any voltage up to 24 V.

A relays power (how strong an electromagnet it has to pull the contacts together) can be achieved in a number of ways using the formula P = VI where P is the power in watts, V is the voltage and I is the current in amperes. You should be able to see that for a given power, say 1 W, we could achieve this using a 5 V, 200 mA coil, a 10 V, 100 mA coil or a 20 V, 50 mA coil. In your case I've selected a commonly available voltage (12 V for automotive) in the hope that you'll find one with < 50 mA current requirement.

The contact ratings are a separate matter. These need to be rated ≥ the voltage and current they are switching. This isn't an issue at all in your application as the meter is probably passing a milliamp or so through the contact.

12 volt planet might be a good place to learn more on the topic.

Have a look through your junk collection for a wall-wart power supply with a 12 V DC output. Cut the DC plug off, check the polarity with a multimeter and you're in business - and safe!

  • \$\begingroup\$ Thanks! Didn't know about these devices! Why 12V if the sensor specify "24 Vca/Vcc" ? \$\endgroup\$
    – nicari
    Jan 21, 2021 at 22:07
  • \$\begingroup\$ See the update. \$\endgroup\$
    – Transistor
    Jan 21, 2021 at 22:21
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    \$\begingroup\$ I wouldn't do that. if your power supply fails the other counter stops working. you could end up getting sued by whoever owns the other counter. \$\endgroup\$
    – Jasen
    Jan 22, 2021 at 0:00

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