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I have a very simple circuit that is misbehaving in a way that I cannot explain with my very limited experience.

The circuit that consists of a motion detector:

(http://www.digikey.ca/product-search/en?x=0&y=0&lang=en&site=ca&KeyWords=255-1809-ND)

hooked up to an inverter:

(http://www.digikey.ca/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=TC74HC04APF-ND&x=0&y=0&cur=USD).

I have the output of the sensor hooked up to pin 9 (4A) on the inverter, and I am reading the output on pin 8 (4Y) with a 10X oscilloscope probe.

What I am observing is that the output of the inverter just stays LOW at all times. So, even when there is no motion and the sensor is outputting LOW, the output of the inverter is LOW. Upon closer examination there are 5ms spikes in the inverter output, but that's all. However, if I attempt to read the state of the input with another oscilloscope channel, the circuit immediately starts working as it should. I can see the output of the sensor going HIGH/LOW when expected and I see the inverter properly mirroring that.

I looked up the capacitance of my probe, which states 22pF, so I tried to put a 27pF capacitor between inverter input (i.e. sensor output) and ground, which simply caused those 5ms spikes to go away. Now the inverter is really stuck on LOW.

Next I tried to put a 400K pull-down resistor between inverter input and ground. This indeed makes everything work again, except that I am wasting current by sinking the output of the sensor into the ground. I'd say about 40uA, which is really serious, considering that my whole circuit is supposed to run on 50uA. I suppose I could increase the pull-down resistor to 2000K to match the probe, but I'd like to understand why this is necessary before I go out buying such strangely high valued resistors...

Any advice would be appreciated.

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Look at the sensor's output stage in the datasheet. It is an open drain output on the high side. This means that its output is either high or floating. The pull-down resistor resolves the floating state, and that is why hooking the probe to it makes it work.

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