Hi again,
How are you testing this? Have you got a pot (voltage divider) connected to the input and the +5v supply (one lug 0v, one 5v, and the wiper as the input), so you can vary the input voltage form 0-5V just by turning it?
quote: Maybe that is my problem and I should put a potentiometer at the input as well and set the clamp to 5v but turn the input down all the way.
Yep. Although the pot at the input would 'simulate' the output of your sensor. So you set the clamp votlage pot, and slowly turn the input from 0v to 5v, and it 'should' hold the output at a certain threshold. if the input is unconnected, god knows what it will do...probably just pick up noise, so the voltage could be anything.
I think that the reason your circuit refuses to put out less than 1.6V is because of the opamp itself. As I said, opamps have an output voltage swing less than the supply voltage, it will never output 0V or Vcc (your power supply voltage), I guess, for your opamp, the lowest outptut with respect to ground is 1.6V.
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/a741p2.html
for a different opamp, but has useful info
The LF358 apparently allows the output voltage to be very close to the negative supply (GND in this case) are you using this one? Theoretically, it should allow it to drop below 1v.
From that 'DIY FCD' page, he does explain it rather well, and I've been thinking about that circuit, but my analogue knowledge is not what it should be. All it seems to be is a voltage follower (a buffer) with a low pass filter on the input to attenuate noise pickup (but not eliminate), and a comparator. The comparator uses a diode in reverse bias so that the output swtiches from one input, to the other. Ie: from your signal, to your reference, set by the pot. (bit similar to my idea, but simpler, and rather clever
)
If your app (your sensor) is identical to the one in the page mentioned above, then I don't think you need to worry, as the graph he has shows the lowest output of the sensor to be in the order of 2v.
Let me know how you get on.
Blueteeth.