I'm implementing an additional pulse counter connector now. The reed connector will be a 4pin connector - 2 pins for a reed sensor, 2 pins for a pulse counter.

Canique Ambience will be able to count pulses, e.g.

• from a rain gauge or
• from gas/water/electricity meters or
• from a fan's tacho signal to calculate the RPM

 with very low quiescent current.

Technically I will be pulling up the input pin via a 1Mohm resistor to 1.8 volt and current will only flow when a pulse is driving the input low. So if there is a 10ms pulse, then 1.8 µA will flow for 10ms.

The pulse counter itself will draw about 10nA.

If we assume that a pulse is coming in every second (1Hz), the average quiescent current that this circuit introduces would be 18nA + 10nA = 28nA. If pulses are coming in at a lower rate, then this figure will approach 10nA. If they come in at a higher rate, then average current will be above 28nA.

Another factor is the pulse duration. The higher the pulse duration, the higher the current consumption.

And don't forget the cable capacitance. A long cable will have a high capacitance. So even after releasing the pulse counter input, it can take a few ms for the capacitance of the cable to charge up - during that period current will flow to charge the cable capacitance.

Let's do a small calculation here. Assumption: Cable Capacitance = 1.6nF (80pF/meter, 20 meter cable)

1.6nF * 1.8V = 2.88 nanoCoulomb

2.88 nC = 2.88 nA * s = 2.88 nA for a 1 second period => we have to provide a current of 2.88 nA for 1 second to fully charge the cable capacitance

If the pulse rate is 1Hz, then we'd have an additional 2.88 nA of consumption due to the cable capacitance.

Next thing to consider is a debouncing circuit. The 1M pullup resistor will form an RC filter with a small capacitor (~1nF) so as to not count any bouncing in the switch. The reed sensor input also has an RC filter.

Until now I've written about driving the input low during a pulse. Is the opposite possible too? Driving the input high? If Canique Ambience is configured for that kind of operation (means: no pullup resistor), then yes.

An external device, sharing the same ground, can send pulses with up to 5V. The quiescent current of the pulse counter in Canique Ambience will be 10nA with no additional current since there is no pullup resistor active.

What are the differences between the reed sensor and the pulse counter?